Hi

 

My builder saying he will change the walls to 140mm celotex pir at 0.023, what sort of u values will this produce?

 

Currently 0.22 with 140mm of frametherm.

 

Cheers

 

Much Thanks 

 

Mike 

If this is Frametherm 32 then the PIR alternative will be U=0.160W/m² .K

So do you know what the qall u valur will be?

The wall u values will be.

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 .  

Im looking for those figures in picture above to shpw 140mm PIR instead of the wool.

 

Im hoping to have better walls than 0.22! 

PiR and service void on the inside.

20 minutes ago, dpmiller said:

PiR and service void on the inside.

Whats that?, they are swapping the wool for pir.

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. 

agree with @dpmiller and @Russell griffiths. we're simply copying what a lot of the major TF manufacturers do in the factory. here's the MBC example.

 

https://www.mbctimberframe.co.uk/closed-panel-wall-options/

In the calculation above layer 4, TF200, needs setting to zero. The 0.77 value for this is only valid when against an unventilated airspace. This increases U-value to 0.26.

To clarify, a straight swap of 140mm PIR for 140mm of Frametherm32 would reduce the U value of a wall of 0.22 to 0.16. As ADLIan comments about the wall build up posted I had previously said the same. https://forum.buildhub.org.uk/topic/19745-sap-calcs-advise/?tab=comments#comment-318670

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? 

14 minutes ago, Mike_scotland said:

you're right. that is their 0.18W/m2K wall. if you look at their other walls (0.14 and 0.11) you'll see they do. e.g.

 

 

all 3 walls are shown on the link I posted.

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.

No it’s a service batten to run pipes and cables through. 

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!

 

28 minutes ago, Russell griffiths said:

No it’s a service batten to run pipes and cables through. 

Of course omg, its been a long day! Im not that daft haha

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

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

So even at 0.19 if they have the layer 4 in, it would actually be higher? (0.23 or something)

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)

I have passed it to architect,  thanks for bringing this to my attention