Insulating an attic roof

[Ian Bond]

Following on from another thread, and looking for advice and experiences from PPUKers...

I need to insulate my attic roof. The attic was always habitable (not by todays standards!) with a Queen Post Truss structure, suspended floor and lath and plaster walls to the eaves. The ceiling slope is underdrawn with a lath and plaster ceiling, fixed to the underside of the rafters between the eave walls and apex.

The rafters are just 3 1/2 ins deep, so there is not much space between the ceiling and the stone roof tiles.

If I try to insulate between the rafters (when the roof tiles are re-layed) there's not much depth if I allow 2ins (or 50mm) for ventillation under the tiles.

I can insulate the bedroom ceilings no problem, and the back of the attic lath & plaster walls too. Just the ceiling slope is not so easy.

Is the only way to achieve this to lose my lath and plaster ceiling under insulated board? It would also mean losing some of the visible depth of the principle rafters and purlins, which is a shame, and slight loss of headroom.

Please let me know if you've solved a similar problem, and what you've used. Also, have you managed to keep the character of the room and roof structure?

Ta very much,

Ian

 

[chuckey]

If the tiles are coming of anyway then a breatable,reflective membrane should be used. This will have the immediate benefit of reducing solar gain -only applicable when the suns shines on it in summer. As you have this very tight height limit, ordinary breathable insulation (glass fibre etc.) would not work very well, indeed I doubt that you can buy it in 2" thickness, but if you can, use it. If you use a non breathable foam type slab insulation, while you will get about twice the insulation effect, you will risk condensation on the underneath of it, due to the lathe and plaster ceiling letting through water vapour.
If the room is costing too much to heat, then later on the lathe and plaster ceiling will have to go. The 2" of glass fibre can stay any way. Then fill in the gaps between the joists with foam insulation, install a plastic vapour membrane and plasterboard the ceiling. No real reason to loose any appreciable head room.
Frank

 

[Ian Bond]

Thanks Frank.

The rafters and purlins project about 5 inches below the ceiling so there's room to board under, but I'd want to leave as much timber as possible exposed. Shame about the lath & plaster ceiling, but I can't see an alternative that doesn't risk condensation.

I'll have a look at the insulation boards available, but hope to find something eco friendlier than urethane, if possible.

Cheers

Ian

 

[Nemesis]

And something rather more conservation oriented?

 

[Geoff Flegg]

Ian, I had the same problem with my Victorian house. There are several options:-

I presume that as your tiles are coming off you are intending on putting a breathable membrane on before the tiles go back - what would at one time have been roofing felt, now superseded. I used Proctor Roofshield, and like you I wanted to get the maximum amount of insulation in between the rafters.

On speaking to Proctor's technical department, there are 2 ways of doing this. In both the important thing is to ensure that water cannot "pond" behind the roofing battens - which can cause them to rot - so you can either:-

1 - Fill the entire rafter depth with a solid foam insulation (Kingspan, Celotex or similar), put the membrane directly over the top, then put counter battens along the top of the rafters before nailing your roofing battens across in the normal fashion - so creating a gap between the bottom of the roofing battens and the top of the membrane for any water to run down. This will make your roof a little higher than it is currently - fine if you're detached, no good for me as my house is a semi, so it would have made my roof higher than my neighbours.

2 - Fill the rafters with insulation to no more than 15mm from the top of the rafters, then allow the membrane to sag into the gap as you lay it - don't pull it taught - then put roofing battens on as normal. The sag in the membrane allows any water to run under the battens. This is the method I used, roof is the same height as it was originally but you get 15mm less insulation than with method 1.

You need building control approval for major re-roofing jobs now, in my case I should have had to underclad the ceilings with insulation as well to get the required U-value - building control actually signed it off as it was, an error on their part. If your building is listed you'll need the relevant permission for that as well, which may affect if you can remove the ceilings anyway.

One option is the multi-layer foil insulations, which claim to offer high performance for very little thickness, but there is lots of controversy as to how well they actually work in reality - just search for foil insulation on here for lots of debate........

 

[Ian Bond]

Thanks Geoff,

Option 1 is out for me for the same reason you had (almost, mine's mid terrace)

Option 2 appealed and I intended to do that until today, using thermafleece and a membrane draped to allow water run-off.

Another thread "an alternative to thermafleece" has put me off - have been advised that the small air gap would be insufficient, and risk condensation and rot of the roof timbers.

Hence the quest for an eco-friendly, period building friendly, probably not bank account friendly solution.

I've not given up on your option 2 yet, as to me it appeals the most. The underboarding solution would mean hiding the lath and plaster ceiling and losing depth of the lovely roof timbers. But they're aesthetic issues, and the environmental consideration will force some change, if not by me by future occupants. And perhaps Building Control, seeing as I live on the same road as the Town Hall where they all work

Frank's suggestion (with a little modification) is the only realistic alternative. But I would want to use the most eco friendly insulation possible, and maybe cover with something than can have a lime plaster finish.

Thanks for your input so far, it is all useful and going to be considered carefully.

Don't worry - I won't remove any historic fabric.

 

[worms]

Where does all this leave those of us with no felt, proper timber sarking and slates nailed to the sarking without cross-battens? ie the traditional set-up in a Scottish stone and slate building? This set-up seems to be good for a little over 100 years before the nails start to rust out and the wood deteriorates. If condensation on the underside of the slates was a problem, I would imagine the longevity would be considerably less.

So what is the difference between sarking touching the slates and insulation material touching the slates as in the op's situation? Does the sarking act as a "breathable" layer by absorbing the moisture and releasing it again either internally or once the RH drops? What is the insulating value of a half inch of pine? And if I line the sarking internally with insulation, do I still need a ventilation gap?

 

[Ian Bond]

Good question. I've had another look at Second Nature's website, and their BBA certificate is available as a PDF.

They refer to Thermafleece sarking, and show the wool batts between the rafters without by the looks of it a 50mm ventilation gap. They do however recommend a vapour control barrier on the warm side of the insulation and breather membrane over the rafters.

 

[chuckey]

Worms, its all down to the quality of your sarking timber. I have removed an old purlin from a pig sty, it has actually eroded sections due to water running down it and yet in general its a good piece of wood. If it was a piece of wood from a big shed it would have rotted away 100 years ago. I suspect its douglas fir?
frank