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Can anyone help me :)


Mike_scotland

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I think @A_L means that the new wall value if using PIR will be

11 hours ago, A_L said:

U=0.160W/m2 .K

but if there are any changes to the geometry it could be different.

 

I just tried it on PHPP and got 0.229 for the Frametherm and 0.157 for the PIR so that agrees with @A_Ls figures.

 

PS Also do not forget the hydrothermal (moisture migration) changes going from Frametherm to PIR will make and those figures are the U value for just the insulation not the whole wall buildup so the wall value will be less, assuming the whole wall is not just PIR O.o.  

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5 hours ago, Russell griffiths said:

You still need insulation on the inside to mitigate against the cold bridges through the studs, just fitting 140 between studs does not make a good wall build up. 

Im sure the mbc diagram i seen doesnt have insulation on the inside? 

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Yeah sorry i dont mean to call amyone out, i just wanted tp make sure my builder not trying a fast one.

 

So would it be worth having 140mm wool inbetween stud then 50mm of pir then cavity batten? 

 

I take it this cavity batten is to keep the cold hard off the plasterboard? 

 

Do you require one if you have one on the outside? Obviously we have a 50mm cavity after blockwork.

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2 hours ago, Mike_scotland said:

So would it be worth having 140mm wool inbetween stud then 50mm of pir then cavity batten?

in my opinion I believe it is as it reduces U-values AND reduces cold bridging. we're going for 80mm to get to around 0.11W/m2K. Also, for us, it's easier to install the mineral wool insulation between the studs and layer the PIR over the top and as we're doing the insulation ourselves this is very important for me!

 

obviously, each person needs to decide what's important for them though!

 

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48 minutes ago, Mike_scotland said:

Architect saying if we change to PIR the u values change from 0.22 to 0.19?? That sound correct? 

 

Both values include a thermal resistance for layer 4, and as @ADLIan and myself have indicated this is contrary to convention. See section 4.8.6. page12 of the attached pdf.

 

Suggest you point your architect to this.

 

 

BR_443_(2006_Edition).pdf

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Yes, a U value of 0.19 implies a thermal resistance of 1/0.19 or 5.263m2K/W, removing layer 4 (R=0.77) and adding in the value for increased external surface resistance (R=0.29) makes the thermal resistance 4.783 - 0.77 + 0.29 = 4.783m2K/W which gives a U value of 1/4.783 = 0.209W/m2K (0.21)

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