Thursday, March 18, 2010

Mechanical properties of the porcine bile duct wall

Abstract

Background and Aim

The function of the common bile duct is to transport bile from the liver and the gall bladder to the duodenum. Since the bile duct is a distensible tube consisting mainly of connective tissue, it is important to obtain data on the passive mechanical wall properties. The aims of this study were to study morphometric and biomechanical wall properties during distension of the bile duct.

Methods

Ten normal porcine common bile ducts were examined in vitro. A computer-controlled volume ramp infusion system with concomitant pressure recordings was constructed. A video camera provided simultaneous measurement of outer dimensions of the common bile duct. Wall stresses and strains were computed.

Results

The common bile duct length increased by 25% from 24.4 ± 1.8 mm at zero pressure to 30.5 ± 2.0 mm at 5 kPa (p < 0.01). The diameter increased less than 10% in the same pressure range from 8.6 ± 0.4 mm to 9.3 ± 0.4 mm (p < 0.01). The stress-strain relations showed an exponential behavior with a good fit to the equation: σ = α . (exp(βε) - 1). The circumferential stress-strain curve was shifted to the left when compared to the longitudinal stress-strain curve, i.e. the linear constants (α values) were different (p < 0.01) whereas the exponential constants (β values) did not differ (p > 0.5).

Conclusion

The porcine bile duct exhibited nonlinear anisotropic mechanical properties.

Background

The function of the common bile duct is to transport the bile from either the gall bladder or the liver to the duodenum by passing it through the sphincter of Oddi. The duct has been described as a passive conduit consisting mainly of connective tissue with a high collagen content and only few smooth muscle cells . Contractions have been reported but these may be retrograde projections of contractions from the sphincter of Oddi .
Contractions of the gall bladder and relaxation of the sphincter of Oddi facilitate bile flow . Hence, the bile duct can be considered as a pressure vessel without the ability to generate active forces by itself. Since the biliary tract is a distensible pressure vessel, it is important to obtain data on the mechanical wall properties. The mechanical properties will determine the behavior of the duct during loading and is likely to change in diseases of the biliary system. Few papers have focused on the biomechanical and morphometric properties of the bile duct wall. The literature on bile duct mechanics mainly contains data on ducts tested uni-axially in vitro . Uni-axial testing can not be done with preserved tri-dimensional structural integrity of the organ wall. Distension of intact segments provides a more physiological-like condition of testing. When the force of inflation is applied, the intact segment deforms . This approach was used in two studies of the normal porcine bile duct . However, these studies were limited to measurements only at high pressures or to circumferential tension-strain relations where tension was computed from the transmural pressure and radius but where the wall thickness was not measurable. The stress-strain relation is, however, a more valid measure of the biomechanical properties . Stress is force per unit cross-sectional area. Strain refers to the resulting deformation of the material and is usually expressed as a fraction of the initial length. Strain is non-dimensional which favors comparison between different experiments. The proportionality constant between stress and strain for a linear relationship is called the elastic modulus and is a measure of wall stiffness . For non-linear stress-strain relations, an incremental modulus can be computed or mechanical constants determined. In cylindrical tubes the normal stress and strain components are in radial, longitudinal and circumferential directions. If the wall is thin, then the radial component can be ignored and the mechanical problem can be reduced to a two-dimensional one.
The aims of this study were to provide morphometric measures of the wall changes during distension of the bile duct and to derive the circumferential and longitudinal stress-strain relations in order to determine the mechanical properties.

Material and methods

Anesthesia and blood samples

Ten four-months-old LY-strain female pigs (a mixture of Danish Country breed and Yorkshire) weighing 43.9 kg ± 0.7 kg were studied. They were fasted overnight and received as im. premedication 4.8 mg kg-1azaperone (Sedaperone®) and 0.6 mg kg-1midazolam (Dormicum®). Thirty minutes later 0.4 mg kg-1etomidate (Hypnomidate®) was administered iv. and the pigs were intubated and connected to the respirator. Anesthesia was maintained by continuous intravenous infusion of 10 mg kg-1 h-1ketaminol (Ketamine®), 0.6 mg kg-1 h-1midazolam and 0.12 mg kg-1 h-1parvulon (Pancuron Bromide®). A blood sample was taken for analysis of bilirubin, alanine transaminase and alkaline phosphatase to reduce the probability of the pigs having illnesses of the liver and biliary system. The values for bilirubin, alanine transaminase and alkaline phosphatase were 7.6 ± 0.99 μmol l-1, 48.4 ± 4.3 U/l and 247.8 ± 19.8 U/l, respectively. Thus, all pigs were considered normal and included in the study. The study complied with the Danish regulations for care and use of laboratory animals.

Isolation of the common bile duct

An upper midline abdominal incision was made and the bile duct was exposed. The pressure in the bile duct was measured by inserting a needle into the duct and then connecting it to a low compliance perfusion system using external pressure transducers (Baxter uniflow™). The pressure signal was analog-to-digital converted with a sampling frequency of 5 Hz and acquired on-line to a computer using dedicated software (SuperMingo™, Gatehouse Aps, Aalborg, DK). The mean pressure in the common bile duct was 0.81 ± 0.06 kPa. The pressure fluctuated with respiration with approximately ± 0.05 kPa. Next the bile duct was dissected from the adjacent tissue. Proximally, the duct was cut at the level where the cystic duct intersects with the hepatic duct to create the common bile duct. Distally, the duct was cut at the duodenal wall. The duct did not seem to change length during excision. The pig was euthanized by an intracardiac injection of KCl after the segment was removed.

 Figure 1. Schematic drawing of the in vitro setup. The bile duct is placed in the organ bath and attached to the volume infusion/pressure measurement system. A video camera connected to a monitor and VCR provides pictures of the bile duct for external bile duct morphometric measurements.

In vitro procedures

After excision the common bile duct was immediately transferred to an organ bath containing 22°C oxygenated (95% O2 and 5% CO2) calcium-free Krebs-Ringer solution with 95 mg l-1 EGTA and 60 g l-1 Dextran at pH = 7.40. EGTA was added to abolish muscle contractions. The pressure measuring system was calibrated and the zero pressure level was set with the transducer at the same level as the surface of the solution in the organ bath. The proximal end of the segment was ligated to a tube connected to an infusion system and the transducer. The other end was ligated as close to the sphincter of Oddi as possible. Air was removed from the lumen of the segment. The above-mentioned Krebs-Ringer solution was used for infusion into the segment after equilibrium to zero pressure. The data acquisition software also controlled a rollerpump with infusion and withdrawal functions (Ole Dick, Instrumentmakers Aps, Denmark). The pump was programmed to infuse or withdraw volume so that inflation to 5 kPa lasted approximately 1.5 min. The volume rate was between 0.5 and 1.5 ml min-1, depending on the size of the bile duct in each pig. When the pressure reached 5 kPa, the flow was reversed for the same time period as the infusion to assure that the withdrawn volume equaled the infused volume. The bile duct was preconditioned by six cycles of volume infusions and withdrawals up to 5 kPa pressure (the number of cycles determined by pilot studies). After the preconditioning procedure, the distension experiment was done as one more cycle to the same pressure level. A Sony CCD camera and a VCR provided recordings of the outer dimension of the bile duct under the pressure changes induced. The experimental set up is illustrated in figure 1. The segment was removed from the organ bath after the distension series and four rings from each segment were cut at 20, 40, 60 and 80% length with the proximal end of the segment being 0%. The rings were 2 mm wide and were immersed in neutrally buffered formaldehyde. The formaldehyde fixed rings were visualized under microscope (Zeiss, Stemi 2000-C). The views were frame grabbed, displayed on a monitor and the wall thickness was measured at four locations of each ring. Since there was no variation in thickness along the circumference, the thickness was given as the average of these four measurements (h0).

 Figure 2. Pressure recording versus time during volume infusion. The pump was reversed at a pressure of 5 kPa. The overshoot may be caused by inertia in the infusion system.

 

Validation of system

Pump infusion and withdrawal precision were tested as the repeatability in the range from 0.5 to 1.5 ml min-1 by infusing and withdrawing over 2 minutes for each volume setting 10 times . The coefficient of variance was between 2.2% and 6.3%, which was found acceptable. No difference in volume was found between when the pump was set at infusing or withdrawing volume (p > 0.2).
Repeatability of the morphometric measurements done using the digital image analyzing software SigmaScan Pro (Jandel Scientific, Germany) was tested by measuring a precision scale 10 times and the result was given as the coefficient of variance . The resolution was evaluated as the two-point discrimination. Evaluation was done with magnifications corresponding to the experimental conditions. When the video camera was not mounted on the microscope, a calibration scale of 10 mm was measured ten times. The measured value was 9.97 ± 0.03 mm with 0.86% coefficient of variance. The two point discrimination was 0.096 ± 0.001 mm evaluated over ten experiments. When the video camera was mounted on the microscope (magnification 20×), a calibration scale of 5 mm was measured as 4.99 ± 0.01 mm with 0.74% coefficient of variance. The two point discrimination was 0.023 ± 0.002 mm over ten experiments.
The dynamic parameters of the pressure measuring system were evaluated by a pressure chamber test . The pressure recording system behaved as an overdamped second order system. The rise time was 0.08 sec (10 – 90% of the pressure drop from 9 to 0 kPa) meaning that the system was capable of measuring a pressure rise of 56 kPa sec-1. This value was magnitudes higher than the pressure changes measured in the experiments (between 0.21 and 0.67 kPa sec-1 close to the end of the inflation where the pressure rise was steepest (Figure 2)).

Morphometric and mechanical data analysis

Still pictures from the VCR were frame grabbed at pressures 0, 0.5, 1, 1.5, 2, 3, 4 and 5 kPa for each segment during infusion. The length and diameter were measured from the digitized images using SigmaScan Pro. The length was measured between the 20 and 80% locations to avoid edge effects. The outer diameter was calculated by measuring the area of the segment between 20 and 80% length and dividing it by this length to obtain a mean diameter of the segment. The internal diameter and wall thickness could not be measured experimentally with this preparation but was calculated based on the following assumptions : 1) the wall was incompressible, i.e. the volume of the wall did not change during distension, 2) the shape of the common bile duct was cylindrical, 3) the wall thickness at no-load conditions and outer diameter and length at various inflation pressures were measurable, and 4) the wall thickness-to-radius ratio was small. The volume of the wall (V) at pressure 0 kPa was calculated as
V = (ro02 - (ro0 - h0)2)πl0     (1)
where ro0, h0 and l0 were the measured outer radius, the wall thickness and the length at pressure 0 kPa. The wall thickness was calculated for all pressure steps as:
where l and ro were the measured length and outer diameter during loading.
Calculations and measurements as described above were used for computation of the mechanical parameters assuming equilibrium conditions . The Circumferential stress was defined as:
where ΔP was the transmural pressure and ri the inner radius (ri = ro - h). As the segment was isolated and the surface of the Krebs-Ringer in the organ bath was the zero pressure reference, the transmural pressure was equal to the applied pressure in the segment.
The Longitudinal stress was defined as:
Cauchy strains were calculated for simplicity, though the deformation was quite large .
Circumferential strain was defined as:
where rm was the midwall radius (by subtracting 1/2h from the outer radius) at the various pressure loads. r0m was the reference midwall radius calculated from the radius measured at a pressure of 0 kPa.
The Longitudinal strain was defined as:
where l was the measured segment length under the imposed pressure loads and l0 the reference length measured at a pressure of 0 kPa.
The Radial strain was defined as:
where h was the calculated wall thickness of the imposed pressure loads and h0 the measured wall thickness of the formaldehyde fixed rings.
The circumferential and longitudinal stress-strain relations were compared using curve-fitting software (TableCurve 1.12®, Jandel Scientific, Germany). The exponential equation σ = α . (exp(βε) - 1) was used since the elasticity of biological tissues often exhibits exponential behavior [9]. The α and β values were derived from the curve fit regression done for each segment separately. The slope of the non-linear stress-strain curves is called the incremental elastic modulus and is a measure of wall stiffness. The exponential equation was differentiated, yielding σ' = (αβ) . exp (βε), to define the incremental elastic modulus. The circumferential incremental elastic modulus at high and low strain was correlated to the diameter at 0 kPa and to the wall thickness-to-radius ratio (determinant for the stress). The circumferential incremental elastic modulus was calculated at the highest and lowest strain in common for all bile ducts.

 Figure 3. Bile duct morphometric parameters as a function of pressure. All parameters varied as function of pressure (p < 0.01). n = 10. Mean ± SEM is shown.

Statistical analysis

Results are expressed as mean ± SEM unless otherwise stated. The data distribution was tested for normality by inspecting probability plots and for variance homogeneity by Bartlett's test. Student's t-test and in case of non-parametric distribution of data Mann-Whitney Rank sum test was used for statistical analysis. The α and β values were compared statistically by using one-way analysis of variance (ANOVA) or Kruskal-Wallis one-way analysis of variance on rank if the normality test or the test of equal variance failed. Association between the morphometric and biomechanical parameters was evaluated by Pearson Product Moment correlation. The results were considered significant when p < 0.05.
Figure 4. Bile duct mechanical parameters. The strain-pressure data are from circumferential (circles), longitudinal (squares) and radial (triangles) directions. Note in the stress strain graph (B) and the elastic modulus graph (C) that the circumferential curves are shifted to the left of the longitudinal indicating that the bile duct is stiffest in circumferential direction. n = 10. Mean ± SEM is shown.

 

Results

The pressure increased only slightly during the first minute of inflation. This was followed by a gradually steeper increase until the maximum pressure of 5 kPa was reached. Thus, the inflation curve had an exponential course with a long toe region and a steep region (Figure 2). Though the pump was reversed at a pressure of 5 kPa, overshoot was observed before the pressure decreased again.
The variation of length, diameter and wall thickness of the bile duct as function of pressure were non-linear. The most pronounced change occurred in the pressure range from 0 to 2 kPa (Figure 3). The length increased by 25% from 24.39 ± 1.75 mm at 0 kPa to 30.54 ± 2.08 mm at 5 kPa (p < 0.01). In the same pressure range the diameter increased less than 10% from 8.61 ± 0.40 mm to 9.33 ± 0.41 mm (p < 0.01). The wall thickness decreased by 31% from 1.00 ± 0.06 mm at 0 kPa to 0.69 ± 0.04 mm at 5 kPa (p < 0.01). The wall thickness-to-radius ratio decreased by 38% from 0.24 ± 0.02 at 0 kPa to 0.15 ± 0.01 at 5 kPa (p < 0.01).
The longitudinal and circumferential strains were positive and with the largest strain in the longitudinal direction (Figure 4A). The radial strain was negative. All three strains were non-linear when expressed as a function of pressure. The longitudinal and circumferential stress-strain relations were non-linear (Figure 4B). The determination coefficients for the equation: σ = α . (exp(βε) - 1) were 0.90 ± 0.05 and 0.92 ± 0.01 in circumferential and longitudinal directions, respectively. The circumferential stress-strain curve was shifted to the left when compared to the longitudinal stress-strain curve. The linear constant differed (p < 0.01) whereas the exponential constant did not differ (p > 0.5) between the circumferential and longitudinal direction. The incremental elastic modulus as a function of strain was non-linear and the circumferential curve was shifted to the left of the longitudinal curve (Figure 4C).
An association was found between the circumferential incremental elastic modulus and the diameter at a pressure of 0 kPa. The association was significant for the incremental elastic modulus at both high and low strain values (r = 0.884 and 0.894, p < 0.01 for both, figures 5A and 5B). An inverse association was found between the circumferential incremental elastic modulus at low strain and wall thickness-to-radius ratio at 0 and 5 kPa (r = -0.764 and -0.677, p < 0.05). No association was found between the elastic modulus at high strain value and the wall thickness-to-radius ratio.

 Figure 5. Regression curves between the unloaded diameter at 0 kPa and the circumferential incremental elastic modulus at low (A) and high (B) strains. An association was found at both low strains (r = 0.894) and high strains (r = 0.884) (p < 0.01).

 

Discussion

The major findings were that in the pressure range studied 1) the increase in bile duct length was much larger than the increase in bile duct diameter, 2) the circumferential and longitudinal stress-strain relations and the incremental elastic modulus showed an exponential behavior and the circumferential curves were shifted to the left of the longitudinal curves, and 3) an association was found between the circumferential incremental elastic modulus and the diameter at the initial unpressurised state and between the circumferential incremental elastic modulus and wall thickness-to-radius ratio.

Methodological aspects

Data on bile duct stress-strain relations were obtained by volume infusion into the common bile duct under simultaneous pressure recordings. At the same time outer dimensional changes were recorded. Analysis of diameter and length showed a high degree of accuracy based on the evaluations performed in this study. One of the assumptions made in this study for the stress calculations was that the geometric configuration of the lumen was circular. This was confirmed by vision and further validated by determining a second diameter in 90° planes by a prism in a few pilot experiments (unpublished data). Other assumptions made in this study were that the bile duct wall thickness-to-radius ratio was relatively small in order to use thin shell theory for analysis and that the wall was incompressible. Such assumptions are commonly made in biomechanical studies because it simplifies the analysis. Thin shell theory can be applied as long as the membrane is so thin that bending rigidity can be neglected . The wall thickness-to-radius ratio should be around 0.10–0.20. In this study wall thickness was measured at the unpressurized state and then calculated for the pressurized states. At the unpressurized state the wall thickness-to-radius ratio was 0.24 and decreased to 0.15 at 5 kPa. The stress-strain relation under the circumstances in this study reflects mainly the passive elastic properties. Preconditioning was done to obtain repeatable results . Thus, after the specimen was mounted in the organ bath, the loading cycles were repeated until the stress-strain relationship became stabilized. The interpretation of preconditioning is that the tissues are disturbed in the preparation process by cutting, temperature changes, chemical environment, hypoxia and smooth muscle contractions and need to be restored to a stabile condition .

Physiological and mechanical aspects

Exponential mechanical behavior has been observed in the bile duct in vitro as well as in other tissues in vivo and in vitro . The exponential behavior is expedient for organs with reservoir function since low wall stiffness at physiological pressures facilitate wall stretch to accommodate the gall. The steep increase in wall stiffness with higher loads provides a mechanism to avoid overstretch and damage to the tissue. This is in agreement with a previous study of compliance in the intact bile duct where high compliance was found at low pressures and low compliance at high pressures . The normal pressure range in the bile duct is associated with the migrating motor complex (MMC) of the intestine and the pressure waves are transmitted from the sphincter of Oddi . In phase I and II of the MMC the pressure range in the bile duct is between 0.6 and 1 kPa and rise to 1.3 kPa under phase III . With bile duct obstruction the pressure stabilizes at about 3 kPa . The most pronounced changes in dimension seen in this material were in the pressure range from 0 to 2 kPa, i.e. in the physiological pressure area. As commonly done in biomechanical studies, these experiments also superseded the physiological range.
Positive association between the incremental elastic modulus and the diameter of the segment at 0 kPa pressure at both low and high strain values was demonstrated. Hence, the bigger the initial diameter, the bigger the elastic modulus. This is likely due to that a bigger duct needs to be stiffer to counteract the higher force exerted by the pressure. A thicker wall or a stiffer material in the wall can contribute to the increased stiffness.
The common bile duct shows an anisotropic behavior with a much larger capacity to stretch in the longitudinal direction compared to the circumferential direction. A previous study in the intact bile duct showed that stress in the longitudinal direction was lower than in the circumferential direction which supports our finding. Anisotropy also characterize other biomaterials such as arteries where it was found that the elastic modulus in the circumferential direction was less than in longitudinal direction . The fact that the bile duct more readily elongates than increases its diameter may have several, yet hypothetical, functions. First, elongation may mechanically affect the sphincter of Oddi, facilitating leakage of gall through it at high biliary tract pressures. Second, a rather stiffwalled organ in circumferential direction reduce the wall stress but increases the shear stress and the resistance to flow. Elongation further contributes to higher shear stress and resistance to flow. Hence, during obstruction a pressure will build up faster, resulting in inhibition of bile production. These hypotheses obviously need further study using more advanced biomechanical approaches.

Tuesday, March 2, 2010

Your Home Naturally,cool

Keeping cool indoors when it is hot outdoors is a problem. The sun beating down on your home causes indoor temperatures to rise to uncomfortable levels. Air conditioning provides some relief. But the initial costs of installing an air conditioner and the electricity costs to run it can be high.
In addition, conventional air conditioners use refrigerants made of chlorine compounds, suspected contributors to the depletion of the ozone layer and global warming. But there are alternatives to air conditioning. This article provides some common sense suggestions and low-cost retrofit options to help you "keep your cool" and save electricity.

Staying Cool

An alternative way to maintain a cool house or reduce air conditioning use is natural (or passive) cooling. Passive cooling uses non-mechanical methods to maintain a comfortable indoor temperature.
The most effective method to cool your home is to keep the heat from building up in the first place. The primary source of heat buildup (i.e., gain) is sunlight absorbed by your house through the roof, walls, and windows. Secondary sources are heat-generating appliances in the home and air leakage. Specific methods to prevent heat gain include reflecting heat (i.e., sunlight) away from your house, blocking the heat, removing built-up heat, and reducing or eliminating heat generating sources in your home.

Reflecting Heat Away

Dull, dark-colored home exteriors absorb 70% to 90% of the radiant energy form the sun that strikes the home's surfaces. Some of this absorbed energy is then transferred into your home by way of conduction, resulting in heat gain. In contrast, light-colored surfaces effectively reflect most of the heat away from your home. The most effective method to cool your home is to keep the heat from building up in the first place.

Installing a radiant barrier

Radiant barriers are easy to install. It does not matter which way the shiny surface faces - up or down. But you must install it on the underside of your roof - not horizontally over the ceiling. and the barrier must face an airspace.
For your own comfort while in the attic, install the radiant barrier on a cool, cloudy day. Use plywood walk boards or wooden planks over the ceiling joists for support. Caution: Do not step between the ceiling joists, or you may fall through the ceiling. Staple the foil to the bottom or side of the rafters, draping it from rafter to rafter. Do not worry about a tight fit or small tears in the fabric; radiant transfer is not affected by air movement. The staples should be no more than 2 to 3 inches (5 to 8 centimeters) apart to prevent air circulation from loosening or detaching the radiant barrier. Use a caulking gun to apply a thin bead of construction adhesive to the rafters along the seams of the foil barrier. This will make the installation permanent.

Roofs

About a third of the unwanted heat that builds up in your home comes in through the roof. This is hard to control with traditional roofing materials. For example, unlike most light colored surfaces, even white asphalt and fiberglass shingles absorb 70% of the solar radiation. One good solution is to apply a reflective coating to your existing roof. Two standard roofing coatings are available at your local hardware store or lumberyard. They have both waterproof and reflective properties and are marketed primarily for mobile homes and recreational vehicles. One coating is white latex that you can apply over many common roofing materials, such as asphalt and fiberglass shingles, tar paper, and metal. most manufacturers offer a 5-year warranty.
A second coating is asphalt based and contains glass fibers and aluminum particles. You can apply it to most metal and asphalt roofs. Because it has a tacky surface, it attracts dust, which reduces its reflective somewhat. Another way to reflect heat is to install a radiant barrier on the underside of your roof. A radiant barrier is simply a sheet of aluminum foil with a paper backing. When installed correctly, a radiant barrier can reduce heat gains through your ceiling by about 25%.  Radiant-barrier materials cost between $0.13 per square foot ($1.44 per square meter) for a single-layer product with a kraft-paper backing and $0.30 per square foot ($3.33 per square meter) for a vented multiflora product with a fiber- reinforced backing. The latter product doubles as insulation.

Walls

Wall color is not as important as roof color, but does affect heat gain somewhat. white exterior walls absorb less heat than dark walls. and light, bright walls increase the longevity of siding, particularly on the east, west, and south sides of the house.

Windows

Roughly 40% of the unwanted heat that builds up in your home comes in through windows. Reflective window coatings are one way to reflect heat away from your home. These coatings are plastic sheets treated with dyes or thin layers of metal. Besides keeping your house cooler, these reflective coatings cut glare and reduce fading of furniture, draperies, and carpeting. Two main types of coatings include sun-control films and combination films. Sun-control films are best for warmer climates because they can reflect as much as 80% of the incoming sunlight. Many of these films are tinted, however, and tend to reduce light transmission as much as they reduce heat, thereby darkening the room. Combination films allow some light into a room but they also let some heat in and prevent interior heat from escaping. These films are best for climates that have both hot and cold seasons. Investigate the different film options carefully to select the film that best meets your needs. Note: do not place reflective coatings on south-facing windows if you want to take advantage of heat gain during the winter. The coatings are applied to the interior surface of the window.
Although you can apply the films yourself, it is a good idea to have a professional install the coatings, particularly if you have several large windows. This will ensure a more durable installation and a more aesthetically pleasing look.
Landscaping is a natural and beautiful way to shade your home and block the sun.

Blocking the Heat

Two excellent methods to block heat are insulation and shading. Insulation helps keep your home comfortable and saves money on mechanical cooling systems such as air conditioners and electric fans. Shading devices block the sun's rays and absorb or reflect the solar heat.

Insulation

Weatherization measures - such as insulating, weather stripping, and caulking - help seal and protect your house against the summer heat in addition to keeping out the winter cold. For more information on weatherizing your home, see our weather stripping article The attic is a good place to start insulating because it is a major source of heat gain. Adequately insulating the attic protects the upper floors of a house. Recommended attic insulation levels depend on where you live and the type of heating system you use. for most climates, you want a minimum of R-30. In climates with extremely cold winters, you may want as much as R-49. again, check the DOE fact sheet insulation on how to determine the ideal level of insulation for your climate.
Wall insulation is not as important for cooling as attic insulation because outdoor temperatures are not as hot as attic temperatures. also, floor insulation has little or no effect on cooling.
Although unintentional infiltration of out-side air is not a major contributor to inside temperature, it is still a good idea to keep it out. Outside air can infiltrate your home around poorly sealed doors, windows, electrical outlets, and through openings in foundations and exterior walls. Thorough caulking and weather stripping will control most of these air leaks.

Shading

Shading your home can reduce indoor temperatures by as much as 20 degrees F (11 degrees C).
 
Landscaping

Landscaping is a natural and beautiful way to shade your home and block the sun. A well-placed tree, bush, or vine can deliver effective shade and add to the aesthetic value of your property. When designing your landscaping, use plants native to your area that survive with minimal care. Trees that lose their leaves in the fall (i.e., deciduous) help cut cooling energy costs the most. when selectively placed around a house, they provide excellent protection from the summer sun and permit winter sunlight to reach and warm your house. The height, growth rate, branch spread, and shape are all factors to consider in choosing a tree.
Vines are a quick way to provide shading and cooling. grown on trellises, vines can shade windows or the whole side of a house. Ask your local nursery which vine is best suited to your climate and needs. Besides providing shade, trees and vines create a cool microclimate that dramatically reduces the temperature (by as much as (9øf []5øc]) in the surrounding area. During photosynthesis, large amounts of water vapor escape through the leaves, cooling the passing air. and the generally dark and coarse leaves absorb solar radiation. You might also consider low ground cover such as grass, small plants, and bushes.a grass-covered lawn is usually 10øf (6øc) cooler than bare ground in the summer. If you are in an arid or semiarid climate, consider native ground covers that require little water. For more information on landscaping, see the erec fact sheet landscaping for energy efficiency

Planning Your Planting

Placement of vegetation is important when landscaping your home. The following are suggestions to help you gain the most from vegetation:
  • Plant trees on the northeast-southeast and the northwest southwest sides of your house. Unless you live in a climate where it is hot year-round, do not plant trees directly to the south. Even the bare branches of mature deciduous trees can significantly reduce the amount of sun reaching your house in the winter.

  • Plant trees and shrubs so they can direct breezes. Do not place a dense line of evergreen trees where they will block the flow of cool air around or through them. * Set trellises away from your house to allow air to circulate and keep the vines from attaching to your house's facade and damaging its exterior. Placing vegetation too close to your house can trap heat and make the air around your house even warmer. * Do not plant trees or large bushes where their roots can damage septic tanks, sewer lines, underground wires, or your house's foundation. * Make sure the plants you choose can withstand local weather extremes.
Shading Devices

Both exterior and interior shades control heat gain. Exterior shades are generally more effective than interior shades because they block sunlight before it enters windows. When deciding which devices to use and where to use them, consider whether you are willing to open and close them daily or just put them up for the hottest season. You also want to know how they will affect ventilation. Exterior shading devices include awnings, lovers, shutters, rolling shutters and shades, and solar screens. Awnings are very effective because the block direct sunlight. They are usually made of fabric or metal and are attached above the window and extend down and out.
A properly installed awning can reduce heat gain up to 65% on southern windows and 77% on eastern windows. A light-colored awning does double duty by also reflecting sunlight. Maintaining a gap between the top of the awning and the side of the house helps vent accumulated heat from under a solid- surface awning. If you live in a climate with cold winters, you will want to remove awnings for winter storage, or by retractable ones, to take advantage of winter heat gain.
Roughly 40% of the unwanted heat that builds up in your home comes in through windows. The amount of drop (how far down the awing comes) depends on which side of your house the window is on. An east or west window needs a drop of 65% to 75% of the window height. A south-facing window only needs a drop of 45% to 60% for the same amount of shade. A pleasing angle to the eye for mounting and awning is 45ø. Make sure the awning does not project into the path of foot traffic unless it is at least 6 feet 8 inches (2 meters) from the ground. One disadvantage of awnings is that they can block views, particularly on the east and west sides. However, slatted awnings do allow limited viewing through the top parts of windows.
Louvers are attractive because their adjustable slats control the level of sunlight slats control the level of sunlight entering your home and, depending on the design, can be adjusted from inside or outside your house. The slats can be vertical or horizontal. Louvers remain fixed and are attached to the exteriors of window frames.
Shutters are movable wooden or metal covering that, when closed, keep sunlight out. Shutters are either solid or slatted with fixed or adjustable slats. Besides reducing heat gain, they can provide privacy and security. Some shutters help insulate windows when it is cold outside. Rolling shutters have a series of horizontal slats that run down along a track. Rolling shades use a fabric. these are the most expensive shading options, but the work well and can provide security. many exterior rolling shutters or shades can be conveniently controlled from the inside. One disadvantage is that when fully extended, the block all light.
Solar screens resemble standard window screens except they keep direct sunlight from entering the window, cut glare, and block light without blocking the view or elimination air flow. They also provide privacy by restricting the view of the interior from outside your house. Solar screens come in a variety of colors and screening materials to compliment any home. Although do-it-yourself kits are available, these screens will not last as long as professionally built screens.
Although interior shading is not as effective as exterior shading, it is worthwhile if none of the previously mentioned techniques are possible. There are several ways to block the sun's heat from inside your house.
Draperies and curtains made of tightly woven, light-colored, opaque fabrics reflect more of the sun's rays than they let through. The tighter the curtain is against the wall around the window, the better it will prevent heat gain. Two layers of draperies improve the effectiveness of the draperies' insulation when it is either hot or cold outside. Venetian blinds, although not as effective as draperies, can be adjusted to let in some light and air while reflecting the sun's heat. Some newer blinds are coated with reflective finishes. To be effective, the reflective surfaces must face the outdoors. Some interior cellular (honeycombed) shades also come with reflective mylar coatings. But they block natural light and restrict air flow. Opaque roller shades are effective when fully drawn but also block light and restrict air flow. Ventilated attics are about 30 degrees F (18 degrees C) cooler than unventilated attics.

Removing Built-Up Heat

Nothing feels better on a hot day than a cool breeze. Encouraging cool air to enter your house forces warm air out, keeping your house comfortably cool. However, this strategy only works when the inside temperature is higher than the outside temperature. Natural ventilation maintains indoor temperatures close to outdoor temperatures close to outdoor temperatures and helps remove heat from your home. But only ventilated during the coolest parts of the day or night, and seal off your house from the hot sun and air during the hottest parts of the day.
The climate you live in determines the best ventilation strategy. In areas with cool nights and very hot days, let the night air in to cool your house. A well-insulated house will gain only 1 degree F (0.6 degree C). By the time the interior heats up, and the outside air should be cooler and can be allowed indoors. In climates with day time breezes, open windows on the side from where the breeze is coming and on the opposite side of the house. Keep interior doors open to encourage whole house ventilation. If your location lacks consistent breezes, create them by opening the windows at the lowest and highest points in your house. This natural "thermosiphoning," or "chimney," effect can be taken a step further by adding a clerestory or a vented skylight.
In hot, humid climates where temperature swings between day and night are mall, ventilate when humidity is not excessive. Ventilating your attic greatly reduces the amount of accumulated heat, which eventually works its way into the main part of your house. Ventilated attics are about 30 degrees F (18 degrees C) cooler than unventilated attics. Properly sized and placed louvers and roof vents help prevent moisture buildup and overheating in your attic.
Often-overlooked sources of interior heat gain are lights and household appliances, such as ovens, dishwashers, and dryers. Because most of the energy that incandescent lamps use is given off as heat, use them only when necessary. Take advantage of daylight to illuminate your house. and consider switching to compact fluorescent lamps. These use about 75% less energy than incandescent lamps, and emit 90% less heat for the same amount of light.
New, energy efficient appliances generate less heat and use less energy. Many household appliances generate a lot of heat. When possible, use them in the morning or late evening when you can better tolerate the extra heat. Consider cooking on an outside barbecue grill or use a microwave oven, which does not generate as much heat and uses less energy than a gas or electric range.
Washers, dryers, dishwashers, and water heaters also generate large amounts of heat and humidity. To gain the most benefit, seal off your laundry room and water heater from the rest of the house. New, energy efficient appliances generate less heat and use less energy. When it is time to purchase new appliances, make sure the are energy efficient.
All refrigerators, dishwashers, and dryers display an energy guide label indicating the annual estimated cost for operating the appliance or a standardized energy efficiency ratio. Compare appliances and buy the most efficient models for your needs.
Using any or all of these strategies will help keep you cool. even if you use air conditioning, many of these strategies, may not be enough. sometimes you need to supplement natural cooling with mechanical devices. fans and evaporative coolers can supplement your cooling strategies and cost less to install and run than air conditioners.
Ceiling fans make you feel cooler. their effect is equivalent to lowering the air temperature by about 4 degrees F (2 degrees C). Evaporative coolers use about one-fourth the energy of conventional air conditioners. Many utility companies offer rebates and other cost incentives when you purchase or install energy saving products, such as insulation and energy efficient lighting and appliances. Contact your local utility company to see what it offers in the way of incentives.

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