Insulation Overview for New Pacific NW Homes
Levin Nock Dec 2008 www.JLAF.biz 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

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,
www.villagegreenhomes.org 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.