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Ideal roof insulation…


matthyde83

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I’m trying to make some decisions on roof insulation on my new build…
 

So far I’ve increased the cavity to 150mm and using EPS Beads, increased floor to 200mm PIR.  Now I’m tackling the roof. 


I have a few different sections.  Mono-pitched on ground floor currently specced as 150mm rafters at 400 centres, dormers with similar and a hipped section specced with 400mm loft roll. 
 

Reading through building regs submission it’s suggesting 0.16 u value for mono pitch using 200mm celotex and 0.11 u value for the loft wool sections. 
 

I’d really like to hear people thoughts / advice…

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6 hours ago, Iceverge said:

209FF0E1-D2B5-4685-9AB5-44A4A73C0FB2.thumb.jpeg.530586e616942c1499d4e09fef31c343.jpeg

 

 

This for the pitched sections. 400mm blown cellulose on the flat sections.

 

 

Thank you.  So this achieves the stated 0.16 for the pitched sections.  I can imagine this being quite costly though?  9’x2” instead of 4, OSB layer, glasswool and 220mm blown cellulose…

Can you tell me why the recommendation?

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7 minutes ago, SteamyTea said:

Along with the insulation (warm roof helps all over) have you considered integrated PV?

It can help reduce temperatures in the summer as the energy is converted to electricity and does something useful.

I’ve looked but my roof really does not lend itself!

 

54a1 f.pdf

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3 minutes ago, matthyde83 said:


What build up would you suggest for a warm roof?  And if I’m insulating on top of rafters do I need to set rafters lower so that overall height of building doesn’t contravene planning?

@joe90 knows a fair bit about warm roofs.  Hopefully he will be along.

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2 hours ago, matthyde83 said:

Thank you.  So this achieves the stated 0.16 for the pitched sections.  I can imagine this being quite costly though?  9’x2” instead of 4, OSB layer, glasswool and 220mm blown cellulose…

Can you tell me why the recommendation?

 

The timber price would logicaly be more but 100mm rafters I'd expect them to be very closely spaced. The increased spacing and reduced labour should somewhat compensate. 

 

My non exhaustive list of why blown cellulose trump's PIR. 

 

Installed cost of cellulose is half the material cost of PIR for the same U value but with zero waste. 

 

Fire performance. 

 

Noise performance. 

 

Waste during install. 

 

Heat protection. 

 

No thermal bypass. 

 

No shrinkage. 

 

Performs just as well in cold weather. (Foam doesn't) 

 

Blown cellulose contributes lots to airtightness. 

 

It's carbon negative. 

 

No thermal bridging through the aluminium foil. 

 

No gas migration leading to poorer performance over time. 

 

No mess on site. 

 

 

 

 

 

 

 

 

 

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9 minutes ago, joe90 said:

Yes mine is a warm roof, built with I joists 400mm deep full filled with rockwall batts, lined inside with OSB bubble glued together to give an airtight (but breathable) loft.

Thanks.  Is the 400mm to allow for Rockwool rather than PIR?  Is that due to cost?  

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1 minute ago, matthyde83 said:

Thanks.  Is the 400mm to allow for Rockwool rather than PIR?  Is that due to cost?  

Yes, all my 200mm wall cavity’s is rockwall batt filled (PIR Is a petrochemical insulation and rockwall is not, fitting PIR is difficult to get airtight and yes more expensive;)

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Just now, joe90 said:

Yes, all my 200mm wall cavity’s is rockwall batt filled (PIR Is a petrochemical insulation and rockwall is not, fitting PIR is difficult to get airtight and yes more expensive;)

Are they lined outside or just membrane? 
I think I joists may be the way to go but not sure given my roof layout. 

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7 minutes ago, Iceverge said:

 

The timber price would logicaly be more but 100mm rafters I'd expect them to be very closely spaced. The increased spacing and reduced labour should somewhat compensate. 

 

My non exhaustive list of why blown cellulose trump's PIR. 

 

Installed cost of cellulose is half the material cost of PIR for the same U value but with zero waste. 

 

Fire performance. 

 

Noise performance. 

 

Waste during install. 

 

Heat protection. 

 

No thermal bypass. 

 

No shrinkage. 

 

Performs just as well in cold weather. (Foam doesn't) 

 

Blown cellulose contributes lots to airtightness. 

 

It's carbon negative. 

 

No thermal bridging through the aluminium foil. 

 

No gas migration leading to poorer performance over time. 

 

No mess on site. 

 

 

 

 

 

 

 

 

 

I imagine with I or metal web it would be even better?  How about vermin?  When I demolished the bungalow that was there the roof was chock full of every kind you could imagine, and some you couldn’t!!

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Most roofs just have the membrane under the tiles. I suppose any wildlife could eat its way through if it wanted. I don't see how cellulose would be different from anything else though.  It's treated with boric acid which insects don't like. 

 

That's all we have, but I can hear it flapping in the wind (insulation on flat). Time over again think I'd splash for an OSB layer. 

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15 minutes ago, matthyde83 said:

Are they lined outside or just membrane? 

Breathable membrane under slates, as slates are flat there is no gap for wildlife to get access, my OSB is on the inside to a/ hold insulation in place b/ create an airtight layer. 

 

9 minutes ago, Iceverge said:

I can hear it flapping in the wind

Again as slates are flat no gap for much wind to enter, we are in a windy location but never heard any flapping.

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29 minutes ago, joe90 said:

Breathable membrane under slates, as slates are flat there is no gap for wildlife to get access, my OSB is on the inside to a/ hold insulation in place b/ create an airtight layer. 

 

Again as slates are flat no gap for much wind to enter, we are in a windy location but never heard any flapping.

I’ve got handmade clay tiles so plenty of gaps!

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On 18/04/2022 at 09:17, joe90 said:

Yes, all my 200mm wall cavity’s is rockwall batt filled (PIR Is a petrochemical insulation and rockwall is not, fitting PIR is difficult to get airtight and yes more expensive;)


Which rock wool product did you use and at what joist centres?  Majority seems to be for cavity walls and therefore 455mm wide?

 

How was the cost at 400mm depth?

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3 minutes ago, matthyde83 said:


Which rock wool product did you use and at what joist centres?  Majority seems to be for cavity walls and therefore 455mm wide?

 

How was the cost at 400mm depth?

Can’t remember the actual product (specified by me but supplied by my builder), yes was used in my wall cavity.

 

Roof Joist centres were 400mm so tightly fitted. cost was part of a fixed price fir the main structure so don’t know.

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6 hours ago, joe90 said:

Can’t remember the actual product (specified by me but supplied by my builder), yes was used in my wall cavity.

 

Roof Joist centres were 400mm so tightly fitted. cost was part of a fixed price fir the main structure so don’t know.


How come you stuck with 400mm centres when you have 400mm deep joists!?  I’m expecting if we’re changing from 150mm and 175mm joists to 400mm engineered then one of the savings to offset will be increased centres?  Maybe it’s because of the cavity batts only being in 445mm sections though?

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