Would you notice much difference 0.20 versus 0.17 U value?

[colin7777]

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

[epsilonGreedy]

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

[ToughButterCup]

23 minutes ago, colin7777 said:

[...]

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

[...]

 

Thats the point of PHPP. Or @JSHarris spreadsheet.  Links available with a bit of googling. 

Ian

[MikeSharp01]

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

Edited June 20, 2019 by MikeSharp01
Typo 15 not 18

[colin7777]

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 

[epsilonGreedy]

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.

[TerryE]

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 June 20, 2019 by TerryE

[A_L]

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 m^2.

[MikeSharp01]

1 hour ago, MikeSharp01 said:

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

Yes I could have worded that better couldn't I.