Insulation sloping roof

[joatex]

The farmhouse (not listed) has had a large extension added more than a hundred years ago. The tiled roofing has a considerable slope and the inner surface lined with lath and lime mortar plaster. I wish to provide insulation for this large area. There is no access apart from lifting and relaying the tiles or breaking through the fragile lath/plaster. In the latter case it would be difficult to make sure that ventilation was adequate even if insulating boards could be inserted without damage due to the irregular surface of the laths. The state of the rafters is unknown without lifting tiles but the roof shows no sagging.

The previous owner of my own house (circa 1970) has lined the inner surfaces of the outer walls with polystyrene sheeting apprximately 3/8th thick, then papered those with artex. Prone to damage but effective. Is polystyrene sheeting/boards a 'no-no' these days? If not, such sheets (or similar) could be stuck to the plastered ceiling of the extension then covered with lining paper and a breathable paint applied.

 

[biffvernon]

3/8 inch of polystyrene will not add much insulation. Whatever you use it will have to be a much thicker layer to be worthwhile. Is it important to insulate the roof rather than the loft floor or is the loft a heated space so needs to be insulated above?

 

[Nemesis]

I can't see the point of a breathable paint on top of polystyrene.

 

[Lime]

I presume the plaster and lath is mounted on the underside of the rafters.
If this is so there is little you can do except to remove the tiles and fit the insulation directly under them.
Removing the tiles would give you an opportunity to clean, inspect and repair within the hidden void.
It depends on how large this void is (dependent on the thickness of the rafters) as to what amount of insulation you can use.
I would use an insulation that allows moisture to pass through it, that is resistant to wet/damp and will not rot under any conditions.
Leave a gap between the tiles and insulation for ventilation.
It also needs to stay where you put it.

The polystyrene sheet is not permeable so would hold any dampness within the plaster against the rafters and laths. How damp it is depends on the condition of the roof and it's exposure.
3/8 inch polystyrene is very effective if condensation is a big problem although it does not give much insulation.

 

[Mark]

Is polystyrene allowed to be used like this ?

I am thinking of fumes in the event of a fire ?

 

[joatex]

Yes Biff, it's a large area slates above then rafters followed by the lath/plaster ceiling. lifting the slates looks a major job yet getting into the cavity which cannot be more than 4" deep would disturb the lath/plaster. I'll email the pic to you, haven't got the hang of the photobucket system

 

[biffvernon]

Then really the only way to do it without wrecking the existing roof is to put the insulation below the lath & plaster and then another ceiling. This obviously reduces the headroom so may be impossible. How much is the room used? If it's only occasionally I'd go for improving the insultion under the floor and leave the loft room cold.

 

[Geoff Flegg]

Exactly the same problem that I've had with my house, which has attic rooms - the lath & plaster ceiling being directly attached to the rafters. In my case I needed to replace all the roof covering anyway, so insulating between the rafters was not a problem whilst the tiles were off - but this only allows 2" of insulation, I've gone for the rigid foam type to get best performance for the thickness. Big claims are made for the multi-foil type insulation, which is very thin, but there is a lot of doubt over the true performance of this type - Biff has posted up information about this before if you have a search.

For a total roof strip and renewal you now have to get building regs approval, which means means you have to insulate to a high level - 0.2U I think - which in my case just wasn't possible by only insulating between the rafters, so I'm going to have to underclad the existing ceilings with more insulation, and then plasterboard over that, which I could have done without !

However, as yours is just an extension, you can probably get away without building regs approval - I think it's only if you strip more than 25% of the total house roof, check with your local authority if you're worried. In my case I can almost guarentee that they would have spotted the work if I hadn't applied for approval, so had no choice!

As for ventilation - my old tiles had no felt underneath them at all, perfectly normal for a roof this old, my new tiles have a breathable membrane underneath them (Deltex Roofshield) - no one seems to use felt any more, which was impermeable and can apparently cause condensation problems. Building inspector has insisted on vent tiles being put in various locations, and there is a 1" gap above the insulation - principally there to make sure that the membrane drapes into that gap and therefore that no water can get trapped behind the roofing battens. You can instead completely fill the rafter depth and use counterbattens under the main roofing battens - I couldn't do that as my house is a semi and it would make my roof 1" higher than my neighbours where they join.

When the ceilings are underclad with more insulation that will incorporate a vapour barrier - the foil face on the insulation, plus you tape the joints, prior to covering with plasterboard.

Sympathetic to an old building - no, not really, but I've got no choice but to comply with the regs. It will be hidden, and the old ceilings are being left in place, covered up by the insulation.

In your case I'd agree with Biff and just underclad and put a new ceiling in, if headroom is OK, you defintely want to insulate that room and don't want the hassle of taking the tiles off. I'd go for rigid foam insulation rather than polystyrene though - have a web search for Celotex or Kingspan, there's always quite a few sellers on Ebay if you're into that. I think you can also get plasterboard with insulation already attached to it, ask a local builders merchant.

As far as ventilation goes, the idea is that the vapour barrier (foil either on the front face of the insulation or the back of the plasterboard) stops moisture entering from within the house. If the roof is over 100 years old and has never been stripped then the chances are it has no felt underlay currently, so ventilation from the outside into the roofspace shouldn't be an issue.

 

[joatex]

Thanks Greg for the comprehensive and helpful reply. The roof area is large and as it is inclined to the north quite frigid in the winter. The added insulation under the ceiling is the most attractive solution but we shall need to check the rafters before adding more weight. I'll try for a pic of the roof and the ceiling to show the situation more clearly. Cheers Ross

 

[Geoff Flegg]

No problem Russ ! :wink:

Rigid foam insulation weighs very little by the way - in fact I wouldn't be surprised if the plasterboard weighs more per square yard.

Geoff

 

[Roger H]

You might want to have a look at insulation bonded to plasterboard such as gyproc thermaline. I am putting 55mm of that under my rafters with a multifoil (don't mention it..) above them. Not exactly cheap at circa £30 a board though.

Best of luck.

 

[biffvernon]

with a multifoil (don't mention it..) above them.

But you did mention it. The saga continues here.

 

[joatex]

Greg, the pics show the area involved, some 30sq.m. The ceiling is sagging slightly but intact. Forty years ago slates had been covered with tarred netting, interestingly enough the netting/tar is coming away cleanly.

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