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Can you double layer insulation?

ABOUT INSULATION The thermal barrier of a home should consist of a continuous layer of insulation on all sides—including the lowest floor, the exterior walls, and the ceiling or roof. Doubling the thickness of insulation will double the insulation's R-value, cutting heat loss in half.

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ABOUT INSULATION Thicker is better In cold weather, a puffy parka holds in your body heat. Insulation does the same thing for a house. The thicker the insulation, the better it works to reduce heat flow from the inside of a home to the outside during winter, and from outside to inside during summer. The thermal barrier of a home should consist of a continuous layer of insulation on all sides—including the lowest floor, the exterior walls, and the ceiling or roof. Doubling the thickness of insulation will double the insulation’s R-value, cutting heat loss in half. Each time the insulation layer is doubled in thickness, this rule applies. The energy saved per year by doubling insulation from R-10 to R-20, however, will be considerably more than the energy saved by doubling insulation from R-20 to R-40, because of the law of diminishing returns. In some cases, like an attic, it’s worth piling on more insulation because there is plenty of room. It’s much more expensive to add that much insulation to exterior walls. It takes more than just insulation to slow heat Stopping air leaks is just as important as — maybe more important than — adding insulation. Unless builders prevent air from leaking through walls and ceilings, insulation alone won’t do much good. Not only are drafts uncomfortable, but air moving through insulated cavities can cut the efficiency of the insulation by as much as 50%. Some insulation types make good air barriers, and some don’t. In all cases, it’s best to keep the insulation tight to the air barrier. THERMAL BRIDGING IS CONDUCTION IN ACTION When there is no insulation in a roof or wall, the framing is the most insulated part of the assembly. It has the highest R-value. Softwood lumber has an R-value of 1.25 per inch, so a 2×6 stud has an R-value of almost 7. As soon as you put insulation between the studs or rafters above R-7, however, the framing becomes the weak thermal link. If the framing cavities are filled with closed-cell spray foam insulation, the insulation has an R-value of about 36. At that point, the studs or rafters become a glaring weakness in the design. Building scientists call this phenomenon “thermal bridging” because the studs or rafters bridge the space between inside and outside the thermal envelope. If you look for it, thermal bridging can sometimes be seen from either inside or outside. Inside, it can cause a problem called ghosting, or cold stripes behind the drywall during winter. These cold stripes can encourage condensation that leads to the accumulation of dust particles on the drywall; eventually, visible vertical stripes may form. Outside, you can see the effect of thermal bridging in snow-melt patterns on roofs and drying patterns on walls. A continuous layer of rigid foam installed on the inside or outside of a wall or roof drastically reduces thermal bridging through the framing. R-VALUE MEASURES HOW WELL INSULATION WORKS Heat flows from hot to cold; it can’t be stopped, but it can be slowed If we measure the rate at which heat flows through a building material or building assembly—for example, a wall or a roof—we can calculate a number (the R-value) to indicate its insulating ability. The higher a material’s R-value, the better the material is at resisting heat flow through conduction, convection, and radiation (outlined above). Insulation manufacturers report R-values determined by tests following ASTM standards (for example, ASTM C518). Common insulation types and their R-values Residential insulation materials have R-values that range from about 3 to 7 per inch. The amount of insulation installed in any given building assembly depends on the climate, the part of the house being insulated, the project budget, and local code requirements. Batts and blankets: R-3.1 to R-4.1 per in.

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Blown-in and loose-fill insulation: R-2.6 to R-4.2 per in.

Rigid foam: R-3.6 to R-6.8 per in.

Closed-cell spray foam: R-6 to R-6.8 per in.

Open-cell spray foam: R-3.5 to R-3.6 per in. Green homes go beyond code minimum The U.S. Department of Energy has developed a list of recommended insulation levels for different climate zones. The climate zones are represented on the map (click to enlarge). For more information on climate zones, see All About Climate Zones. Houses heated by natural gas, fuel oil, or an electric heat pump should use the R-values set out by the DOE and listed below as a base. Because electric heat is relatively expensive, houses with electric resistance heat need more insulation than shown in the table below. In some parts of the country, minimum code requirements for insulation already (or may soon) exceed these DOE recommendations. For example, the 2009 International Residential Code requires cold-climate builders to include a minimum of R-20 wall insulation and R-15 basement wall insulation. DOE recommended R-values for various parts of a house Zone Attic Wall Floor Slab edge Basement wall (framing cavity insulation) Basement wall (continuous rigid insulation) 1 R-30 to R-49 R-13 to R-15 R-13 R-4 R-11 R-10 2-3 R-30 to R-60 R-13 to R-15 R-13 to R-25 R-8 R-11 R-10 4 R-38 to R-60 R-16 to R-21 R-25 to R-30 R-8 R-11 R-4 5 R-38 to R-60 R-16 to R-27 R-25 to R-30 R-8 R-11 to R-19 R-10 to R-15 6-8 R-49 to R-60 R-18 to R-27 R-25 to R-30 R-8 R-11 to R-19 R-10 to R-15 In any case, green builders almost always exceed minimum code requirements for insulation thickness. Many energy consultants, including Betsy Pettit and Joseph Lstiburek, now recommend that cold-climate homes include R-60 ceilings, R-40 above-grade walls, R-20 basement walls, and R-10 basement slabs. Some builders go further; for example, an Illinois home designed to meet the rigorous German Passivhaus standard is insulated to nearly R-60 on every side—even under the slab.

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