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I have a room made of hollowed concrete blocks that I framed by mounting 2x4s, wide side against the wall. This gives me a 1.5in depth cavity to fill with insulation.

Should I use an r19 fiberglass insulation and compress it down from 6" to 1.5"? Or should I use a 1" foam board (r5)? I know compressing insulation lowers the r value. I'd like to get the r value as high as possible (recommended is r13 in Florida)

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    Long answer == pros who know their stuff telling you R-values of 23 different things. But short answer is don't compress - you are wasting material, wasting $, and not getting anywhere near the R-value/insulating efficiency. Find something that is designed to be used in that size space. – manassehkatz-Moving 2 Codidact Mar 6 '20 at 0:23
  • @manassehkatz what can be used in a 1.5" cavity? I've looked around and I couldn't find anything, besides foam, meant to fit in that cavity. Short of aerogel, I haven't heard of anything more insulating than the foam neither, not for that space – Tyler M Mar 6 '20 at 0:32
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    Then use foam. Or build out with another layer of 2x4s. – manassehkatz-Moving 2 Codidact Mar 6 '20 at 0:35
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    Most of the time, a lot of the benefit comes from the air being held in place by the fibers/cells/whatever of insulation, so I would be careful about assuming that compressing insulation would result in it providing the same effect. There's also the issue of putting up a wall with tons of compressed insulation behind it, which means that if you ever drill or cut a hole in that wall you're going to have huge amounts of fiberglass trying to spew itself out of the hole and get aggressively caught on your drill bit/saw blade/whatever. – Tungsten Wizard Mar 6 '20 at 4:13
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If you use anything Use foam, fiberglass gets its R value in the volume. The block wall actually has an R value, I think when I poured my shop walls there was foam on outside and inside and I think the wall was rated at R 50. Since you have a solid wall I might just have an air space. As 1” of foam is expensive for the R value and the air space is actually an insulator. I would base my choice on the r value of foam panels vs the cost.

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    R value of still air is .29 per inch. R value of say 1" iso board is 7. I'd go with the foam. I would not put fiberglass in it maybe roxul. Fiberglass batts are suprisingly not optimized for R value for their density. So compressing them might actually result in more R value per inch. Fiberglass also looses R-value if it gets wet. – Fresh Codemonger Mar 6 '20 at 1:40
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    Owens Corning would disagree with your assumption of compressed Fiberglas being better. – Ed Beal Mar 6 '20 at 3:31
  • they do not. Here is their technical bulletin. R15 rated product compressed to 2.5" is R11 which is R4.4 per inch. R15 product non compressed for 3.5 = R4.28 per inch. dcpd6wotaa0mb.cloudfront.net/mdms/dms/Residential%20Insulation/… – Fresh Codemonger Mar 6 '20 at 7:22
  • @FreshCodemonger your comment should be an answer itself. According to that, R-13 insulation can be compressed down to a 1.5" cavity and still retain R of 6.6, which is higher than the foam's R5, suggesting I should compress it if i did use fiberglass – Tyler M Mar 6 '20 at 13:44
  • But as I said you loose insulation value when compressed, fiberglass insulation is recycled glass and sand , how good is glass as an insulator? The insulation value is based on the ability to hold the air volume or insulating materials thermal conductivity. All of the info is based on ideal conditions, no moisture, even in the desert a home has moisture in the air space or wall cavity that affects the R value these things really come into play in a damp environment this I know from living in Oregon and California (Mohave) – Ed Beal Mar 6 '20 at 15:09
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Spray foam may be hazardous to the health if the chemical reactions are not finished due to wrong application, dry air, too thick layer. And of course, a good protection is needed during the spraying work.

A ventilated air layer is sometimes part of insulation constructions in order to get rid of moisture, depending on the location of the layer, location of the due point, R values, drying time etc.

A reliable, fast and easy way to get all necessary information is to input each layer into the professional "Ubakus"-Site, which is free for private use: Ubakus.de

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