Insulation Overview for New Homes


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What kind of insulation should I use in my home? Here is a brief overview of some construction methods used in the Pacific Northwest, in terms of how they eliminate air leaks, manage moisture, and minimize thermal bridging from interior to exterior.

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Insulation Overview for New Homes

  1. 1. Insulation Overview for New Pacific NW Homes Levin Nock Dec 2008 Copyleft CC-BY v2.0 Besides looking good and being durable, some of the main goals for the walls of a home, are to eliminate air leaks, manage moisture, and minimize thermal bridging from interior to exterior. To save money on heating and cooling, air sealing has a major impact. Spray foam works very well at air sealing. Blown-in insulation can also work well if it’s thick enough—7.5” is great, 5.5” is probably ok. Fiberglass insulation does not air seal well, so housewrap is usually used on conventionally built homes. Wall cavities that will be filled with fiberglass insulation should be spot-sealed first with spray foam or caulk, to form a redundant airsealing system to complement the housewrap. Unless walls are absolutely, perfectly airtight and watertight everywhere, they need a way to dry out when they get wet inside. For moisture management, pretty good air sealing can be worse than none. Many old, drafty, uninsulated walls manage moisture very well, as long as there’s plenty of heat inside the building. In the winter, the wall cavities are constantly dried out by all the hot air leaking through them to the outdoors. If a wall is airsealed, but there are a few air leaks remaining, then each leak can quickly attract condensation and mold growth. If you airseal at all, it’s important to do a detailed, thorough job. For instance, if housewrap is used, it should be carefully, meticulously taped and sealed around all the doors and windows. West of the Cascade Mountanis, a new wall should include an external rain screen—5/8” of space between the sheathing and the siding, maintained by vertical lathe or special housewrap, with screening at the top and bottom to keep out pests. This enables moisture on the back of the siding to dry out, instead of entering the wall cavity. Thermal bridges attract condensation and leak heat energy. The aluminum frame of an old double-glazed window is an obvious example
  2. 2. of a thermal bridge. A less obvious example is a standard 2x6 stud in a conventional stickframe wall, because wood has a very low R-value compared to most types of insulation. This becomes obvious in an infrared image of the exterior of a conventional wall on a cold day. The studs are clearly visible, as heat radiators. Many conventional 2x6 walls with R-19 insulation have an overall R-value below 10, because of thermal bridges. One way to minimize thermal bridges and air leaks, is to add a 2” external layer of closed cell polystyrene foamboard that’s taped and sealed. This layer is impermeable to water and water vapor. For moisture management, if the wall cavity gets wet, it must dry to the inside. So, unlike typical local construction, there is no vapor barrier at the wallboard. Ideally, the interior finish would be breathable, such as American Clay Plaster. Water vapor can enter the wall cavity from the interior of the building, but the exterior polystyrene keeps the wall cavity warm in the winter, so the vapor never condenses (assuming that interior humidity is managed at a reasonable level). This technique is used more commonly in the Midwest, where winter temperatures are more severe. It works with a standard 2x6 wall filled with blown-in insulation that is less expensive than the foamboard. Most walls around Portland are built to dry to the outside, so some negotiation might be needed to get approval from the building inspector for the unusual vapor barrier design. Locally, used this technique. Another way to minimize thermal bridges is called “advanced framing” or “staggered framing”. This technique uses an interior row of 2x4’s on 2-foot centers, along the interior edge of an 8 inch sill plate, plus an exterior row of 2x4’s offset by 1 foot, along the exterior edge of the sill. This eliminates many of the thermal bridges, while providing extra space to fill with blown-in insulation. Electricians and plumbers love it, because they don’t need to drill many studs. At the windows and corners, careful attention to detail can minimize the thermal bridges and also the amount of wood used.