Insulating attic bedroom in a Victorian cottage

[Andy Davies]

Our roof has no insulation making our attic bedroom freezing in winter and boiling in summer (there's also no felt under the front pitch, but I'm aware this isn't necessarily a problem). I'd like to insulate it but feel I need to know more about our options before getting a builder or roofer involved.

I can see how we can add a decent layer of insulation in the horizontal areas but for the larger sloping sections, I'm struggling to understand what's possible (there's a rough sketch of the cross-section of the house below).

As I understand it our options are to remove the existing lathe and plaster ceiling and replace it with an insulation backed plasterboard, or strip the roof off and insulate between the rafters - although as they are only 80mm deep, there's not much room for insulation once an air gap of 50mm is allowed.

Has anyone any thoughts or recommendations?

Thanks

Andy

 

[Penners]

Your problem is very similar to this recent thread on this forum, except that you've got rather more angled rafter ceiling area to cope with.

As you say, if you want to get an effective depth of conventional insulation, plus an air gap, between your rafters (and assuming you don't want to use a multi-foil insulation - see the above thread for more on this) then you really only have two choices - remove the ceiling and pack out the bottom faces of the rafters or remove the roof and pack out the top faces.

Best conservation practice suggests that you should strip the roof and work from above, because the orignal fabric of the roof is replaceable (less any tile breakages). Doing this would also give you the opportunity to incorporate a breathable damp-proof membrane to improve both the ventilation and the water-resistance of the roof (which I assume has no membrane or felt at present). However, this is the more expensive option. If you were prepared to remove the lath-and-plaster ceiling and packout the rafters from below, then replacing with new laths and lime plaster would probably still be cheaper than stripping and re-roofing. Particularly if you are able to do a DIY job.

Is the house listed? If so, you'll need to talk to your CO about getting consent for this work.

 

[Andrew Bodenham]

Penners advice to strip the external roof covering, insulate and replace is very sound.

If you can stomach losing the original interior then an alternative would be to strip the lath & plaster, then fit say 30mm PU slab insulation (e.g. kingspan ) between the rafters, then insulate across the rafters with a further 50mm of the same, before boarding over with standard plasterboard and skimming. This approach lends itself to DIY, though you would probably want someone to do the plastering for you. Not a very 'period house' approach, though you could do a similar thing with more trad / wholemeal ingredients.

The external approach has the great advantage that you can continue to use the attic space, though in my expericence of roofers I'd expect and fair bit of 'breakthrough' damage. If your roof has other problems or is reaching the end of it's life then tackling the job this way could kill two birds with one stone. External appearance would be altered - at the end of the day it comes down to cost and where your conservation priorities lie.

 

[Andy Davies]

Thanks for your comments...

I'd read that post and a few others before I posted plus building regs, stuff from the Energy Saving Trust, Kingspan, Celotex, TLX etc. (probably too much!)

I'm quite happy to go for the option of stripping the roof and insulating from above but given the rafters are only 80mm deep I'm struggling to see how we get anything like a decent amount of insulation in.

Everything I've read seems to recommend a ventilated roof for loft conversions (presumably for condensation reasons) so I presume the same recommendation applies for existing attic rooms, once the 50mm air gap is allowed for then that only leaves 30mm for insulation.

I've also had a read around the multi-foil literature, ignoring the debate on it's effectiveness for a moment, the Triso literature recommends a counter batten over the top to achieve and air gap but as we're in a terrace of several cottages we can't raise the roof height.

Fortunately our house isn't listed, so just got the BCO to deal with and I have asked them what they would find acceptable but not had an answer yet.

Thanks

Andy

 

[citizensmith]

We had our roof replaced last year - with 3 attic bedrooms just like you have shown. It was indeed freezing in winter and boiling in summer. We went for breathable membrane and multifoil over the entire roof and put 2ft of fibre wool on the flat ceilings. result - amazing transformation of the whole feel of the house and massively reduced fuel (oil) consumption. If you need to re roof then its worth doing well as you wont do it again in your lifetime (hopefully).

 

[DaveBrigg]

put 2ft of fibre wool on the flat ceilings.

Just checking - did you really put this much on?? (from someone who had the roof replaced two years ago and knew nothing about multifoil at the time - doh )

 

[citizensmith]

Yes - but the ceiling areas are fairly limited, plus we already had the fibre wool and i wanted to use it up. i think it was actually a bit less than 2ft as it was 170mm cross layed three layers thick ( so about 1ft 8inches?). We couldn't use the loft areas for storage as the cieling joists were too thin to take the kind of weight we would put in a loft. It may seem a bit "belt and braces" but I am pleased with the results and the change in the "feel" of the upper parts of the house.

 

[Penners]

as we're in a terrace of several cottages we can't raise the roof height.

Now, you see, if you'd included that little nugget of information in your original post, you could have saved a lot of time involving posts that suggest packing up the rafters from above!

 

[Andy Davies]

So is there any point in removing the roof covering and fitting say 30mm of insulation between the rafters?

 

[Andrew Bodenham]

If you were reslating / retiling the roof anyway then yes, I'd take the opportunity to add some insulation between the rafters, and I'd probably compromise on the air gap and put in say 50mm. With a breathable membrane under the slates / tiles then condensation is less of a problem than it was with impermeable roofing felt, so ventilation is less critical. In my opinion.
But I don't think I'd take the roof covering off just to put in that much insulation.
Maybe you're back to tackling this from the inside.

 

[DaveBrigg]

So is there any point in removing the roof covering and fitting say 30mm of insulation between the rafters?

I assume that the only insulation of this thickness is multifoil? I've wondered whether it is possible to fit this retrospectively, maybe by getting into the loft space and using one of those litter picking devices to push it down the gap between ceiling and membrane. But then I remember the manufacturers being hauled up by advertising standards because the insulation claims relied on the foil being sealed airtight, something which could not be achieved if feeding strips down into the space between the rafters. I assume that a more solid form of insulation, such as kingspan, would damage the ceiling plaster if slid down over the laths?

 

[Andy Davies]

If you were reslating / retiling the roof anyway then yes, I'd take the opportunity to add some insulation between the rafters, and I'd probably compromise on the air gap and put in say 50mm. With a breathable membrane under the slates / tiles then condensation is less of a problem than it was with impermeable roofing felt, so ventilation is less critical. In my opinion.
But I don't think I'd take the roof covering off just to put in that much insulation.
Maybe you're back to tackling this from the inside.

The front pitch definitely needs work, not so sure about the rear but I'll ask some roofers what they think (the rear is also smaller and the dormer on the rear needs at least re-cladding, if not more work)

I'll give my local BCO a call and see what they think about the 50mm idea - from the emails I've swapped with them so far they seem quite helpful and flexible.

I've also got some further questions...

Does it matter what line the insulation follows, for example should it follow the line of the roof, or the line of the ceilings and dwarf walls, or even could it just follow the line of the roof and then instead of going up to the right, the line of bedroom ceiling (like an A with the point chopped off)

Apart from cleaning the rafters, should they be treated with anything?

As the ceilings won't be coming down, there won't be a vapour barrier between the ceilings and the insulation, is this likely to cause any problems?

And lastly I read some where (can't remember where) that there shouldn't be a gap between the lathe and plaster and the insulation, but don't see how this is completely possible as the back of the lathe and plaster obviously isn't smooth.

I haven't discounted the idea of pulling down the ceilings but as the front roof needs to come off, it seems more sensible to do it from the outside if possible.

Thanks

Andy

 

[Feltwell]

Andy, I was in a very similar position to yourself, but in my case the roof had to come off anyway.

Victorian semi, so like you raising the roof height was a non-starter, with 2 attic bedrooms with no insulation at all. Not a listed property. Original roof at the point where so many tiles were shot it was time to completely strip and start again with new tiles. Rafters around 80mm deep.

As I was stripping more that a certain percentage of the roof (think it was 25%?) I had to comply with building regs and so had to insulate to a certain standard. I got away with less than 50mm of air gap between insulation and the membrane, as I got an email from the technical department of the membrane manufacturer to confirm this was OK, the BCO accepted this. The membrane I used was Proctor Roofshield

http://www.proctorgroup.com/index.asp?lm=266

Apparently the main point of the gap is so that the membrane can be draped between the rafters so that water is not trapped behind the roof battens - it's nothing to do with condensation, the breathability of the membrane takes care of that. 50mm is not needed.

I put 50mm of Kingspan/Celotex type insulation between the rafters, which I knew wasn't enough to get the required U-value for the building regs so I knew I'd have to underclad the ceilings with more insulation and plasterboard at some point - however, there's no time limit with building regs, so I planned to do it as and when convienient after the main roof was finished. I didn't really want to do it as there was nothing wrong with the original lathe & plaster ceilings and headroom was already tight in places.

However - my regular BCO was on holiday when I called to get sign-off before the scaffolding came down. His stand-in came out, congratulated me on the standard of the job and signed it off without asking about insulation!

The 50mm of insulation I put in has made a substantial difference, it was much warmer up there last winter and cooler this summer. I probably will underclad the ceilings anyway at some point, when I get round to it!

By the way, as you'll be doing lots and lots of cutting anyway to get insulation between the rafters, this firm supplies seconds of the rigid foam type boards, which often means just a wrinkle in the foil facing or a tapered edge which you can just cut off:-

http://www.secondsandco.co.uk/

No need to treat the rafters with anything. My insulation follows the ceiling and walls, so the eaves in my case are un-insulated - they are only used for storage. There's no hard & fast rules though, whatever works for you & your house.

 

[Andy Davies]

Thanks that's very helpful...

 

[Penners]

Does it matter what line the insulation follows, for example should it follow the line of the roof, or the line of the ceilings and dwarf walls, or even could it just follow the line of the roof and then instead of going up to the right, the line of bedroom ceiling (like an A with the point chopped off)

You could adopt either of those approaches, but if possible you're best to avoid leaking heat into any area that isn't lived in - it's just a waste. So best follow the line of the walls, rafters and ceiling if you can.

 

[Feltwell]

You could adopt either of those approaches, but if possible you're best to avoid leaking heat into any area that isn't lived in - it's just a waste. So best follow the line of the walls, rafters and ceiling if you can.

Plus, to add further weight to that argument, for the flat section of ceiling at the top it means you can just use ordinary loft insulation - which is much cheaper than rigid foam - and easier to fit.