Between rafter insulation

[James94]

Hi all, I’m building a storey and a half house and  trying to find the best way of insulating the sloping part of the ceiling.  

 

Here is the specification we currently have, I would like some advice on the best (easiest and most cost effective) way to achieve this u-value. 

 

All advice welcome.

 

Thanks James 

[Pocster]

Hey

 

I put pir insulation between rafters . Foam filled any gaps then 50mm over the lot taping all the joints 

 

cheers 

[Pocster]

Oops sorry ! Just read your ‘spec’ . I put 150mm inbetween rafters and 50mm over it all . From memory this gave around 0.15. So with plasterboard aswell should meet your spec .

More intelligent people will be along shortly !

[Temp]

I have a 1.5 storey house with 200mm deep rafters, PIR between. I wouldn't do this again. Too hot in summer.

 

I would probably build a warm roof with most of the insulation above the rafters.

 

I have a shed that has a ventilated cavity between the cladding and insulation. This shed stays surprisingly cool in summer. I have a hunch that something like the following might be worth investigating...

 

Tiles

Ventilated void (eg vents at eaves and ridge)

Membrane (vapour permeable)

Multifoil insulation

Rockwool between rafters

PIR below rafters.

 

Yes three different types of insulation.

 

 

[the_r_sole]

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Edited September 26, 2019 by the_r_sole

[James94]

Rafters are 220mm deep

Edited August 22, 2019 by James94

[the_r_sole]

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Edited September 26, 2019 by the_r_sole

[the_r_sole]

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Edited September 26, 2019 by the_r_sole

[dimpsy]

Although we did something completely different in the end.

 

Construction: BTC 0580

Tiles on battens on Tyvek Supro installed draped over counter battens. 100mm PIR insulation is installed on a 9mmOSB board that sits atop the rafters. 230mm Knauf Earthwool 032 is installed between rafters. Internal lining is a DuPont Airguard Reflective AVCL and a 12.5mm plasterboard.

 

Condensation:

No condensation is predicted

 

U-value:

0.09 W/m²K

 

We hope this information helps to resolve all queries. If you require further assistance please contact us again on the details below.

 

[nod]

1 hour ago, pocster said:

Oops sorry ! Just read your ‘spec’ . I put 150mm inbetween rafters and 50mm over it all . From memory this gave around 0.15. So with plasterboard aswell should meet your spec .

More intelligent people will be along shortly !

 

[nod]

1 hour ago, pocster said:

Hey

 

I put pir insulation between rafters . Foam filled any gaps then 50mm over the lot taping all the joints 

 

cheers 

I do a lot of these 

We normally counter baton the ceiling to stop the insulation sitting on the airtight barrier and fix the airtight barrier taught onto the counter batons The board with duplex 

Again with the slopes Baton then run your barrier 200 mil onto the 

upright  

[AliG]

Are architects all on commission from Kingspan!

 

K107 "Kooltherm" has conductivity of 0.02 W/m K and costs, according to a quick Google, around £28 a square metre at 100mm depth. Cheaper prices may be available. You really should only use this stuff if you have a ridge height issue.

 

As an aside, 75mm is a totally non standard thickness which would be hard to get hold of and be a bit more expensive for any given volume than 100mm boards.

 

100mm of Celotex GA4000 costs around £12 a square metre. Its conductivity at 0.022 is 10% worse, so it would have be be 10% thicker to provide the same insulation value.

 

You could also substitute the Kooltherm based plasterboard for PIR based which would be about 10-15% cheaper at the same thickness.

 

In this case substituting 100mm Celotex for the 75mm of K107 and PIR based plasterboard would save roughly 40% om the material costs whilst giving roughly the same u-value.

 

I think at this rafter thickness you could also use 150mm of rockwool between the rafters and PIR below and be even cheaper again. You would have to check condensation issues.

 

Alternatively you could use 150mm of PIR between the rafters and plasterboard below.

 

Also if the rafters are 220mm deep is the proposal really to have a 25mm gap then 75mm of insulation then a 100mm gap then plasterboard/PIR below. That seems v odd.

 

Also what do they mean, install vapour barrier on the warm side of the insulation, between it and the plasterboard, half the insulation is attached to the plasterboard, so that is just impossible!

[PeterW]

190mm of Earthwool between the rafters, 25mm PIR over secured with 25mm battens at the joists and then 25mm infill, taped at all joints will work for you from both uValue and airtightness perspective and is also good for decrement delay. 

[the_r_sole]

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Edited September 26, 2019 by the_r_sole

[A_L]

3 hours ago, James94 said:

I would like some advice on the best (easiest and most cost effective) way to achieve this u-value. 

 

200mm of frametherm 35 or RWA45 between your rafters, and your K118 will give U = 0.13

 

 

[the_r_sole]

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Edited September 26, 2019 by the_r_sole

[PeterW]

21 minutes ago, the_r_sole said:

 

Where are you getting that calc from? 

I've just looked at doing a room in the roof with a similar build up, but could only get to 0.14 with 200mm Earthwool and 50mm PIR on the inside (that was just through Knauf insulation direct) Could definitely do with a better uvalue for less material!?

 Use FrameTherm32 for the rafter fill, gives 0.14 when you take into consideration the 25mm PIR stopping the bridging of the rafters. This was on a 600mm rafter spacing. 

[the_r_sole]

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Edited September 26, 2019 by the_r_sole

[James94]

I would like to thank everyone for their help and great advice. 

 

I was considering using two layers of frametherm with PIR underneath, but the trusses aren’t spaced out evenly due to the dormers. So I was wondering how easy is  frametherm to cut and work with?

 

When using the celotex calculator it asks for rafter spacings, most are at 600mm centres but there’s are a few which are less. Should I choose the 600mm option or will I recieve  incorrect calculations? 

 

See attached photo.

 

 

Thanks James.

 

 

 

 

 

 

 

[Oz07]

As a note most commercial contracting builders pay absolutely no attention to brand of pir round here only thickness. It's whatever's cheapest and avail at the time 

[AliG]

The spacing is to calculate the bridging losses from the trusses. It will not make a meaningful difference.

 

You can cut Frametherm to width pretty easily whilst it is still rolled up.

[James94]

Can anyone tell me  what thickness PIR i would require if I were to use 180mm frametherm  32 or 35 between rafters to achieve 0.14? 

 

Thanks 

[AliG]

50mm should get you there.

 

You need to work in R-Values to add up materials.

 

A U-value of 0.14 is equivalent to an R-Value of 1/0.14 = 7.14.

 

180mm Frametherm 35 has an R-Value of 5.1 (From datasheet) - The calculation is thickness in m/thermal conductivity = 0.18/0.035 = 5.14.

 

So to get to a U-Value of 0.14 you need to add another 2 to the R-Value.

 

To calculate this you multiply the conductivity by the R-Value that you need = 0.022x2 = 0.044mm.

 

So 50mm will get you there.

 

To do the calculation properly you need to allow for bridging by trusses and small gains from the other materials in the roof. This will be close enough, the supplier should be able to give you an exact value.

 

 

 

 

[A_L]

1 hour ago, AliG said:

So 50mm will get you there.

 

Not quite, when you allow 15% for timber fraction, on same build-up as before U=0.157. To get 0.14 or better you need at least 70mm, U=0.136