Basement U Value Calculation

[SBMS]

Exploring a mad idea to extend what was going to be just a 1.5m hole for an indoor swim spa, to an actual basement with a cinema room..

 

Am trying to work out what insulation to use. I am thinking of 100mm PIR stuck to the walls.  300mm waterproof concrete walls.  Is there a U Value calculator that'll approximate this, taking into account that the walls are underground? I used ecotherm IWI calculator and with 215mm block walls and 92mm insulated Plasterboard is around 0.23.. Would this be much improved with another 85mm of concrete wall, and by virtue of being underground?

[JohnMo]

U value calc will be pretty much the same as a floor.

  On 01/03/2025 at 09:20, SBMS said:

Would this be much improved with another 85mm of concrete wall, and by virtue of being underground?

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No. Concrete isn't an insulation it a good conductor of heat, add another 100mm of PIR, would bring your U value closer to 0.1. The ground is always around 6 to 8 degs. So heat loss will be constant all year around. You don't get the extreme cold of air temp but you don't get the warmer temps either.

[SBMS]

  On 01/03/2025 at 09:27, JohnMo said:

U value calc will be pretty much the same as a floor.

No. Concrete isn't an insulation it a good conductor of heat, add another 100mm of PIR, would bring your U value closer to 0.1. The ground is always around 6 to 8 degs. So heat loss will be constant all year around. You don't get the extreme cold of air temp but you don't get the warmer temps either.

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Thanks @JohnMo. Wonder if its cheaper to insulate the external walls with 200mm of EPS (bigger hole, bit of labour), or internally with 200mm PIR...

[JohnMo]

 

 

  On 01/03/2025 at 09:32, SBMS said:

insulate the external walls with 200mm of EPS (bigger hole, bit of labour), or internally with 200mm PIR

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200mm PIR is nearly equal to 300mm EPS. For the same internal space both options are similar.

[ADLIan]

Basement U-values are very complicated calculations. The basement floor U-value is based on the P/A ratio (as normal ground floors), the depth below GL and the type/thickness of insulation. The basement wall U-value is based on the depth below GL and the type/thickness of insulation in both the floor and the wall. Perhaps contact one of the XPS manufactures (Polyfoam?)

[Thorfun]

I did in depth calculations for the insulation for our basement taking in to consideration the depth of the basement and the insulating effect of the earth. 
 

but it was so long ago I can’t remember it! But I did have a thread on here about it which a search might yield. 
 

I’ll try and take a look if I have some time. 

[Thorfun]

found it. 
 

 

[Alan Ambrose]

>>> Basement U-values are very complicated calculations

 

Not that complicated to get a reasonable approximation though. Just take the insulation u-value and assume everything else doesn’t contribute much. The heat loss calc is a bit more forgiving as instead of say sub-zero worst case temperatures for walls and roofs the temperature a metre or so down is fairly constant. Say 9 or 10C depending where you are in UK.