Andrew

Determining target u-values

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We've been talking to a few timber frame suppliers and there's a bit of a chicken and egg situation going on insomuch that they are asking us for the target u-values we want to achieve. We know we want a low-energy home with high levels of airtightness. We know we want MVHR. We also know that we need to strike a balance in terms of cost and performance as we don't have an endless budget. 

 

What I'm struggling with is what is the real world difference between having 0.12 W/m2K and 0.14 W/m2K. 

 

I know I need to model the house and I've had a look at the free PHPP spreadsheet which looks unnecessarily complex.  What I'd like to find is a simpler model which basically says we have so many meters squared of wall / roof at a certain u-value, for each wall / roof and then the same for glazing but recording the compass direction it is facing so some allowance for solar gain can be made. Out of the back I simply want to know the kWh of heating required per year and then I can change the u-value and see the effects. I know this won't be perfectly accurate but should allow me to the see the relative values of different wall constructions. 

 

Does anything like that exist? Or are we looking this from the wrong perspective?

 

Thanks.

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what you need to look at is the SAP calculation for the new house which balances out your heating system/insulation etc and that will give you max u-values for the elements which you can decide how and where you want to exceed

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40 minutes ago, Andrew said:

Does anything like that exist?

 

Yes, it’s on @JSHarris blog ....!

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41 minutes ago, PeterW said:

 

Yes, it’s on @JSHarris blog ....!

 

 

Here's a direct link:

 

Heat loss calculator - Master.txt

 

and, whilst I'm at it, here's a link to a simple spreadsheet for calculating heat floor loss and UFH output:

 

Floor heat loss and UFH calculator.txt

 

Both of these are Excel spreadsheets, which the forum software won't allow, so they have been renamed to fool the system into thinking they are text files.  Download them somewhere, then rename them, replacing the .txt suffix with .xls and they should open in most spreadsheet software OK.

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2 hours ago, Andrew said:

What I'm struggling with is what is the real world difference between having 0.12 W/m2K and 0.14 W/m2K. 

 

@Andrew for the specific example in central England the saving is around 0.78kWh/m2/yr

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3 hours ago, Andrew said:

What I'm struggling with is what is the real world difference between having 0.12 W/m2K and 0.14 W/m2K. 

 

In practical terms not a lot really. You'll need downstairs heating at least even with triple glazing and good airtightness. In my experience from running PHPP with different U factors etc if you're in the south of England you'll need a U factor of very close to 0.1 W/m2K to not need a conventional heating system.

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Having now spent a few hours playing with @JSHarris heat loss calculator, just wanted to say thank you, it was pretty much exactly what I was looking for and has really helped me understand the impact of the different u-values. As others have said the difference aren't that great and were much less than I expected. 

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