Convective looping in fiberglass batts?
A big knock against fiberglass batts is that even when a building exterior is well sealed and the batts are installed correctly there can still be additional heat loss due to convection within the building cavity. I usually hear this called ‘convective looping’. I have heard this a few times as a justification for using spray foam, cellulose or BIBS. Does anyone know of any impartial research done into this phenomenon? Is it actually a significant form of heat loss in wall assemblies with fiberglass batts?
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It's a problem if you don't fit the batts perfectly for length &/or don't tuck in the edges & corners first then tug the batt out sligthtly proud of the stud edges for a compression-fit. If you are obsessive in how you fit the batts you can get the full performance out of them.
Building Science Corp. has measured this and documented it. You can probably poke around on their site and dig up the the particulars.
It's also easy to spot most batt insulation defects using infra-red cameras in assemblies:
There have been quite a few studies; I don't have them at my fingertips.
Convective looping is not a problem in walls if the batts are well installed. If there are large vertical cavities from bottom plate to top plate -- gaps where the insulator forgot to fit the insulation -- you can obviously have a problem. Of course, denser batts are better than the low-R-per-inch cheapo batts.
Fluffy blown-in fiberglass insulation on an attic floor can have convective looping problems in very cold weather, although fiberglass manufacturers claim that newer types of insulation have reduced the effect. In any case, cellulose performs so much better than blown-in fiberglass on attic floors that it is usually a mistake to specify fiberglass for this location.
Here's one of the original studies. It shows that thick (R-38) loose fill fiberglass in attics, with no cover on top, can have a problem. However, even with no cover, fiberglass batts work find in that application.
Most of the problem with convection in fiberglass is one-way airflow, leaking in or out of the building, or from the basement to the attic, through a fiberglass cavity. Fiberglass does little to stop that. But looping within a cavity is rarely a problem if its installed well. In fact, any such looping should show up in the testing that is done to determine the R-value on the package.
When air sealing a home well, use dense fiberglass batts for walls and cellulose for attic floors. There is way too much fear yap about batts. The problem is supervision. The absolute easiest part of building a home is installing batts or being a super. It takes no skill only caring. Dense packing walls with cellulose is fine, what I find in my area is it's expensive, costs more. With air sealing and natural gas at the street it just doesn't make sense. Example... cold cold winter this year, yet a standard batt new build with 9' ceilings and a gas fireplace burning for ambiance all the time, the Nat Grid bill is $175/month deep winter... How yaa gonna beat that? How? How much yaa gonna spend to beat that?
I'm batty bout batts, seems many of us are... such evil... pink evil... panthers that attack in the night... stealing our heat and looping it... loopy I say.
Thank you all!