Attic Insulation Quantity
Do We Need More Attic Insulation than in Walls?
INSULATION LOCATION & QUANTITY for ATTICS - CONTENTS: Why do we need more insulation in the attic than in building walls?Extra insulation is most cost-effective added in top floor ceiling. Comparing benefit of adding cathedral ceiling insulation to adding wall insulation? How much attic or cathedral ceiling insulation should be installed?Solar Age Magazine Articles on Renewable Energy, Energy Savings, Construction Practices
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Attic insulation quantity guide: this article discusses the comparative benefits of placing more insulation in a building attic than in building walls, and the comparative benefits of adding ceiling versus wall insulation for cathedral-ceiling areas.
Is it Still Necessary to Insulate the Attic More than Building Walls?
Buildings have always had more insulation in the attic than in the walls or below ground level. This I believe is due to warm air rising. With the advent of airtight construction and mechanical ventilation with heat recovery, is it still necessary to insulate the attic more than the walls?
I believe that pre-fab homes imported in Sweden have equal R-value insulation around the entire building shell. This sounds like the logical way to go. -- Philip Cyr, Sunrise Technologies, Caribou ME
Why We Put More Insulation in Attics than Walls of Buildings?
The reason for putting more insulation in the attic is twofold:
First, in older insulated homes, air near the ceiling may be as much as 10 degF. hotter than air at the floor. So the rate of heat loss was [and may still be] greater at the ceiling.
Our photo (left) show how simply observing uneven snow melt on the roof of an older home can indicate areas of uneven building insulation, air leaks, or other causes of heat loss.
The second reason is that after the first 4 to 6 inches of insulation (in 2x4 or 2x6 construction wood framed walls), it is a lot cheaper to add insulation to the ceiling (see our page top photo) than to the walls (where added framing or other tricks will be required).
So up to a point, extra insulation is most cost-effective in the ceiling.
In a well-insulated home, the first reason may no longer be valid [depending on how heat is distributed in the building] since there may be very little temperature difference from floor to ceiling.
[This is true, at least in theory. But even a "well insulated" older home where an insulation retrofit has been extensive, may have air leaks and unexpected temperature variations.
Field measurements made during building inspections using infra-red to compare temperatures at floors, bottom of walls, center and top of walls, and ceilings, can still find surprising variations in temperatures even in some new, well-insulated homes
The second reason, though, still holds as long as you can cheaply stuff insulatin into your attic. [Just don't block attic ventilation intake at the building eaves or you may, while adding attic insulation, also create an attic moisture problem in some buildings.
In buildings where idoor air temperatures are found to be uniform, floor to ceiling, then cathedral ceilings are an exception to the building insulation quantity and placement logic discussed just above.
Once the space between the rafters of a cathedral ceiling has been filled with insulation
See ROOF VENTILATION SPECIFICATIONS where we warn about "hot roof" designs"
or where no more insulation can be added in a catheral ceiling without blocking its ventilation design, then the incremental cost of adding insulation to the cathedral ceiling is comparable to adding insulation to the building walls.
In both cases, where the total R-value of the building walls or cathedral ceiling are considered inadequate, rather than costly framing changes, we often laminate a layer of 1" or 2" high-R solid foam insulation on the ceiling and/or walls, covering the new layer with drywall.
But even this approach is more trouble than first meets the eye: Electrical outlets, switches, windows, doors, trim all need to be built-out to cover the edges of the new insulating material. --DF
The question-and-answer article about the comparative benefits of adding attic insulation or cathedral ceiling insulation versus wall insulation in buildings, quotes-from, updates, and comments an original article from Solar Age Magazine and written by Steven Bliss.
Original article in PDF form:
Q&A on Even [building] Envelope Insulation - do we still need to add more insulation in the attic of modern airtight homes? - PDF version, use your browser's back button to return to this page
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