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Encapsulated Fiberglass in Old Attics


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Old house - 1905-ish.

4 sidewall attics in the upper floor - one at each corner of the home.

Guy installs the encapsulated fiberglass batts in the ceiling rafters and on the floor of the attics. Then the flooring (ceiling joists) is covered with ply for storage.

There is no insulation on the sidewalls themselves.

Somehow this doesn't sit right with me. Something about trapping moisture against the roof sheathing. . .

Any thoughts?

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It seems they just conditioned the attic space. Fiberglass within rafters will create a moisture problem because there is no ventilation now. The attic floor and knee wall should have been insulated and nothing in rafters. I suppose you could sheath the rafter and created a dead air space and that would work but not obstruct the air flow.

Nothing scientific, purely anecedotal - never seen properly insulated rafter bays done by a skilled homeowner.

FWIW- don't like encapsulated fiberglass.

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Hi Randy,

When you say "ceiling rafters" I take it to mean the rafters over the sloped ceilings on the upper floor of an older home. With these, the sidewalls, floor of the kneewall attics and the rafter (collar) ties above the flat ceiling in the middle of a second floor are typically insulated too. It sounds like that is what you are describing. However, perhaps the sloped underside of the roof is insulated over the kneewall attics as well and that's what you meant. (Obviously, I'm cornfoosed).

If it's as I first described it, I see a couple of issues. The first is going to condensation on those uninsulated kneewalls. If he doesn't want condensation on the interior side of those kneewalls, he needs to either insulate those too, or turn the kneewall attic into conditioned space. That means insulating the roof plane from the eaves to the ridge and that's impossible to do well in any of these older homes as a retrofit using fiberglass. Blown foam, maybe, but fiberglass? No.

The next is vapor diffusion from the home into those kneewall attics and the roof plane. Unless he's got an air gap above the insulation all the way from the kneewall attics to the little cavity above the flat ceilings, and damned good upper and lower vents so that convection works well, water vapor from the home is going to pass through the insulation, condense on the underside of that cold roof plane, collect there and eventually rot the roof.

It's pretty rare that I see good airflow over those old sloped ceilings, because the builders have usually used blocking at either the top or bottom of the sloped ceiling rafter bays, and it's impossible to get airflow through there without prying out or drilling holes through every single one of the blocks. Now, if he'd used dense-packed cellulose in the sloped ceiling rafter bays, that'd be different. That stuff makes a terrific air barrier when it's packed well, and you won't find condensation problems where it's been used. Warranty problems with a roof due to lack of ventilation, maybe, but no condensation.

Tell the guy to go here and study these:

Oak Ridge National Laboratories Retrofit Best Practices Guide

Oak Ridge National Laboratories Insulation Fact Sheet

Oak Ridge National Laboratories Moisture Control Handbook



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