Warm pitched roof

[Ben1984]

Considering options for roof insulation - having watched Robin Clevett do a warm pitched roof on his channel:

 

 

I am seriously considering doing the same. He uses 100mm PIR on top of the roof and 100mm PIR between the rafters. I would be looking at 100mm PIR on top and ~150mm mineral wool between the rafters (mainly to avoid having to cut the PIR between the rafters) otherwise I would look to copy everything he did on the video.

Does anyone have any experience of doing this? Part of me thinks it would actually be easier to cut the PIR between the rafters and go for a more standard cold roof. I can see pros and cons to both options.

[nod]

Which ever you decide I’d avoid the mineral wool Prone to sweating 

[Ben1984]

17 minutes ago, nod said:

Which ever you decide I’d avoid the mineral wool Prone to sweating 

Thanks, that's useful insight. If I need to use PIR between the joists either way I would be more inclined to stick with a cold roof. 

[ADLIan]

57 minutes ago, nod said:

Which ever you decide I’d avoid the mineral wool Prone to sweating 

If by 'sweating' you mean prone to condensation this statement is rubbish. Bad roof design causes condensation problems not the type of insulation.

[nod]

16 minutes ago, Ben1984 said:

31 minutes ago, ADLIan said:

If by 'sweating' you mean prone to condensation this statement is rubbish. Bad roof design causes condensation problems not the type of insulation.

Your lack of experience makes your statement rubbish

[ADLIan]

3 hours ago, nod said:

 

 

Rash statement as you know nothing about me, my background, knowledge or experience.

[Crofter]

Manners please chaps.

 

'Sweating' is the product of poor design, poor understanding of condensation risk, and poor application of a vapour control barrier.

[ADLIan]

And not choice of insulation type!

 

Can we put 'sweating' in the same bin as 'thermal mass'?

[G and J]

2 minutes ago, ADLIan said:

And not choice of insulation type!

 

Can we put 'sweating' in the same bin as 'thermal mass'?

Well I'm not going to.  Whilst neither terms are purely scientific they convey concisely an issue which has relevance.

 

@nod, I'm putting mineral wool in my timber frame stud walls.  Inboard of that will be a vapour control layer (AKA an expensive and approved equivalent to airtight polythene). Presumably that will mean steam from indoors won't get there and won't get the wool wet.

 

But humidity can come from outside too - so is it an external source of 'sweat' in a warm roof?  In which case do I need to be worried about my walls?

[Crofter]

1 hour ago, G and J said:

Well I'm not going to.  Whilst neither terms are purely scientific they convey concisely an issue which has relevance.

 

@nod, I'm putting mineral wool in my timber frame stud walls.  Inboard of that will be a vapour control layer (AKA an expensive and approved equivalent to airtight polythene). Presumably that will mean steam from indoors won't get there and won't get the wool wet.

 

But humidity can come from outside too - so is it an external source of 'sweat' in a warm roof?  In which case do I need to be worried about my walls?

You want to design your layers in such a way that you eliminate condensation risk.

 

So on the inside you have a vapour barrier, then your insulation layers need to, ideally, become progressively more vapour permeable as you progress towards the outside (e.g. PIR first, then mineral wool). Then you have a breather membrane which should keep most moisture out of the structure, but which will allow any moisture that does accumulate to escape. And finally your cladding which acts as a bulk rain shield.

 

What you must avoid doing is creating a completely sealed volume from which moisture cannot ever escape. So you have one VCL, and either side of that a pathway for moisture to get out.

[Ben1984]

6 hours ago, Crofter said:

You want to design your layers in such a way that you eliminate condensation risk.

 

So on the inside you have a vapour barrier, then your insulation layers need to, ideally, become progressively more vapour permeable as you progress towards the outside (e.g. PIR first, then mineral wool). Then you have a breather membrane which should keep most moisture out of the structure, but which will allow any moisture that does accumulate to escape. And finally your cladding which acts as a bulk rain shield.

 

What you must avoid doing is creating a completely sealed volume from which moisture cannot ever escape. So you have one VCL, and either side of that a pathway for moisture to get out.

 

It sounds like my original idea of PIR on top of the roof and 150mm mineral wool between the rafters could be problematic then. If I have a VCL immediately behind the plasterboard and then PIR ontop of the roof would this design fundamentally fail? Or are there ways I could make this work?

[Crofter]

2 hours ago, Ben1984 said:

 

It sounds like my original idea of PIR on top of the roof and 150mm mineral wool between the rafters could be problematic then. If I have a VCL immediately behind the plasterboard and then PIR ontop of the roof would this design fundamentally fail? Or are there ways I could make this work?

It's certainly a higher risk for interstitial condensation. But I believe it's possible to make it work.

On my build I did full fill wool between studs/rafters, and a layer of PIR on the inside. Seemed the safest method.

 

I'm in no way a qualified or trained builder or engineer though.

[Ben1984]

10 hours ago, Crofter said:

It's certainly a higher risk for interstitial condensation. But I believe it's possible to make it work.

On my build I did full fill wool between studs/rafters, and a layer of PIR on the inside. Seemed the safest method.

 

I'm in no way a qualified or trained builder or engineer though.

 

Thanks. I'm starting to think there is a good reason why warm pitched roofs are fairly uncommon!

[ProDave]

I have a warm pitched roof, or more correctly a hybrid roof, with 100mm wood fibre board over the rafters and 200mm mineral wool (Frametherm 35) between the rafters, then an air tight layer then plasterboard.

 

I swear by this method and it is infinitely better and easier to detail that any other method.

[Ben1984]

4 hours ago, ProDave said:

I have a warm pitched roof, or more correctly a hybrid roof, with 100mm wood fibre board over the rafters and 200mm mineral wool (Frametherm 35) between the rafters, then an air tight layer then plasterboard.

 

I swear by this method and it is infinitely better and easier to detail that any other method.

 

Thanks ProDave. I was hoping to hear from someone with personal experience doing this. Do you happen to know what U-value this design achieved? I am not familar with wood fibre boards, is there a particular reason you chose these over PIR?

[Crofter]

4 hours ago, Ben1984 said:

 

Thanks. I'm starting to think there is a good reason why warm pitched roofs are fairly uncommon!

I do have a pitched warm roof. But I don't have trusses, as all the cross beams would represent too many penetrations and thermal bridges. I've got a ridge beam and cut rafters instead.

 

I wouldn't discount the idea of layering your insulation the way you describe, but I would seek more advice on it. It's a higher condensation risk but it may not be a show stopper.