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I'm currently choosing the thickness of the PIR insulation I will use for the ground floor (first floor US) in my house. It's a traditional (UK) suspended timber floor build with concrete foundations under the suspended floor (about 50cm of crawl space). Joists are 130mm thick

According to some online U-value calculators having 100mm thick PIR insulation I will have 0.22 W/m^2k U-value. 130mm PIR insulation will have 0.18 W/m^2k U-value.

The current price of them is £50 for 100mm PIR board and £75 130mm PIR board and I'm trying to decide whether the extra 30mm is worth it.

Does anyone know what the real world difference will be between the 100mm and 130mm PIR and whether it's worth the extra 50% price?

abdnChap
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    You really might want to give PIR board a smoke test before you install it in your home. They actually had a horrible fire in the Sunshine Mine (hard-rock copper/silver mines do not contain anything that would burn) because of foam, and then you have the Grenfell fire of course. – Harper - Reinstate Monica Jul 05 '22 at 22:46
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    To be clear, this insulation is only in the floor? Or the walls too? – LShaver Jul 06 '22 at 00:53
  • They did not like my answer so I deleted it. Basically calculate how much energy will you save between 100 and 130. It comes out to 0.04 W/m^2k U-value saving. That is 20% per meter square. – Ruskes Jul 06 '22 at 03:14
  • It's only for the floor in between joists – abdnChap Jul 06 '22 at 07:29

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So the difference in insulation value is (0.04 W/m^2k U-value) ~20%

The 0.18 U-value insulation will save you about 2% in heating/cooling cost, calculated as follows:

Assume the floor represents 10% of all heating and cooling energy consumption as indicated here.

Let's say you have a 300m2 floor. You would need 100 boards. The price difference would be 2,500. Let's say you have annual energy bill of 2,000. With a 2% saving it will take you over 60 years to break even.

source: insulation

My numbers might not be 100% accurate, but in any case, it say that the additional investment is not worth it.

Ruskes
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    I like calculations. But the problem here is that the ~20% (correct) increase in insulation will *not* result in a 20% decrease in energy costs. (a) only HVAC, not all energy costs, (b) other parts of the house - e.g., windows, doors, roofing, etc. all have impacts on energy usage too. Even if the insulation determines 1/2 the total HVAC energy usage (which would be a lot), that would extend (in this example) the payback time to 12 years instead of 6 years. Not as simple as it seems. – manassehkatz-Moving 2 Codidact Jul 06 '22 at 01:47
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    @knowitall - So by your logic, if you replace one 100 watt light bulb with a 75 watt light bulb, the entire home's energy cost will be reduced by 25%. That's nonsense. Your answer is incorrect which is why I downvoted it. – Mark Jul 06 '22 at 04:17
  • @Mark You are correct I made mistake. The floor represents 1/3 of all energy consumption (walls and ceiling). If you reduce that energy consumption by 20% you reduce overall consumption. One light bulb has about 0.0001 % of total energy consumption. Whereby the flooor has ~33%, reducing it by 20% =6.6 % of total. – Ruskes Jul 06 '22 at 04:30
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    @knowitall - Where did you find the 1/3 figure for energy consumption due to the floor? I've never seen an estimate even close to that. Most values I've seen are close to 10%. I think the conclusion in your edited answer is now correct, though it could use a little more explanation. – Mark Jul 06 '22 at 04:47
  • Just a guess, where did you see the 10%, that would make the ROI even worse. If you know it just offer the correct answer please. – Ruskes Jul 06 '22 at 04:49
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    Although the math is questionable I really appreciate your answer. Thank you for using your free time to help me with my issues. – abdnChap Jul 06 '22 at 07:35
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    Agree with @abdnChap. While the actual numbers may or may not be completely accurate, the thinking and logic are on track: insulating the floor _will_ help reduce heating & cooling, but not _significantly_. – FreeMan Jul 06 '22 at 12:39