by SIM » Sat Jan 02, 2010 9:20 pm
The exterior wall top plate is an area that is difficult to insulate in older homes.
Typically there is a 3-4" space above the attic floor and the roof decking. Most homes have fiberglass batts rolled into the joist cavity that extends out onto the exterior top plate. The problem being that air from the soffit is free to circulate through the fiberglass batt, thus greatly reducing the R-value. Some estimates project the R-value of un-faced fiberglass batt that is exposed to horizontal air flow to be less than 1.0. Other homes with blown in cellulose or fiberglass insulation have next to no insulation near the exterior wall top plate. This is usually the result of the insulation being tapered off near the top plate, or because it fell into the soffit area.
A solution to this problematic area is to dense pack a mesh bag with cellulose and stuff it down between the attic floor joists and roof rafter channels. This will assure maximum the thickness of insulation while greatly reducing the amount of air flow through it. Remember, you can pack cellulose but you can't pack fiberglass. Packing fiberglass reduces its R-value to almost zero, especially if there is any kind of air flow through it.
You can purchase the bags from Energy Strategies: 1-920-207-9481 for about 55 cents a bag.
Many people, primarily roofers, oppose this practice of blocking a roof rafter channel because it stops the air flow in and out of the attic. But with proper air sealing of the attic floor insulation always trumps ventilation. With that said, some homeowners insist on having ventilation even at the expense of less insulation. In this case we keep the existing baffles in some channels (or even replace attic baffles with a more rigid type so the dense packed cellulose bag does not crush it when put into place).
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