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.

[Greeners]

When reroofing, we converted to a warm roof from a cold one by adding 18mm OSB then 150mm PIR on top of the rafters, taped, membraned and counter-battened. We are planning on EWI so increasing soffit depth also allowed adding sprockets to the rafter end to accommodate the additional depth of insulation so we should be able to connect the wall insulation to the roof insulation, albeit intersected by the OSB at the transition.

 

There's a lot of cutting angles into PIR for hips and ridges so it's messy and laborious and sealing the eaves is not for the fainthearted on an old building. It was cool in the Robin Clevett video how he insulated a simple shaped roof, then added dormers (or some feature roofs) on top with cold roof construction - very nice.

 

Also, make sure you keep track of where the rafters are, it's not trivial to hit them through a batten and 150mm of insulation, not to mention the interesting cost of 300mm structural screws (every second screw angled).

 

I like the principle of this approach, and would do it again, but only on a very simple roof shape - my roof was overly complex, especially for a rookie

 

 

[Rishard]

I’ll chime in on this as what you’re proposing is very similar to what I’m about to undertake. Robins method requires a lot of skill and accuracy. I’ve done dozens of roofs with cut PIR between. It’s horrible. Not to mention difficult to achieve the level of accuracy. Gapo tape does help with this somewhat but comes at a very high cost. Cutting 100mm pir is hard to maintain accuracy, I have the fancy festool insulation saw and even this isn’t perfect. I’ve done 70mm sheets on my table saw where I can make repeat cuts. Doubling up sheets can help achieve your thickness. Even when every rafter is meticulously fixed and spaced there always seems to be a slither of light that gets through somewhere. If you’re prepared for cutting lots of pir and buying a boat load of gapo tape then his method is great. For me, buying Pir is by far the cheapest method of insulating sadly, combined with its uvalue it’s very appealing. Using the full sheets over the top seems to be the best solution for pir, minimal cuts and thermally breaking the rafters.

 

 

I have 180m2 of warm pitched roof to insulate and I’m working to a similar principle to you. It won’t be everyone’s cup of tea but I’ve researched and costed out multiple versions of insulating this roof. I’ve had advise from my phpp designer and condensation risk calcs done. They all state the most breathable material to the outside however the VCL changes everything. Obviously it needs to be bullet proof which may be hard if your contracting this work out. For me, being the builder on this, I’m very confident I can achieve an excellent vcl in my roof build up. Height is also and issue on my project as some areas already have low eves ceilings on plan and the ridge height is already decided upon on planning. (Had I known what I know now when I put in planning 6 years ago….)
 

My build up is 120mm pir over rafters in 2 layers to stagger the joints. 
145mm rafters full filled with Knauf mineral wool. 
11mm osb below

vcl

service void. 
 

I know the risks. I’m willing to take it. The condensation risk came back ok with an effective vcl. For me, it’s mainly cost and simplicity of installation that ruled. I got a uvalue of 0.1 with the above. Also have mvhr planned. 
 

I costed out dozens of alternative blown cellulose and wood fibre options. They bother required much greater rafter depths which double my timber price. Wood fibre came in at twice the price of pir for half uvalue for thickness. I know all of the above have different characteristics and achieve varying uvalues and that uvalue isn’t only thing to consider.
 

Everyone’s circumstances are different on their project, sometimes a bit of context may help. Who’s installing the roof? Who’s installing the insulation?  What do you have on your plans? What u-value do you need? What’s your budget? 
 

If I had the money and roof height weren’t an issue I would go wood fibre over engineered rafters with blown cellulose allday. 

[Crofter]

@Rishardare you going to build that from the top down? I e. osb on the inside, and then add the insulation layers from the top?

My roof was osb sarking outside with breather felt, then wool between rafters installed from underneath (fun! Somebody needs to invent lighter than air mineral wool!), then 100mm PIR boards forming a complete layer on the inside, and VCL after that. 

I was mostly working on my own and adding the wool and VCL was one of the worst bits of the whole build. If I'd have been braver I'd have installed the wool from above but I wasn't certain I could get it done quickly enough to do it in dry weather.

[Rishard]

@Crofter I will indeed nail up the osb first when I get a couple of clear dry days. I’ll then cut my rafter roll insulation to my rafter spacings and roll it in, followed by the pir over the top. Working from above as you say. Mineral wool still isn’t great but at least it won’t be flapping down in my face with reduced ventilation. 

[Ben1984]

2 hours ago, Rishard said:

I’ll chime in on this as what you’re proposing is very similar to what I’m about to undertake. Robins method requires a lot of skill and accuracy. I’ve done dozens of roofs with cut PIR between. It’s horrible. Not to mention difficult to achieve the level of accuracy. Gapo tape does help with this somewhat but comes at a very high cost. Cutting 100mm pir is hard to maintain accuracy, I have the fancy festool insulation saw and even this isn’t perfect. I’ve done 70mm sheets on my table saw where I can make repeat cuts. Doubling up sheets can help achieve your thickness. Even when every rafter is meticulously fixed and spaced there always seems to be a slither of light that gets through somewhere. If you’re prepared for cutting lots of pir and buying a boat load of gapo tape then his method is great. For me, buying Pir is by far the cheapest method of insulating sadly, combined with its uvalue it’s very appealing. Using the full sheets over the top seems to be the best solution for pir, minimal cuts and thermally breaking the rafters.

 

 

I have 180m2 of warm pitched roof to insulate and I’m working to a similar principle to you. It won’t be everyone’s cup of tea but I’ve researched and costed out multiple versions of insulating this roof. I’ve had advise from my phpp designer and condensation risk calcs done. They all state the most breathable material to the outside however the VCL changes everything. Obviously it needs to be bullet proof which may be hard if your contracting this work out. For me, being the builder on this, I’m very confident I can achieve an excellent vcl in my roof build up. Height is also and issue on my project as some areas already have low eves ceilings on plan and the ridge height is already decided upon on planning. (Had I known what I know now when I put in planning 6 years ago….)
 

My build up is 120mm pir over rafters in 2 layers to stagger the joints. 
145mm rafters full filled with Knauf mineral wool. 
11mm osb below

vcl

service void. 
 

I know the risks. I’m willing to take it. The condensation risk came back ok with an effective vcl. For me, it’s mainly cost and simplicity of installation that ruled. I got a uvalue of 0.1 with the above. Also have mvhr planned. 
 

I costed out dozens of alternative blown cellulose and wood fibre options. They bother required much greater rafter depths which double my timber price. Wood fibre came in at twice the price of pir for half uvalue for thickness. I know all of the above have different characteristics and achieve varying uvalues and that uvalue isn’t only thing to consider.
 

Everyone’s circumstances are different on their project, sometimes a bit of context may help. Who’s installing the roof? Who’s installing the insulation?  What do you have on your plans? What u-value do you need? What’s your budget? 
 

If I had the money and roof height weren’t an issue I would go wood fibre over engineered rafters with blown cellulose allday. 

 

Thanks for your reply. It sounds like you have given this subject a lot of thought.

I (expeienced builder) will be doing all of the work on the roof myself, including the insulation.  I don't have the technical drawings or structural calculations yet, so I'm not sure what depth rafter I will be working with - undecided on whether I will stick with a cut roof or use trusses.
I think 0.12 would be a good target for the roof, but there's a degree of flexibility here.

I have a degree of flexibility on the ridge/ceiling heights - this is unlikely to be a deciding factor for me.

I am currently favouring 100mm PIR on top of the rafters and either 100mm PIR or 150mm mineral wool between the rafters - if you have any suggestions that would improve on this without increasing costs significantly I would be very keen to hear them.

[Rishard]

I would start with finding out what size your rafters need to be. Mine had been oversized by the architect to accommodate more insulation. Moving to a warm roof allowed me to reduce these and thermally break them with insulation over the top. I’d think full filling your rafter depth with a mineral or blown material will ultimately be easier to do. Mines a gable to gable with 1 small valley section. If there were dormers or hips, pir in between would become even more tricky. 

[ProDave]

I fitted my insulation from underneath.   I used Frametherm 35 which is a lot less nasty than some other forms of mineral wool insulation, and it is quite stiff, so cut a bit wider than the gap and it just pushed in and stays where you put it.

 

[Rishard]

@ProDave Did your frame therm come in slabs or on the roll? Did you need to make these shorter lengths to handle or could you put up large sections in one? Did it require any temporary support before you put the osb up? Good to know there is an alternative way if I decide to insulate from below. I’m also using Knauf as I find it much nicer to handle. I’m using 150mm Knauf batts in my cavity so I can also use up some of my remaining bits of insulation for between my rafters. 

[Crofter]

I had to add some temporary support to my wool when installing from below. I had a big roll of polythene warning tape left over from doing the electrical supply, so just used a staple gun to tack on a couple of runs of that. It might confuse and alarm anybody who ever has to open up the roof in the future I suppose 😂