air gap needed between roof sark boards and insulation?

[Lulunoclue]

Hi
We want to make our loft a usable room and at the moment there is no insulation (or ventilation) in the roof space at all. We are however, getting a couple of Velux windows fitted this week.

I understand we should have 100mm (fibreglass wool) insulation between rafters then 170 horizontally across that, but I am really confused about the 25 or 50mm air gap I've read about!!

We're in Scotland so there's no roof felt - the tiles are just attached onto butted sark (?) boards held up by rafters.

The 1920's/30's bungalow is quite prone to damp so I don't want to cause any additional problems. How on earth do you create this gap? Loft is very big so I don't fancy adding multiple battens to every single rafter cavity! Surely a batten would block airflow anyway?? I also don't want vents in every single cavity either - there would be hundreds and it would look awful outside.

I've also considered using only the YBS super quilt over the rafters but I don't know if this would be warm enough. Does anyone know the equivalent depth in comparison to traditional style?

Any advice would be much appreciated!

 

[Alastairreid]

Are you planning on making this usable room B/R compliant?

Are you installing a fixed staircase?

Anyhoo your insulation would be of the rigid type with 100mm fitted between the rafters and a 50mm airgap between the insulation and sarkin/tiles. You increase the depth of the rafters by running battens down the length of them.
You would also need to fit rigid insulation to the underside of the rafters approx 30mm to prevent cold bridging.
Then a vcl, followed by the plasterboard.

Ventilation across the eaves and ridge would be ideal.

 

[Lulunoclue]

Thanks for that. So you recommend fixing battens 50mm deep vertically down insides of each rafter then slim rigid insulation ( is that like kingspan?) between them, topped with fibreglass roll over the outside?? Just want to make sure I've understood!!

The cost of Kingspan & time involved in cutting it plus the job of fixing hundreds of battens is a little terrifying TBH! Shoving a bit of mineral wool (pre perferated to correct size) between rafters seems so easy in comparison!

Food for thought! Thanks again.

 

[Lulunoclue]

What is the consensus of opinion on spray foam insulation?? It's supposed to be impervious to moisture and can apparently go straight onto sarking boards (without dreaded air gap)
Has anyone used it and was it a success?

 

[Alastairreid]

You fix the battens to the underside of the rafters to increase their depth!

150mm is what you need.

Spray foam is a waste of money and damages your roof, its designed to stick to the underside of the slates/tiles but you have sarkin boards.

Kingspan/celotex or similar rigid boards has much better u values than glasswool insulation.

 

[Lulunoclue]

You fix the battens to the underside of the rafters to increase their depth!

Then how do you get air gap? Sorry we are complete novices. Or are you saying Kingspan is breathable/damp proof and can touch boards?

Also, the stuff I've seen is thin, will it have same insulation properties as the 270mm I wanted or would I still need to add more before plasterboard?

Kingspan seems to work out at least double the price of loft roll too.