How to insulate an enclosed garage ceiling?
In my work as a building analyst here in Southeast PA, I meet lots of homeowners having comfort issues in conditioned spaces above garages. Typically these spaces are cold in the winter, though not necessarily warm in the summer. Quite often, there is fiberglass batt in the ceiling cavity of the garage (covered by drywall). In many cases, the batt has kraft paper and is facing down towards the unconditioned garage, sometimes up against the floor and sometimes lying on top of the ceiling. I’ve shied away from recommending a dense pack approach in these situations, concerned that the assembly would have less chance of drying out if it were to ever get wet. Between plywood, carpet/flooring padding, carpet, etc. above and the kraft paper below I get a little nervous!
However, in reading this article on GBA, my thinking is shifting a bit… https://www.greenbuildingadvisor.com/blogs/dept/building-science/should-paper-facing-batt-insulation-face-inside-or-outside
Since the kraft paper is a vapor retarder and not a vapor barrier, should I really be so cautious, even if it is technically installed incorrectly? Are there even that many ways for moisture to get into the ceiling cavity? Outside of the garage wall assembly/rim/band joist I can’t think of many, other than a pipe breaking or some sort of flood from above. Nor can I see much diffusion from the interior of the house making its way downward into the floor/garage ceiling assembly…
So with all that said, is there a hard and fast approach to all/most garage ceilings or should I keep being paranoid in PA? (Any recommendations on techniques, materials, etc. would be greatly appreciated!)
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Replies
Greener,
Here is a link to an article that discusses the type of floor assembly you are talking about:
How to Insulate a Cold Floor.
In general, floor assemblies are dry, while cathedral ceilings are damp. Why? Because of the stack effect. Air leaks in floor assemblies draw in dry outdoor air in winter. Dry outdoor air is safe.
Air leaks in cathedral ceilings draw in humid indoor air in winter. Humid indoor air contributes to condensation and moisture accumulation when the air contacts cold surfaces.
Your guess concerning kraft paper is correct. It won't trap moisture, especially in a floor assembly. Kraft paper is a smart vapor retarder with variable vapor permeance. If it ever gets damp, it becomes vapor-permeable.
If you know a cellulose installer with a lot of experience filling voids in older buildings, that contractor can help you decide how to proceed with the type of floor assembly you are talking about. If you can get a hose into the floor and aim the hose at any air spaces or gaps, these floor assemblies can be dense-packed with cellulose (compressing any existing fiberglass insulation).
Thanks for the quick and thoughtful response Martin! Any specific reason to dense pack cellulose here vs. a loose fill fiberglass? And regardless of insulation material, does it make more sense to blow the insulation in over the top of the batt and force the paper down to the garage ceiling? Or under, compressing the batt upward leaving paper somewhere in the middle? Finally, any concerns if the dense packed insulation is in contact with CMU's out towards the edge of the joist bays where they meet the concrete block garage wall?
Greener,
Q. "Any specific reason to dense pack cellulose here vs. a loose fill fiberglass?"
A. There are several possibilities: either cellulose or blown-in fiberglass can be dense-packed (the best approach), or cellulose or blown-in fiberglass can be installed without dense packing. Regardless of whether your insulation contractor chooses cellulose or fiberglass, a dense-packed installation reduces air flow more than an installation that is less dense -- meaning that dense packing is desirable.
Why do I prefer cellulose? Because dense-packed cellulose does a better job of reducing air flow than dense-packed fiberglass.
Q. "Regardless of insulation material, does it make more sense to blow the insulation in over the top of the batt and force the paper down to the garage ceiling? Or under, compressing the batt upward leaving paper somewhere in the middle?"
A. I can't think of any reason why it matters.
Q. "Any concerns if the dense-packed insulation is in contact with CMUs out towards the edge of the joist bays where they meet the concrete block garage wall?"
A. Probably not, unless the building has no roof overhangs and the wall is exposed to lots of wind-driven rain. Factors that lessen the risk include wide roof overhangs, a sheltered location, and the presence of siding on the exterior of the CMUs.
Many moons a go I learned from BSC that in order to avoid any chance moisture problems above garages, I needed to seal everywhere around the under-floor, install sealed rigid foam underneath the framing members and then fill the cavity with blown insulation, or my favorite specification in CZs 3-4, to install 2" of closed cell foam under the floor decking and 2" min. of open cell, and 5"-6" of open cell in CZs 5 or higher.
Melting ice and snow, rain and carbon monoxide, can create dampness and moisture issues on a cold floor, specially if you live in an area that receives a lot of precipitation and your car brings all that wet stuff inside the garage.