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Question DetailsAsked on 6/21/2013

spray foam for attic

for attic ceiling

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Here was the same question by another user a while back, with responses -

https://answers.angieslist.com/Can-I-...

The primary advantages and disadvantages of spray foam insulation in an attic ceiling are:

Advantages:

1) fast and pretty cheap to do

2) fills small voids and cracks better than cellulose, and does not have the perimeter gap common with blanket or batt fiberglass insulation

3) does not pack down or consolidate over the years, so insulation value stays fairly constant (does degrade about 10-25% over many years as the foaming gases dissipate)

4) not as messy to install, though that is usually not a factor because in a retrofit case you have to get the old stuff out anyway

5) acts somewhat as a vapor barrier against household moisture getting into the attic if used in attic floor

Disadvantages:

1) causes or aggravates respiratory problems in some people, so you should research it first if you have infants, very elderly, or respiratory problem individuals in the house

2) not very environmentally "friendly", if that bothers you

3) if used in the attic floor, makes any future work in the ceiling area for wiring or light fixture changes, etc MUCH more difficult, because everything is foamed in

4) fire hazard if light fixtures in the ceiling are not properly vented when used in attic floor

5) you have to be VERY careful to close off ALL openings into the attic from outside with fine bug mesh, because flying termites and carpenter ants love to build their nests in it because it is so easy for them to chew cavities in. There are sprays you can apply on top of it to reduce the attractiveness but does not last forever. Do NOT trust foam filling the voids - they should be blocked with wood, bug screen, or caulk BEFORE foaming, because if foam fills the voids and is exposed to the outside the bug will eat right through it and into the attic.

Note I have referred primarily to use of spray foam in the FLOOR of attics (between the underying ceiling joists or the bottom members of roof trusses, not in the "ceiling" of the attic. Some contractors do spray it on the underside of the roof sheating (the "attic ceiling" I think you are talking about, but professional designers consider this very bad practice in most buildings, for two reasons:

1) it traps the roof heat in the roof, degrading the sheathing and shingles. Many shingle manufacturers void the warranty if the underside of the sheathing is insulated, because it puts the sheathing, felt, and underlying shingles in basically a steam cooker in the summer.

2) it traps any moisture that makes it down to the sheathing (and moisture DOES get into the felt, migrates by diffusion into the sheathing, migrates through that and evaporates into the attic to be moved out with the air circulation under the sheathing and out the ridge or gable vents. This promotes sheathing and felt rot and degradation.

Personally, I am strongly opposed to roof insulation under the sheathing - I favor free air circulation under the sheathing, and putting the insulation in and over the attic floor joists.

If you are talking about an attic with living space in it and a peaked ceilin rather than flat (which I hate from a ventilation and access point of view), then this is a much tougher ventilation situation. The ideal insulation and ventilation cannot be achieved, so about the best you can do is keep open air space between the roof rafters, then put your insulation and vapor barrier on the underside of the rafters. In that case it is usually easier to use styrofoam panels fastened to the rafters with vapor barrier (plastic sheating) stapled to the insulation board below that, then the interior finish wood panelling or drywall below that. In that situation I use a housewrap like Tyvek above the foam panels, to try to keep any winter moisture condensation in the airgap from dripping through the insulation and ceiling. With that type of ceiling you have to be sure to provide maximum airflow under the sheathing and around the rafters to remove any accumulated moisture, and take extra care with sealing any pipe or vent penetrations so household air does not get into that confined space, because it is impossible to get in there to inspect or do any repairs without tearing out ceiling or roof. Prior to any reroof job, do a good inspection of this under-roof area from the ridge vent slot and any bathroom or kitchen vent hood openings to inspect for moisture damsge, rot, etc before doing the reroof.

Answered 7 years ago by LCD




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