Helpful? 0

Adding ceiling insulation...

My son's house has R-38 fiberglass batt (12") above the bedroom ceiling. There is vented attic space above that. While the R-38 may be a decent # in theory (though R-50 would probably be the current zone 6 amount), I'm thinking the air sealing capability here is slim to none (below batts is the kraft facing stapled to joist bottoms and 1/2" sheetrock.

We're considering practical ways to add insulation. One would be blown in cellulose; easy to do and reasonable in cost, but don't think this adds much air impermeability (not dense pack in this application). Is there sense in compressing the batts first, then adding the cellulose? The joists are 2x8 so the batts could easily be compressed to tops of those joists, which I know would reduce the overall R value of that batt (despite an increase per inch), -but wouldn't that compression deter some air flow, or is that too negligible to justify? Are there other/better alternatives?

Is it possible/reasonable to do rigid foam over the batts? The foam boards could be fastened downward to the josit tops, thereby also compressing the fiber batts. But, wouldn't quite a thickness of foam be needed, at considerable expense? This seems a bit like doing it above the roof decking, except for no decking! Could foam board seams be taped from above if this arrangement is otherwise feasible? Or, would it be better to foam board below current ceiling and redo sheetrock? (not a great solution due to loss of ceiling height, already low). Or, I guess foam board could go above ceiling, in between joists, below batts, which could be pulled aside and put back, -cobbled together method. The edges against the joist could be sealed with spray foam or quality tape.

Thanks for any and all advice.

Asked by Howard Gentler
Posted Wed, 02/05/2014 - 16:14
Edited Wed, 02/05/2014 - 16:42


5 Answers

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Experienced insulation contractors address attics like yours all the time. The right way to do the work is to lift up the fiberglass batts (separating the batts from the kraft facing if necessary) and piling them up on one side of the attic. Then air sealing work is performed. Once all of the air sealing issues have been addressed, the batts can be returned to the joist bays (with or without the kraft facing).

Once that's done, it's time to install a layer of cellulose on top of the batts to complete the job.

If you can't find an insulation contractor who understands the necessary work, keep dialing the phone until you find one.

For more information on this topic, see Air Sealing an Attic.

Answered by Martin Holladay, GBA Advisor
Posted Wed, 02/05/2014 - 16:40
Edited Wed, 02/05/2014 - 16:46.

Helpful? 0


Appreciate this advice. It will not be difficult to pick up the batts, and was one of my options. I suspect the best practice would then be a couple inches of spray foam onto the kraft paper, sealing the bays well, then the batts back. I'd like to avoid foam due to the expense, although have considered a froth pak type thing for a DIY project (and also getting a professional estimate). Is the cut and cobble method with rigid foam a good second best choice, taking care to seal all seams? I have a foam gun and could double up with a good tape. More work but less expense.

I have nothing against contractors, but am a DIY'er of many years and would not have what I do if I had used contractors regularly. Here in my rural area there are not many real "specialists", -mostly generalists in order to stay in business. This is not to say there isn't expertise in areas like insulation, but with the internet, sites and blogs like this one, BSC, etc, I should be able to gain needed info and not have to use a contractor just so it is (hopefully) done right. I've made my share of mistakes over the years, but did not benefit from current levels of research and info pre internet. I'm also aware of mistakes greater and more eggregious made by contractors that I avoided.

So, thanks for sharing your expertise.

Answered by Howard Gentler
Posted Thu, 02/06/2014 - 09:53

Helpful? 0

Air sealing work usually doesn't require filling every joist bay with spray foam. Nor does this work require a full layer of rigid foam installed by the cut-and-cobble method.

The best way to air seal the leaks is to focus on the actual cracks and holes. The approach is described in my article, Air Sealing an Attic.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 02/06/2014 - 10:02

Helpful? 0

This info would make the job easier. There is not too much that should be a problem, -the one ceiling light stands out. There are otherwise no ducts or other utilities. Not needing a continuous air sealing barrier above the sheetrock must mean that the taped sheetrock is a considerable air barrier in itself, is that correct?

Answered by Howard Gentler
Posted Thu, 02/06/2014 - 14:47

Helpful? 0

You are correct; taped drywall is an air barrier. Of course, penetrations through drywall often leak air. Moreover, watch out for the cracks between partition top plates and the drywall installed on partition walls; these cracks are responsible for significant air leakage.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 02/06/2014 - 15:11

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