Victorian three storey terraced house with slate roof & top floor partially directly under the roof rafters. Need to get the roof renewed, so new batterns breathable membrane & spannnish slates will be going on in the near future.

Is there anyway of effectively insulating between the rafters whilst the roof is off without removing the ceiling? (lathe & plaster ceiling so really don’t want to do that). I am guessing that rafters are probably only 3″, so with 50mm airgap would only leave 25mm for insulation & I am assuming that we can’t install a “warm” roof as it would affect interface with level of neighbour’s roof?

Only thing I can think of is just lining with insulation board under the ceiling, as there is plenty of height & space in the rooms.

Could you “dry line” inside with Kingspan? It made a massive difference to our son’t attic bedroom, quite dramatic. We did the walls as well.

“dry line” inside with Kingspan

I think this is the conclusion that I am coming to, found a reference to in some historical buildings stuff, what sort of thickness would it need to be to be effective?

There’s a few ways to go about it and some of it may depend on if you want/need to comply with B Regs.

If you’re really keen on keeping the lath & plaster, you could install insulation between rafters to full depth, install your breather membrane on top, fix 38 x 38mm counter battens to the rafter and then install your battens and tiles/slates on top. However 75mm deep insulation wont get you anywhere close to the min requirements.

To increase insulation, remove lath & plaster (or potentially clad over it) with insulated plasterboard- thickness as much as you can really. Have a look at Kingspan K7 and K18 boards. You can download PDF literature from their website.

You don’t have to use counter battens but if you don’t you’ll have to leave a gap which reduces your insulation thickness

As you’ve got plenty of room you’re better off increasing the depth of the rafters by screwing some new timber to the bottoms, insulating with celotex between those and then a 40mm celotex board over the lot, finally plasterboard and skim over that.
Tbh, If I was going to all that hassle, I’d pull the ceilings down anyway, and be putting 100mm celotex with a 25mm air gap between the rafters. (you’re replacing with breathable felt anyhow) and then the 40mm celotex and plasterboard over that. Don’t forget the silver tape over all your celotex joints. This should help.

Insulating a Pitched Roof – When Does it Need Ventilating?

If you’re really keen on keeping the lath & plaster, you could install insulation between rafters to full depth, install your breather membrane on top, fix 38 x 38mm counter battens to the rafter and then install your battens and tiles/slates on top.

Not keen on keeping lathe & plaster ceiling, just don’t want the filthy job of removing it whilst living in the house. Any extra height in roof make up won’t work as house is mid terrace and will run into problems meshing with neighbours slates.

Pocketrocket – what you suggest sounds complicated – is there anything stopping me fixing something like Kingspan K18 boards directly to underside of existing ceiling – so long as fixings go through existing ceiling & into old roof rafters/ceiling joists or do I have to leave a gap / counter battern below ceiling first?

Pocketrocket – what you suggest sounds complicated – is there anything stopping me fixing something like Kingspan K18 boards directly to underside of existing ceiling – so long as fixings go through existing ceiling & into old roof rafters/ceiling joists or do I have to leave a gap / counter battern below ceiling first?

I did this.
Whip off skirting. Sort out window reveals, door frames etc that extra depth will impact. (a real faff…)
Move / replace electrical cables/fittings to allow for extra depth (I mounted sockets on wood blocks to right depth).
Mark on where each rafter or stud is.
Attach insulation (I did it with plasterboard already on) using Fischer fixings.
Skim.
Paint.
Wop back on electrical items.

Pocketrocket – what you suggest sounds complicated – is there anything stopping me fixing something like Kingspan K18 boards directly to underside of existing ceiling – so long as fixings go through existing ceiling & into old roof rafters/ceiling joists or do I have to leave a gap / counter battern below ceiling first?

Weight.

As an old Victorian build it’s likely to be overbuilt and it won’t be a problem, but if I was going to add a fair bit more weight, I’d want to have a good look at the structural timbers first. Replacing the topside of the roof should let you get a good look though.

As regards battening or fixing tight to the old surface, I’d say fit it up tight. The air space in the battened cavity isn’t going to be ventilated to anywhere unless I’m mistaken.

Thanks matt outandabout, not planning on doing walls so should be a bit easier for us.

Midlifecrashes

weight

ah yes will have to look into that, these particular houses weren’t built to last apparently & I wouldn’t like to overload anything especially as neighbouring house already removed other half of the chimney breast

If you’re stripping the roof I can recommend this :
http://www.tlxgold.co.uk
It’s a breathable felt & multi-foil insulation in one. Creates a warm roof & you can improve u values by insulating between rafters from the underside or above using a PIR or even just rockwool. Shouldn’t create any issues with levels where you abut the neighbours roof as the battens will pinch the multi foil ( I have been reliably informed).
I have seen quite a few installations of this over the years (I’m a building control surveyor) & I have been impressed with it.
Their tech helpline is good.

Its really not complicated at all, but to get what I would consider a worthwhile amount of insulation I’d say take the ceilings down, after all when is anything worth doing easy!!? 😀
Anyhow assuming your rafters are 3″. Once you’ve removed the lath and plaster screw a 2″ x 2″ to the bottom of the rafter. Be careful about nailing it unless your using a nail gun, as the hammering is very likely to dislodge tiles!

Your rafter is now 5″ deep, cut the 100mm celotex between the rafters and push it in until flush with the 2×2 and tape with silver tape, this will leave you a 25mm or thereabouts air gap to your breathable felt.
On top of this screw 40mm celotex all over to the 2×2 and tape all joints.
Finally screw plasterboard over this through the 40mm celotex into your 2×2,
skim and jobs a good un!
Just be aware that this is going to lower your ceiling line by around 100mm so check it wont cause problems around windows doors etc, of course you could use 75mm celotex and a 25mm batten instead to save an inch.

I wouldn’t be worrying too much about the weight, you’ve already said its only part of the slope so unless your doing really big areas you wont be adding that much weight, plus its spread out over an area so not exactly point loading anything.
That’s the way we’d do it at work, and I work for a reputable builder.

Finally, yes if you want the easy way out simply screw an insulated plasterboard to the rafters, ( we’ve done that in the past) though you may find its cheaper to buy some celotex and normal plasterboards. Obviously you wont get anywhere near the same U values, though it really comes down to how much of an area it is.

Thanks pedropete will ask our roofer if he has had any experience of using it.

Cheers for the in depth replies pocketrocket – will take a proper look at it in a few days time, its at my gf’s house & I’ve only been up the second floor on a handful of occasions.