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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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1 Answers1

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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.

DIY75
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