Author Topic: Cellulose over Fiberglass  (Read 630 times)

Bud9051

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Cellulose over Fiberglass
« on: January 06, 2012, 07:31:33 AM »
As part of my understanding on the movement of warm air I am watching for places where "it matters" and our practice of blowing a cap of cellulose over fiberglass may be one we need to modify.  As I have done and recommended to others, which doesn't mean it is the right way, before the cellulose is blown in, a batt of fiberglass is folded and stuffed into the end of each rafter bay just under the new baffles to block any cellulose from filling our soffits.  Here's the problem, at least in cold country.

With an existing layer of fiberglass against the ceiling below, the air against the sheetrock is being warmed by the heat conducting through.  But here is the difference.  That warm air wants to go nowhere.  Since it does not rise on its own, sealed under a cap of cellulose it would be quite happy right where it is.  However, is it really sealed?  The moving force that circulates that heated air up into the attic is the invading cold air and that folded batt of fiberglass we used to block the cellulose from filling up the soffits has now provided a path for that very cold air to infiltrate our layer of FG and force that warm air up through the cellulose cap, all-be-it a much more difficult task given the density of the cellulose.

A better solution might be, to either build a solid block to both keep the cellulose out of the soffits and eliminate and extra cold air infiltration, or to pull back the existing FG just inside the batt blocking the soffit and dense pack the new install right the sheetrock.  If that dense pack sealed from the baffle to the ceiling, plus across the attic floor, there would be minimal cold air infiltration to either the FG next to the ceiling or the attic space above.  The baffle of course would still be doing its job.

A simple modification of how we are currently doing this install, but perhaps one that improves the results.  Any thoughts?

Bud
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SLS Construction

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Re: Cellulose over Fiberglass
« Reply #1 on: January 06, 2012, 08:24:30 AM »
Where is that whistling smillee at? http://blog.sls-construction.com/2011/air-sealing-attic-baffles
Oh & just say no to cellulose over fiberglass

Bud9051

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Re: Cellulose over Fiberglass
« Reply #2 on: January 06, 2012, 08:48:29 AM »
Great read Sean, but without going back to read again, I didn't see a "no" reference to cellulose over FG.  On some earlier discussions here on this forum we talked about removing the old nasty FG before installing the new insulation.  One of my arguments was, if we remove it, shouldn't our improvement reflect the entire amount of new insulation as compared to zero, rather than just the addition over the existing.  I think I was suggesting the ho remove the old nasty stuff specifically so the auditor couldn't count it and the installer wouldn't bury it.  As I remember the forum wanted me locked up.  So, is your suggestion that we should not be blowing cellulose over FG, even if the FG is in good shape and even if we properly prepare the eaves as illustrated in your link?

Bud
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David Meiland

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Re: Cellulose over Fiberglass
« Reply #3 on: January 06, 2012, 08:54:50 AM »
My main question would be, can you adequately air-seal the attic floor before you blow the cells. I see minimal downside to the FG remaining assuming that all of the air-sealing is done. Without that you could easily be making a marginal situation worse--with the FG only there is enough heat to prevent condensation in the attic, once you add cells the attic temp drops and the condensation starts.
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SLS Construction

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Re: Cellulose over Fiberglass
« Reply #4 on: January 06, 2012, 09:54:29 AM »
Bud, that was only one of the 4 or 5 articles I did on the attic & the just say no portion wasn't addressed in that one - I mainly was pointing to that one to address your baffle / folded FG trick & thinking

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