non-standard joist size, measuring problem


Old 11-24-10, 05:43 AM
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Join Date: Nov 2010
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non-standard joist size, measuring problem

When measuring the joist size for a fiberglass roll, do you include the boards or just the space between the boards? If I have a non-standard size (not 15 or 24) do I have any options other than using blown-in insulation? This is for an attic.
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Old 11-24-10, 06:11 AM
Join Date: Mar 2005
Location: USA
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Blow in cellulose since it does a better job of insulating. Blown in will fit between any joist spacing and allow coverage over the joists.

Old 11-24-10, 07:57 AM
Join Date: Oct 2008
Location: New England
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I agree with Dick, but if batts are the only option, fit is very important.

The two standard referenced spaces are 16" and 24" with the space between being 14.5" and 22.5" respectively. If you have an older home of "I-joists" and need something other than the above, it gets tricky. First, 14.5" insulation is usually 15" wide to give you some play and a tight fit, same applies to the 22.5, it is a bit wider. If you have 18" gaps, then your options would be to buy the wider option and cut it to fit. Or, cut 18" sections and fit them in across the cavities. It sounds terrible, but it actually goes rather fast.

If you prefer the batt approach, but want something better than the fiberglass, look to see if anyone carries mineral wool batts. It is popular in Canada and starting to show up here in the states. I am currently trying my first small job with it and it sure is dense, no air flow through that stuff and that is what I needed for this application.

Good luck and don't forget to air seal before the new insulation goes in.

Old 11-24-10, 02:07 PM
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My space is 21.5". Would it be okay to stuff the 24" stuff in there?
Old 11-24-10, 03:49 PM
Join Date: Mar 2009
Location: WA
Posts: 1,167
Include one board width. Attic insulation width is either 16" or 24", net, this is to cover the ceiling framing member thickness (1-1/2"). Wall batts are 15" or 23, 23-1/4" for inside the cavity. If yours are 24", they are attic, if 23 or so, wall. Use either. Leaving the tops of the joists exposed to the attic air will cost a 20% reduction in overall R-value.
As said, f.g. is the poorest to use. The box store densities are all low enough to have convective loops, except the one high density batt.

Top it with 3" of cellulose to stop convective loops with all low density insulation: Powered by Google Docs


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