# How do I design a 100 mm thick barrier that maximizes thermal and acoustic insulation?

I need to design a thermal and sound barrier - walls of a cuboid-shaped space. Imagine it is a free-standing bike shed in the fields. The walls can not be thicker then about 100 mm (4 inches). The most-internal layer of the wall will be a 2 mm (0.08 in) thick sheet of stainless steel. The most-outer layer of the wall will be a 1 mm (0.04 in) thick sheet of aluminum (fully exposed to weather).

Now, can you suggest what should I put between these two sheets of metal, to achieve the following properties:

• R-value (thermal resistance) of the walls of at least 2.64 K·m²/W, that is in US units R-15 (15.0 h·ft²·°F/Btu)
• noise reduction of 50 dB - Sound Transmission Class: STC 50

Are such properties possible to achieve in a 10 cm (4 inch) space wall between two sheets of metal? If not, what are the best possible properties to achieve in that thickness, with the budget of about 70 EUR/m² or 10 USD/ft²?

I have read about materials such as polyisocyanurate (PIR) for thermal insulation and QuietRock 545 for sound (is it available in Europe?), but maybe there are materials which offer thermal and sound insulation combined, with more efficient use of the limited thickness?

Please suggest materials to put inside the wall between the two sheets of metal, preferably materials possible to purchase in Europe. Do I also need to worry about a vapor-barrier in such application (free-standing bike shed with steel walls)?

• You only need a vapour barrier if there is a heat differential: eg if the shed is heated or cooled. Without it, you will get condensation inside, which will cause mold growth. May 3, 2011 at 21:35
• @gregmec: the shed will be kept in a constant temperature of 25 degC (77 degF), outside temperature being usually lower, rarely slightly higher. Where would the vapor barrier need to be: between the outer sheet of steel and the insulation, between the insulation and the inner sheet of metal, or in both places? You mean condensation inside the room (not a problem as a dehumidifier will be installed inside), or inside the wall (then its a problem)?
– ria
May 3, 2011 at 22:24
• Regardless of the insulation, the structure connecting the inner and outer wall, e.g. studs, will be transmitting most of the sound.
– BMitch
May 4, 2011 at 11:50
• Now I'm awfully curious. What is this building for? I can think of many good movie plots… May 7, 2011 at 21:34
• @Vebjorn Ljosa: When we got to the sound deadening requirements my mind jumped to Lovely Bones. Jul 3, 2011 at 13:13

For thermal, you'll want closed cell foam of some sort. If the wall has to be structural, I'd frame with vertical 2x4s, then on the interior, horizontal 1x1s, spray foam, then your metal panels.

If it doesn't have to be structural, 4" of XPS foam board should work.

That'd be about the best you could do for a thermal barrier.

As for sound proofing, that's a bit of a different goal...especially with interior metal walls...which will reflect a lot of the sound. 4" of insulation and well sealed door openings should get you a decent level of sound deadening, but It definitely won't be sound proof by any means.

• Why are you telling me to use 4" of XPS, where for example PIR (polyisocyanurate) has 50% better insulation in the same thickness? And I am looking for materials which will squeeze every possible tiny bit of K·m²/W R-value out of the available thickness! And using 4" XPS doesn't leave me any thickness at all to use for sound insulation. I am not looking for "a decent level of sound deadening" , I am looking for a specific dB of reduction, a specific STC. If you are suggesting something, show me the numbers.
– ria
May 4, 2011 at 10:46
• And to clarify the question: the wall doesn't have to be structural. No wood is to be used in the whole bike-shed at all, only steel and aluminum. The insulation materials should be inflammable if possible. The wall is made of steel, so it is reasonably fire-proof.
– ria
May 4, 2011 at 10:54
• Fine, use PIR, then. But you're asking for a lot here. BTW, sheet metal will not make the wall fireproof. It'll simple make the surface less likely to ignite. Keep in mind sheet metal will have an incredibly high heat transference so would quickly attack whatever material it is attached to. Most foam board products are extremely toxic once they start melting.
– DA01
May 4, 2011 at 13:56
• Inflammable is a synonym for flammable (en.wiktionary.org/wiki/inflammable). Nov 21, 2011 at 20:44
• Theoretically speaking, the best possible sound/heat barrier is: An air vacuum between the 2 metal plates.
• To achieve even better heat/sound results you could go underground >= 6meters if possible and desirable in case shed does need daylight(windows), plus you get EM shielding.
• Concrete is also great for sound & EM shielding, but not so great about heat since it will act as time delay for heat transfer. Maybe you could create a triplex 2.5/5.0/2.5cm concrete + rock-wool concrete. Concrete costs ~ 70eur/m3 ~ 7eur/m2/10cm. You could partially weld the 2 plates, create a staggered(alternating) metal stud wall skeleton, put insulation in the middle then fill with concrete from top. Concrete could also act as a skeleton and avoid completely metal studs if reinforced. Water + alkaline nature of concrete is a slight concern but aluminum & stainless steel resist corrosion good enough.