Ventilation and Scottish slated roof

[Blob the Builder]

I'm trying to decide on my roof make up. If i go with tradition and have sarking boards on my rafters, covered with a membrane and slates nailed directly through, then what ventilation should i provide?

My rafters are 245mm depth (i have 1 and 1/2 storey room in roof) so i have scope with insulation.

My original specification involved fibre cement slates, battens and counterbattens, ventilated above membrane, on OSB. Below OSB a 50mm vent gap, then 190 mm of PIR between rafters, 25 mm PIR internally across rafters, then plasterboard.

But now i'm thinking of natural slate.

Do i need ventilation gap beneath sarking boards? I've read some details which say i can full fill between rafters, which could open the way for Frametherm roll or equivalent, and internal PIR across rafters, possibly 50mm.

And do i need ventilation above fascia and in ridge?

[the_r_sole]

.

Edited September 26, 2019 by the_r_sole

[Blob the Builder]

My roof pitch is 37.5 degrees

[the_r_sole]

.

Edited September 26, 2019 by the_r_sole

[Thedreamer]

1 hour ago, Blob the Builder said:

I'm trying to decide on my roof make up. If i go with tradition and have sarking boards on my rafters, covered with a membrane and slates nailed directly through, then what ventilation should i provide?

My rafters are 245mm depth (i have 1 and 1/2 storey room in roof) so i have scope with insulation.

My original specification involved fibre cement slates, battens and counterbattens, ventilated above membrane, on OSB. Below OSB a 50mm vent gap, then 190 mm of PIR between rafters, 25 mm PIR internally across rafters, then plasterboard.

But now i'm thinking of natural slate.

Do i need ventilation gap beneath sarking boards? I've read some details which say i can full fill between rafters, which could open the way for Frametherm roll or equivalent, and internal PIR across rafters, possibly 50mm.

And do i need ventilation above fascia and in ridge?

 

My plans just show 2 x 80mm kingspan after the sarking board.

 

My blog has couple of entries for our roofing which might be of interest.

 

We are having a small flat ceiling just below the ridge beam which will be a cold space, but other have a warm space throughout.

 

[AliG]

I am in Scotland and have full fill with 200mm of PIR between the rafters, then insulated plasterboard below.

 

Instead of sarking boards we have sarking planks with a few mm between then a breathable membrane on top. The ventilation is between the membrane and the tiles(clay), we do not have any ventilation at the ridge or eaves into the roof space. I don't see whether clay or slate would affect ventilation. We have airtightness membrane below the PIR then insulated plasterboard below this. I think one of the key things is to have air tightness below the roof so that moisture does not get up into the roof, thus ventilation is less necessary.

 

It did seem that it was not clear if you could use Frametherm and still have no ventilation gap above it. I tried hard to find out if you could and I think it is probably OK, but it seems there is little information available. As said with Frametherm you would need maybe 50-100mm of PIR below the rafters to get acceptable insulation levels, I didn't look into this more as it would have reduced ceiling heights in some areas. The other issue was we just could not get Frametherm 32 or similar a couple of years ago, but it seems a lot more widely available now.

 

As @Thedreamer did, where we were not using the roof space and had a flat ceiling we changed the 200mm of PIR to 400mm of Frametherm as it was much much cheaper.

Edited October 24, 2018 by AliG

[A_L]

18 minutes ago, AliG said:

I think one of the key things is to have air tightness below the roof so that moisture does not get up into the roof, thus ventilation is less necessary.

 

@AliG , air tightness moisture/vapour resistance are two separate phenomena. It is perfectly possible to have an airthight membrane which has a very low vapour resistance and does not significantly inhibit water vapour movement through it. The process is called diffusion and is different from bulk air movement.

[AliG]

True, I was mixing the various properties. I don't actually know if the air tightness membrane is moisture permeable.

 

The insulated plasterboard underneath which is then plaster skimmed is considered adequate as a vapour control layer.

[Blob the Builder]

22 hours ago, the_r_sole said:

off the top of my head it's below 35 degrees you don't need to ventilate at ridge level but always at eaves

I think tyvek supro membrane has a BBA cert that says you don't need eaves or ridge ventilation (or anything above insulation) with slate roofs

Yes i read the Tyvek technical sheet that says that.....is that because the irregularity in slates allow air to get underneath them?   and because sarking boards, being a couple of mm apart, allow air through to the top of the insulation?

[Blob the Builder]

22 hours ago, Thedreamer said:

 

My plans just show 2 x 80mm kingspan after the sarking board.

 

My blog has couple of entries for our roofing which might be of interest.

 

We are having a small flat ceiling just below the ridge beam which will be a cold space, but other have a warm space throughout.

 

I'll take a look at your blog. I have a triangle of cold space below the ridge also, 1600mm height x 3444mm wide, where i plan to have my mvhr ducting running, covered in plenty of fibre insulation.

[Blob the Builder]

20 hours ago, AliG said:

I am in Scotland and have full fill with 200mm of PIR between the rafters, then insulated plasterboard below.

 

Instead of sarking boards we have sarking planks with a few mm between then a breathable membrane on top. The ventilation is between the membrane and the tiles(clay), we do not have any ventilation at the ridge or eaves into the roof space. I don't see whether clay or slate would affect ventilation. We have airtightness membrane below the PIR then insulated plasterboard below this. I think one of the key things is to have air tightness below the roof so that moisture does not get up into the roof, thus ventilation is less necessary.

 

It did seem that it was not clear if you could use Frametherm and still have no ventilation gap above it. I tried hard to find out if you could and I think it is probably OK, but it seems there is little information available. As said with Frametherm you would need maybe 50-100mm of PIR below the rafters to get acceptable insulation levels, I didn't look into this more as it would have reduced ceiling heights in some areas. The other issue was we just could not get Frametherm 32 or similar a couple of years ago, but it seems a lot more widely available now.

 

As @Thedreamer did, where we were not using the roof space and had a flat ceiling we changed the 200mm of PIR to 400mm of Frametherm as it was much much cheaper.

That's interesting AliG, similar proportions to myself. Looking at thermal resistance, it looks like 230mm of frametherm 32 between rafters, and 50mm PIR on warm side of VCL would give same resistance as 190 mm PIR between rafters and 25mm PIR internally. But i may sitck to PIR betwixt the rafters (was just trying to cut down on cutting PIR)

And, i'll also run 400 mm frametherm across the flat ceiling , covering my MVHR ducting. 

[Thedreamer]

47 minutes ago, Blob the Builder said:

I'll take a look at your blog. I have a triangle of cold space below the ridge also, 1600mm height x 3444mm wide, where i plan to have my mvhr ducting running, covered in plenty of fibre insulation.

 

Above our triangle is framtherm followed by 25mm Kingspan taped and seam.

[AliG]

We got caught out that the Frametherm didn't cover the MVHR ducting which we then had to go up and insulate. This wasted a lot of the saving on the PIR.

 

If you are doing PIR between the rafters, 200mm is probably cheaper than 190mm as 90mm is an unusual size. You can also try to get 200mm Celotex which saves on the labout, although some people prefer the staggered joints of 2 layers. I was worried about it fitting badly, but it was cut with a band saw and fits very neatly.

[Blob the Builder]

24 minutes ago, AliG said:

We got caught out that the Frametherm didn't cover the MVHR ducting which we then had to go up and insulate. This wasted a lot of the saving on the PIR.

 

If you are doing PIR between the rafters, 200mm is probably cheaper than 190mm as 90mm is an unusual size. You can also try to get 200mm Celotex which saves on the labout, although some people prefer the staggered joints of 2 layers. I was worried about it fitting badly, but it was cut with a band saw and fits very neatly.

good points...