What insulation for pitched roof?

[Pappa]

Hi guys

 

Can you help me understand what insulation I should go for on a pitched roof in an attic conversion.

I'm hoping to have the insulation layer as thin as possible because I need to maintain 1.9m head height from middle of the stairs and 1.8m at the end of the stairs.

 

 

I'm hearing good things about spray foam insulation.

Closed cell or open?

Do I still need to maintain a 50mm ventilation gap with spray foam?

 

What about PIR? I understand that its hard to make a tight seal against the rafters. However, what if I fill  the edges using cans of spray foam? I'm not keen on the gap tape, seems very laborious.

 

How does Cellulose spray foam compare in terms of Cost and U-value versus PIR + spray foam around the edges?

Ultimately, what will give me the thinnest insulation layer that is compliant with regs?

 

Thanks in advance

 

P

[SteamyTea]

1 hour ago, Pappa said:

Ultimately, what will give me the thinnest insulation layer that is compliant with regs

Realistically you will need to use PIR.

 

You can easily look up the thermal conductivity if the difference materials, the you can convert them to R-Values (thickness divided by k-Value). Add all the R-Values up, and divide that number into 1 to get your U-Value.

 

Avoid sprayed insulation, lenders are refusing to give mortgages on it, regardless of how good or bad it is.

Edited March 16 by SteamyTea

[SBMS]

3 hours ago, Pappa said:

Hi guys

 

Can you help me understand what insulation I should go for on a pitched roof in an attic conversion.

I'm hoping to have the insulation layer as thin as possible because I need to maintain 1.9m head height from middle of the stairs and 1.8m at the end of the stairs.

 

 

I'm hearing good things about spray foam insulation.

Closed cell or open?

Do I still need to maintain a 50mm ventilation gap with spray foam?

 

What about PIR? I understand that its hard to make a tight seal against the rafters. However, what if I fill  the edges using cans of spray foam? I'm not keen on the gap tape, seems very laborious.

 

How does Cellulose spray foam compare in terms of Cost and U-value versus PIR + spray foam around the edges?

Ultimately, what will give me the thinnest insulation layer that is compliant with regs?

 

Thanks in advance

 

P

What’s your actual rafter depth and then we can help advise?

[Pappa]

Thanks for the replys guys

6 hours ago, SteamyTea said:

Realistically you will need to use PIR.

 

Avoid sprayed insulation, lenders are refusing to give mortgages on it, regardless of how good or bad it is.

Sorry, I had my wires crossed, really, I was referring to blown cellulose/EPS beads.

 

4 hours ago, SBMS said:

What’s your actual rafter depth and then we can help advise?

Rafter depth is 74mm. width is 46mm. 430mm centers.

 

Thanks again

 

P

[mistake_not]

Quick play using PIR if you just fill between rafters gives u value of 0.399.

 

 

To hit the required Building Regs U value you need another 65mm of PIR.

 

 

This assumes PIR of 0.023 R value.

 

As I'm sure others will comment, it's great on paper, but really hard to do well unfortunately.

[mistake_not]

 

Mineral wool / EPS would increase your insulation depth required by 1/3. I.e. a total of 185mm.

 

I think you are stuck with a cold roof buildup, so will need a ventilation gap no matter what.

Edited March 17 by mistake_not

[Pappa]

Thats great, thanks so much.

 

Couple of questions:

 

1) I can see you've factored a 30mm gap, what are the pros and cons of 30 vs 50? Is lower possible?

 

2) What about Aerogel? Is the cost still 10x more than PIR? could it get the depth to within rafter depth? Is Aerogel still difficult to work with?

 

3) The required U Value... Is that not for the entire roof construction? Could I under insulate at the top of the stairs to meet head height requirements but then over insulate everywhere else; install triple glazed velux windows etc etc.

 

4) Is 9.5mm plasterboard adequate for a good skim finish on top?

 

Thanks again

 

P

[mistake_not]

6 minutes ago, Pappa said:

Thats great, thanks so much.

 

Couple of questions:

 

1) I can see you've factored a 30mm gap, what are the pros and cons of 30 vs 50? Is lower possible?

 

2) What about Aerogel? Is the cost still 10x more than PIR? could it get the depth to within rafter depth? Is Aerogel still difficult to work with?

 

3) The required U Value... Is that not for the entire roof construction? Could I under insulate at the top of the stairs to meet head height requirements but then over insulate everywhere else; install triple glazed velux windows etc etc.

 

4) Is 9.5mm plasterboard adequate for a good skim finish on top?

 

Thanks again

 

P

 

Ref airgap size: No idea. I'm a DIY builder and planning a warm roof. Just know an airgap is required.

 

Aerogel: lambda value ranges from 0.015 to 0.023. So not much different to PIR. 

 

Another option is Vacuum insulation panels (aka VIP). They are a lambda value of 0.007 I think, made to measure and approx £200 a square meter. Would mean you stay between joists. Check out kingspan OPTIM-R.

 

Re Q3. Not sure. Maybe.

 

Re q4, it would be ok on 400centres. Much better than just taped (wet skim coat)

 

[Temp]

21 hours ago, Pappa said:

I'm hearing good things about spray foam insulation.

Closed cell or open?

 

I'm sure spray foam done correctly is very good but Im not sure mortgage companies recognise the good from the bad. 

 

21 hours ago, Pappa said:

Do I still need to maintain a 50mm ventilation gap with spray foam?

 

That will probably depend on the roof membrane. Is it vapour permeable or not.

 

Remember another reason for the gap is to allow the membrane to drape so it's not pushed against the underside of tile battens. That's to allow any water blown under tiles to run down and not pool above the battens increasing the chances of rot.

 

Sometimes the gap can be eliminated if the membrane is vapour permeable and counter battens raise the tile battens off the membrane.

Edited March 17 by Temp

[ETC]

Install a roof light where you need the head room. You’ll get an extra 100mm easily.

[Pappa]

17 hours ago, ETC said:

Install a roof light where you need the head room. You’ll get an extra 100mm easily.

 

Excellent suggestion.

I was looking in to just that and then I realised there is a purlin right there!

Back to the drawing board.

[SteamyTea]

On 17/03/2025 at 12:47, Pappa said:

The required U Value... Is that not for the entire roof construction? Could I under insulate at the top of the stairs to meet head height requirements but then over insulate everywhere else; install triple glazed velux windows etc etc.

I believe you can do this. It is the overall losses that really matter. I am not sure if there is a minimum you cannot go below, I think it is in the building regs.

 

Forget products like vacuum panels, aerogel and multifoils, they will fail, not be as good as stated, or just useless.

Tiny air bubbles trapped in a thermally low conducting material is the way to go.