Insulating roof

[MDE]

My house has slate roof which also covers an (18th/19th C?) section at the back of the house containing 3 rooms. This section has no lofts - i.e the rooms have sloping ceilings. There appears to be no insulation between the rafters which are about 2.5" thick. Last winter an icy blast penetrated the whole of the back of the house, so I am wanting to fix it. The roofer has suggested replacing the existing bitumenised felt (which only covers part of the roof anyway) with TLX-gold (a breathable multifoil) to give some insulation plus draught reduction. He says you can't really insulate between the rafters as they are not deep enough - the TLX gold goes over them and the battens go on top. Any comments or alternative approaches (other than lining the ceilings of all the rooms)?
Thanks for any suggestions.

 

[Zebra]

If you are looking at reroofing then it is better, as far as I understand, to insulate over the rafters than under. If you're not reroofing then you have no choice but to insulate under, which can be done - my rafters are not much deeper than yours and I have done celotex/kingspan type inbetween the rafters (with a gap at the top for ventilation so that any moisture which may gather up there can escape) with another continuous layer underneath, taped and sealed to prevent any moisture from getting there from inside the room.
But if you are taking all the roof tiles off, then it is better, so I understand, to do a "warm roof" construction, where the insulation is over the top of the rafters. Then the rafters are kept warm, and don't need to be ventilated as any moisture which may get there does not condense.
Another forum member was kind enough to give me a drawing of how he insulated his roof in just the way you describe, with a different multifoil - Tri-Iso Super 10 - not that there is anything in the choice of product - above the rafters. I can email you the drawing if you would like to send me a message with your email address. He was reroofing anyway and needed to improve thermal performance whilst not lifting the roofline by more than a few mm.
No expert but hope this helps. Hopefully someone with more knowledge will be along soon!

 

[MDE]

Thanks, Zebra, I wasn't too clear, but yes the idea was to re-roof as I am also having a skylight put in. I'm assuming that the multi foil will also provide draughtproofing. I'll PM you with an email address.

 

[Lime]

...........
The roofer has suggested replacing the existing bitumenised felt (which only covers part of the roof anyway) with TLX-gold (a breathable multifoil) to give some insulation plus draught reduction. He says you can't really insulate between the rafters as they are not deep enough - the TLX gold goes over them and the battens go on top. Any comments or alternative approaches (other than lining the ceilings of all the rooms)?
Thanks for any suggestions.

While external insulation is to be preferred, it must be installed in such a way that external draughts do not enter underneath it.
Otherwise the insulation will serve very little purpose.

 

[davidward89]

If you are looking at reroofing then it is better, as far as I understand, to insulate over the rafters than under. If you're not reroofing then you have no choice but to insulate under, which can be done - my rafters are not much deeper than yours and I have done celotex/kingspan type inbetween the rafters (with a gap at the top for ventilation so that any moisture which may gather up there can escape) with another continuous layer underneath, taped and sealed to prevent any moisture from getting there from inside the room.
But if you are taking all the roof tiles off, then it is better, so I understand, to do a "warm roof" construction, where the insulation is over the top of the rafters. Then the rafters are kept warm, and don't need to be ventilated as any moisture which may get there does not condense.
Another forum member was kind enough to give me a drawing of how he insulated his roof in just the way you describe, with a different multifoil - Tri-Iso Super 10 - not that there is anything in the choice of product - above the rafters. I can email you the drawing if you would like to send me a message with your email address. He was reroofing anyway and needed to improve thermal performance whilst not lifting the roofline by more than a few mm.
No expert but hope this helps. Hopefully someone with more knowledge will be along soon!

Hi, I know this is an old post but still relevant!! How has the celotex/kingspan been for you? I'm in the process of insulating in exactly the same way, 50mm between and 80 underneath in a pitched roof, from ridge to fascia. I'm reading so many horror stories r.e. condensation but as I understand it the key is the ventilation.
I have a breathable roof membrane under my tiles, would you still recommend soffit or fascia vents in to the 25mm air gap above the celotex, below the membrane? Or ridge vents perhaps? How has your ventilation performed for you @Zebra?
I also am unsure when to foil tape. Is it worth taping the first between rafters layer and the second layer? I'm tempted to tape the wood of the rafters completely, overlapping on to the celotex between the rafters .
What do you think? Any advice or experience from you would be greatly appreciated. I'm wanting to get this right before ploughing on blindly!!!
Thanks

 

[Zebra]

It's been fine. The rafters seem fine. I have open eaves with comb type thingies to keep bugs out front and back, so lots of ventilation at the eaves but none at the ridge and it's fine.
However, unlike what you're describing this arrangement does not go all the way to the ridge. It goes about halfway up the rafters. That is where the ceiling is, halfway up the rafters. So I have a loft which is an open space for the air to circulate and cross from one set of eaves to the other. I think that's key that the air can come in one side and go out the other. If you take this arrangement all the way to the ridge it would be limiting that flow perhaps.
You don't need to tape the first layer in between the rafters nor onto the rafters themselves. The only place that needs taping is the final underneath layer. This is to form an air tightness barrier.

 

[thebadger7]

What's it like in summer? I have a similar scenario and looking at doing over rafter insulation with wood fibre sarking boards to reduce summer heat gains

 

[Zebra]

My house is not typical in terms of summer temperatures. It's low and deep with windows only on one side so gets almost no solar gain and never overheats.

 

[alexander]

Hi, I know this is an old post but still relevant!! How has the celotex/kingspan been for you? I'm in the process of insulating in exactly the same way, 50mm between and 80 underneath in a pitched roof, from ridge to fascia. I'm reading so many horror stories r.e. condensation but as I understand it the key is the ventilation.
I have a breathable roof membrane under my tiles, would you still recommend soffit or fascia vents in to the 25mm air gap above the celotex, below the membrane? Or ridge vents perhaps? How has your ventilation performed for you @Zebra?
I also am unsure when to foil tape. Is it worth taping the first between rafters layer and the second layer? I'm tempted to tape the wood of the rafters completely, overlapping on to the celotex between the rafters .
What do you think? Any advice or experience from you would be greatly appreciated. I'm wanting to get this right before ploughing on blindly!!!
Thanks

you still need ventilation even when using breathable membrane, breathable membrane only allows air flow out the loft it does not allow air flow in so it needs vented, thats why on new builds now they must cut the membrane at the ridge once the dry ridge is fitted to allow condensation to escape, bit pointless putting holes in your secondary protection but that's the building trade in general one disaster to an other