Roof Insulation type.

[Quam256]

I have a roof of tiles(not slate) but no felt underneath.
I would like to be able to insulate between the rafters but dont know what type to go for as from what I have read the roof needs ventilation to stop any moisture getting trapped.The floor is already boarded and would like to be able to use the loft space.

[heero_yuy]

I think you need to use the rigid foam type, "Kingspan" is one trade name and has a silvered side to face inwards. To ensure correct ventillation of the underside of the slates select a thickness that allows for air flow. I think that you also need to fit ridge tile and (possibly) soffit vents to allow the air to circulate.

[Rob]

Rigid Foam is the best type, common brand names include Kingspan, Celotex.

You need to be careful how you achieve this so you don't run the risk of timber decay. If the underside of your tiles has roofing felt you need a gap, at least 25mm between the felt and the insulation and that gap should be ventilated. Otherwise condensation will be a problem. Soem roofs I saw this winter that were unventilated but well insulated were litterally dripping with moisture from condensation.

In practical terms how you ventilate every space between the rafters is a challenge. Normally there should be a grille or gap at the eaves, or perhaps holes drilled into the soffits corresponding to the rafter spacing. That allows air to enter at the base, but you also need air to be able to exit high up at the ridge. Unfortunately that's the hard part and something I rarely see.

Of course if you don't have underfelt, then there is no problem as gaps in tiles will allow air to circulate. But you should still leave a gap between the insulation and tiles.

The manufacturer's will be able to tell you the total thicknes of insulation you need to get to modern standards. It can be quite a lot. You might need to do this as two layers. Some between rafters and some laid to the undersides.

You should fit a vapour check to the underside (roofspace side) of the installed insulation. This is usually a polythene sheet. That helps to reduce warm moist air moving to the colder space outside of insulation and agains reduces the risks of condensation. With the vapour check in place you can then fix your plasterboard or other linings. Some people will use foil backed plasterboard arguing the foil is a sufficient vapour check, but what about the board joints? OK so any vapour check is broken by nail fixings for the boards, hene check rather than barrier.

Make sure that your rafters are sturdy enough to take the additional wieght of the insulation and boarding. That does add up and most rafters will only have been sufficient for the roof covering. You might need additional prulins, struts and even collars to the roof structure and that might mess up your plans for a useable roof space.

You may have boarded out the loft as a floor, but was this just over the ceiling joists or was a proper new floor put in? Ceiling joists are unlikely to have sufficient strength for anything other than occasional maintenance and light storage use.

[Quam256]

thanks for the replies,The loft is just used to store the odd thing ,I was looking at adding insulation under the tiles just as a extra layer of insulation to the house as it is a very large loft.

[DaiNasty]

@Rob - Sincere thanks for that amazing level of detail. This is something I've also been considering as I've got a vast amount of space up there.

This will make it a lot easier to discuss with my potential builder and work out if *he* knows what he's doing..