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SAP Calculation U-Values vs Manufacturer Values


SBMS
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Hi all

 

We have received our first set of SAP Calcs back for our new build.  The wall U-Values are coming out around 0.22 - this is using 100mm block, 100mm cavity with 75mm PIR, 100mm inner light block (just standard for now).  However, putting this same spec into Kingspan/EcoTherm U-Value calculators, it spits out 0.17.  Does the SAP Software calculate things differently?

 

Thanks

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So the calculators all assume 50mm cavity which you don’t have with the solution there. 
 

Why not go with 125 or 150mm cavity and then use full fill blown bead or similar ..?

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3 minutes ago, PeterW said:

So the calculators all assume 50mm cavity which you don’t have with the solution there. 
 

Why not go with 125 or 150mm cavity and then use full fill blown bead or similar ..?

This is the SAP calcs done by a firm, based on the above spec... Are you saying that they don't adjust the 'As Designed' Spec, to reflect the cavity solution described above?

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standard block outer, aircrete block inner, plasterboard on dab and 75mm PUR partial fill should give about 0.20 W/m2K (25mm or 50mm cavity makes no difference to U-value)

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Got you. So effectively they're going off some 'stock' figures.  I was going off ecotherm's calculator;

 

https://uvaluecalculator.ecotherm.co.uk/calculator/walls/cavity wall/brick/lightweight (0.15)/3mm skim-coated 12.5mm plasterboard/partial fill/75/

 

which is the manufacturer @ our local merchants.  Kingspan's calculator has a lower mark of 0.18 - both of these are better than the U Values used by the SAP calculator.  I assume these would suffice as evidence?

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@Iceverge Quickly read the mentioned topic, but what's the rationale in a nutshell? If you have a good builder who can fit PIR well onto the inner leaf is there any reason you would still recommend full fill mineral?

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12 minutes ago, SBMS said:

If you have a good builder who can fit PIR well

 

Unless you're prepared to pay a brickie twice as much it just takes too much time to get an absolutely perfect fit. Too many mortar droppings, irregularities in the blocks and boards and then having to fit around cavity ties. To get anywhere near the labelled performance you would need to build one leaf first, the apply the boards, foaming the back and taping the joints as you go. Almost like an EWI job. Then build the second leaf, 

 

Preferably you would need to pre age the boards to ensure they had already shrunk before installation. Take care to carefully tape all nicks and cracks in the foil facing to prevent gas migration. Also you would need to account for the thermal bridging of the foil face of the insulation at corners. 

 

image.png.12f742cbe2bcffa04ba1769b1ed3f652.png

 

 

While you were at it it would be prudent to apply the PIR in 2 layers with all joints staggered. It would be tricky to do as they cavity ties will dictate the location of the boards and they might need drilling foaming and taping afterwards. Of course there's the issue then of getting everything to fit tight around windows and doors. Ideally the boards will be fitted and individually foamed to create a continuous insulation layer between all windows doors pile penetrations etc.

 

 

 

If you can find a builder who is willing to do all this for a regular price let us know. 

 

Otherwise just full fill the cavity with diligently placed batts, EPS beads or closed cell foam. 

 

TLDR. 

 

Realistically it's impossible fit PIR to the perform even close to the theoretical design. 

 

Dritherm37 is £7.50/m2 for 125mm and will practically outperform Celotex @75mm for £17.50/m2. 

 

 

 

 

 

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