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Would you notice much difference 0.20 versus 0.17 U value?


colin7777

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I know there are too many factors to produce exact information but in general if nothing else changed would you notice a difference between 0.20 and 0.17 U value used on the external walls.

 

Would there be much cost savings in heating going for 0.17 instead of 0.20.

 

Thanks in advance

 

Colin

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52 minutes ago, epsilonGreedy said:

How big is the house and is it a rectangular 2-story box or a u-shaped single story villa. 

Hi, around 12 metres by 6 metres 1 3/4 floors, just trying to get some views as to benefits of improving U values, never lived in a well insulated house 

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4 minutes ago, MikeSharp01 said:

In big picture terms, all other things being equal, your heating bills will be around 18% higher - for ever!

 

 

18% looks high.

 

An average house built to 2013 thermal standards looses about 14% of it heat through the walls. 0.2 dived by 0.17 = 1.18

 

18% of 14% = 2.5% increase on overall house heating cost.

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34 minutes ago, MikeSharp01 said:

In big picture terms, all other things being equal, your heating bills will be around 15% higher - for ever!

 

Not true,  IIRC in out case the wall losses were only about 30% of our total heat loss so this would on its own represent perhaps a 5% increase in your heating bill forever.  However as Mike says, it is really worth playing with the numbers using a simple spreadsheet approximation such as Jeremy's.  Your heat losses are primarily mix of wall, floor, roof, fenestration and air leakage circulation losses and you need to get a good balance.

 

So for example, IMO, if you had to chose between 0.17 + no MVHR and 0.2 + MVHR, then the second option is by far the best.   I went through all of these trade-offs when I was trying to decide whether to have a single-wall or twin-wall TF.  In the end we went with MBCs twinwall Larson-struct construction.  In retrospect, I have absolutely no regrets -- not for the slight U-value improvement, but for other factors that I didn't even consider in my design trade-offs:

  •  The cellulosic-filled twinwall has a far higher thermal capacity and decrement delay which makes the whole environment a lot more thermally stable and pretty much insensitive to external diurnal temperature variation.
  • The blown-fill open panel cavity is intrinsically more airtight than pretty much any of the construction alternatives.
  • You are also far less vulnerable to quality issues with insulation fit and potential airgaps / cold spots.

So at the end of the day this is all about trade-offs.  A good exercise is do the first design iteration then give yourself a £5K improvement budget  and look in turn at using this to improve any one of the above components.  If one stands out then you've got the balance wrong. 

Edited by TerryE
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1 hour ago, colin7777 said:

Would there be much cost savings in heating going for 0.17 instead of 0.20.

 

In central England (Midlands) SAP says it saves about 1.7kwH/yr per m2.

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