U values

[BarnabyT]

Does anyone know how to workout u-values that might be able to assist

BCO coming on Tuesday and it would be nice to be able to fend him off. My original plans for an extension had the roof done with normal fink trusses. The plans stated 300mm rockwool laid in 2 layers and to pack the eaves. This clearly exceeds the required level of performance.

However, we ended up going with scissor trusses which don't provide the same amount of space for insulation. I have gone with 100mm pir between the trusses and underdrawn in 50mm. Seems to me that 150 pir must be close to, if not better than 300mm rockwool.... but I don't actually know. Is there a way to compare the performance of both?

 

[Mike13]

If you have the manufacturers U value for each item, say for example a U value of .3 for x mm of PIR, and 0.4 for y mm of the other insulation, but you have joists with a U value of 1 between strips of 0.4, and it is 1/6 joists on the roof, you would have 1/6x1 and 5/6x0.4 giving you an average U value of 0.5 for the joist section.
Next you take 1/(U value of either section) to get the R value, add the R values, including the .05 or so for the plasterboard. Then you take 1/(final R value) and you are back to overall U value of roof. In my example you have U1=0.5 U2=0.3, so R1=2 and R2=3.333, so R total = 5.3333 and 1/R total = 0.187, which is not quite enough for a roof.

But see what you get when you use the real numbers for your roof.

 

[mikeey84]

I reckon they will accept that without calcs unless there is something exotic about the roof construction. PIR is reckoned to be twice as good as rockwool, so you should be ok

 

[noseall]

I have gone with 100mm pir between the trusses and underdrawn in 50mm

Standard vaulted spec'. Crack on. Don't forget foil tape.

How are you venting this? BC will want to see eaves to ridge ventilation along with 25-50mm air gap above the 100mm PIR.

We use OFV's and vented dry ridge for vaulted fellas...

Bolt3_LI

• noseall
• 27 Apr 2022

Bolt2_LI

• noseall
• 27 Apr 2022

 

[^woody^]

150mm PIR (100 between and 50 over or under) is compliant.

 

[tony1851]

(Note: U-values are changing for new work registered after 15th June).

 

[Dad'n'lad]

Standard vaulted spec'. Crack on. Don't forget foil tape.

How are you venting this? BC will want to see eaves to ridge ventilation along with 25-50mm air gap above the 100mm PIR.

We use OFV's and vented dry ridge for vaulted fellas...

Bolt3_LI

• noseall
• 27 Apr 2022

Bolt2_LI

• noseall
• 27 Apr 2022

What width is the extension please Noseall?

 

[noseall]

What width is the extension please Noseall?

Dimensions are length 3000mm and width (across the gable elevation) 3405mm

 

[noseall]

What width is the extension please Noseall?

Here is a look at the elevation...

Ost

• noseall
• 1 May 2022

Its only a wee extension and I've loved every minute. We can turn these around in about 30 days. My last one was epic.

 

[BarnabyT]

How do u do the pir into the eaves and do the eaves still need to be packed with rockwool? Never done this before

 

[noseall]

How do u do the pir into the eaves and do the eaves still need to be packed with rockwool? Never done this before

The cavity insulation is extended all the way up to the underside of the roofing felt so that it touches the continuous rafter tray. The PIR is then slotted down the eaves so that it pushes against the vertical cavity insulation.

 

[BarnabyT]

Thanks pal

 

[noseall]

How do u do the pir into the eaves and do the eaves still need to be packed with rockwool? Never done this before

From the inside...

Eavesi

• noseall
• 2 May 2022

Note the continuous rafter vent tray (black).

 

[BarnabyT]

Yeah I've got the same vents fitted and there is still plenty of room above the top of the PIR because they're scissor trusses.