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I am planning to build a 'garden pod' for my office in the back yard. we live in a mild climate, it does not go below 0°C for more than a couple of hours a couple of times a year and rarely breaks 30°C in summer.

I will be on a fairly restrictive budget that will mean I will probably have to avoid all of the high R value insulation. I will be using foil paper to wrap the structure if the budget allows.

My question is, Could I use acoustic insulation with an R value of around 1.2, but double it up? I can get enough to do the whole office for $26 (the closest alternative would be $300+) the cavity will be 90mm thick and the insulation is 50mm, so I could almost fit 2 in without compression.

I have read online, that if you compress it that the R value goes down, but also that that's because of voids, so if the cavity is completely full, would that mean I would be getting an R value of 2.4+?

UPDATE I am located in Australia, we use SI R value measurements, SI 2.4R is around 13.5R US value.

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  • Generally, a wool-like insulation has a relatively stable R value per inch thickness regardless of how much it is compressed (within reason). If you use two layers of the stuff and fit it into a space that's rougly twice as thick as what is speced for a single layer, you should get close to twice the insulating value of a single layer. On the other hand, if you cram two layers into the space for a single layer you'll probably only get a slight increase in insulating value.
    – Hot Licks
    Oct 23, 2022 at 0:06
  • Compression is usually bad, but 10% you should still get R-18 to R-20.
    – crip659
    Oct 23, 2022 at 0:07
  • @crip659 - OP was talking about an R value of 1.2 for one layer; 2.4 for 2 layers. Where does the R-18 to R-20 number come from?
    – SteveSh
    Oct 23, 2022 at 0:13
  • @crip659 nope not a typo. It's acoustic insulation meant for internal rooms. But seeing as all the other alternatives are over 10 times the price for r values of around 7.
    – mike16889
    Oct 23, 2022 at 0:23
  • 1
    @Ruskes $52 SOME thermal insulation and decent acoustic sounds like a good idea.
    – mike16889
    Oct 23, 2022 at 2:12

1 Answer 1

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For fiberglass type insulation, the R-value per inch actually goes up a bit when compressed, though the overall R-value may be less because of the reduced thickness.

Take a 5-1/2" fiberglass batt, designed for a 2x6 (5-1/2" deep) wall cavity. The nominal R-value for that material is R-21. If you compress it to fit into a 2x4 (3-1/2" depth) wall cavity, the R-value goes down to R-15, 71% of the uncompressed value (or a reduction of 29%). This is a bit better than the cavity depth reduction of 36.4%.

This is per a North American Insulation Manufacturer's Association (NAIMA) publication I have in my files. I'm sure anyone can find it with a simple Google search.

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  • Ok. Thanks. Sounds like a good idea then. Better than nothing and nice and cheap. The acoustic insulation will come in handy as it will be near the fence, so if I can block the neighbours a bit, that's a major plus.
    – mike16889
    Oct 23, 2022 at 2:06

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