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Question DetailsAsked on 11/25/2015

Calculating cellulose insulation to blow in attic, want to know recs regarding attic above garage

Live in Ohio and will insulate current attic in house to R49-60 with Greenfiber. Question is: We have one bedroom and part of a bathroom above an unheated garage that stay fairly cold in the winter. There is a minimal amount of cellulose insulation in the garage attic (maybe 3 inches). I've heard differing opinions on insulating a garage attic below living spaces. If you think it would help to insulate the garage attic while I have the machine out for the attic over the house, would I insulate to the same R49-60? Or would it be better to just have an HVAC guy come out and determine if the ductwork is adequate, clean air ducts, etc. Thanks.

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2 Answers


Drill and blow garage ceiling with cellulose. There are white plastic plugs available to close off hole insulation contractor drills. Use high velocity blower and pack full. It will raise the temperature in the basement room next to it by closing off the gigantic uninsulated holes around air ducts running in garage ceiling. It will raise the temperature of the room above the garage by 5 degrees. Used to sell this process as part of reducing air infultration in split level homes.

Jim Casper Old Energy Conservation Guru

ps for ideas on gutters and covers see my blogs

Source: www.heartlandmastershield,com

Answered 4 years ago by jccasper


I have never been a fan of cellulose - picks up water and mildews too easy, even if treated.

My preference - though what you and JC are talking about for winter conditions would probably not cause problems, would be to insulate the entire garage, making it a more temperate adjacent area, which could save more energy in the long run because the occupied part of the house would be "seeing" a much lower temperature differential between "inside" and "outside". This assumes of course that the garage is not a terribly drafty after the insulation installation.

The think that scares me, having seen this a fair amount, is during cooling season (if the ducts run through the garage ceiling) the warm moist air (putting Ohio summer conditions nicely) from the unconditioned garage that gets into the garage ceiling space can condense on the cold ducts, resulting in free water, which can then start a mold farm. This assumes they did not put an effective vapor barrier under the drywall in the garage ceiling, to keep the moist air out.

I would, unless your siding is wide open with no vapor or moisture barrier or wrap, instead go with insulating the garage walls so the garage traps more of the house air conditioning effect (indirectly), and acts as a thermal buffer between house and overlying rooms and the outside. This would also reduce airflow and heat transfer at the walls, which the ceiling iinuslation would not do.

Two thoughts on JC's blowing-in suggestion -

1) drilling holes in the ceiling and insulating will penetrate and significantly reduce the effectiveness of the vapor barrier, if there above the drywall ceiling

2) the plastic plugs violate fire code - you are required to have a fire-rated ceiling above the garage if there is living space above, and plastic plugs do not meet that requirement. Holes should be filled completely with drywall patching compound, or much more expensively at least for the materials themselves, tumenescent caulk or fire door caulk (which is a fireproof caulk that swells in a fire, and does not burn away in a fire). There may be tumenscent hole plugs available - I have never seen them, only once designed to go around pipes and wiring and such, so they have a hole in the middle of them. Some code areas accept tight-fitting press-in metal plugs - international fire code itself does not, though.

Answered 4 years ago by LCD

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