Insulating old 3x2 roof space

CashStrappedTeen89

I’ll try to keep it brief, our loft space was converted by the previous owner to a bedroom and separate bathroom.

since we moved in we’ve had issues with damp in the wall and roof space which is the detached side so most weathered.

we had the roof checked and found a leak and some lead which was done wrong. All fixed and through the summer improved, roll around to winter just gone and back again. So we have literally just ripped all the plaster down which was crumbling, we’ve also gone back to brick on the outside wall and found an old poorly bricked up fireplace which I’ve opened and cleaned out as far as I can reach, my intention is to brick this back up and put a vent in and eventually clear down the old chimney from the bottom when I can open it up from the ground floor.

ANYWAY, our roof timbers are 3x2, considering the current standard is 270mm (recommended) insulation I’m trying to think of the best way to achieve a middle ground, so do I use 3x6 and bolt them alongside the other beams and rebuild the bottom section below the support beam (as half of its loose anyway).

I was hoping to DIY this as I’ve done level of experience like boarding, electrical work but by no means am I an expert and we had no choice but to remove the bad plaster etc to figure out the extent.

Any guidance would be hugely appreciated!

ThisIsWeird

3x2 rafters? Are you sure?

Anyhoo, since you can 'board', you will love this job, and the results even more :-)

For the sloping roof rafters, you need to leave a minimum 50mm space between the insulation you will be fitting in between each rafter, and the felt/sarking layer above. A free passage of air must be able to travel up from that eaves space to the roof apex. Hopefully at the top there is also the same amount of gap back down the other side, or perhaps some roof ridge vents?

So, deduct the required 50mm, and then neatly cut rigid insulation board such as Celotex to fit snugly between each rafter. Press it in until it's flush at the front. Fill any gaps, and between sheet sections, with a gentle bead of spray foam.

For the ashlar wall, since there will be a large void behind it, you can fit whatever thickness of insulation between these uprights as you like.

In both cases, you will then be over-boarding it with insulated plasterboard, screwed as usual to the timbers, but obviously using longer bugle-head screws. Foil-tape all joints. Jobbie jobbed. (If you prefer, you can lightly screw the same Celotex to the timber instead, and then properly screw separate p'board over it - no real difference, and your choice).

Thickness? Whatever you can get away with, whilst not compromising on room space loss too much. I'd suggest you want at least 2" of Celotex-type insulation minimum, and 3 or even 4 would be better, tho' nothing like 'twice' as good.

For the ceiling, if the timbers are really just 3" deep, then all you can fit in between the rafters will be 1" insulation in order to leave the 2" void. In this case, I'd be looking to overboard with a further 2" Celo if possible.

For the walls, if you can get 2 or 3" of Celotex neatly and tightly fitted between the timbers, then you could get away with only one further inch of insulation before the p'board layer.

Seal - draughtproof - the insulation against your room floor - you don't want any annoying coldies coming through.

That section of flat roof - I'd also screw insulation + p'board on to that, or the one-piece insulated pasteboard. Tbh, since you have goner to all this superb effort, I'd be taking down that existing board too, checking how it is behind it, and trying to first add insul board between the joists as before.

The chimney wall is a party wall? In which case insulation isn't so important, but I'd still use insulated p'board here, even if just one inch's worth, simply foam-adhered to the brick. The brick will need to be dust free for this, so best primed using Everbuild Stabiliser (406?) or SBR first.

FreeBear

ThisIsWeird said:

3x2 rafters? Are you sure?

For the ceiling, if the timbers are really just 3" deep, then all you can fit in between the rafters will be 1" insulation in order to leave the 2" void. In this case, I'd be looking to overboard with a further 2" Celo if possible.

For the walls, if you can get 2 or 3" of Celotex neatly and tightly fitted between the timbers, then you could get away with only one further inch of insulation before the p'board layer.

3x2 rafters sounds about right - I have similar sized timber in my roof.

I'd not bother cutting and fitting between the rafters - Just put a minimum of 100mm up (foam and tape the joints). If you want to save on labour, use insulated plasterboard and plenty of long screws, but be warned, the sheets are darned heavy. My preference would be multiple layers of 25mm Celotex interspersed with 2x1 battens at regular intervals.

For the knee walls, yes, fill the gaps with insulation and the whack another layer of insulation across the whole wall. Aim for 100mm or more.

On the brick walls, 75mm should be plenty.

CashStrappedTeen89

ThisIsWeird said:

3x2 rafters? Are you sure?

Anyhoo, since you can 'board', you will love this job, and the results even more :-)

For the sloping roof rafters, you need to leave a minimum 50mm space between the insulation you will be fitting in between each rafter, and the felt/sarking layer above. A free passage of air must be able to travel up from that eaves space to the roof apex. Hopefully at the top there is also the same amount of gap back down the other side, or perhaps some roof ridge vents?

So, deduct the required 50mm, and then neatly cut rigid insulation board such as Celotex to fit snugly between each rafter. Press it in until it's flush at the front. Fill any gaps, and between sheet sections, with a gentle bead of spray foam.

For the ashlar wall, since there will be a large void behind it, you can fit whatever thickness of insulation between these uprights as you like.

In both cases, you will then be over-boarding it with insulated plasterboard, screwed as usual to the timbers, but obviously using longer bugle-head screws. Foil-tape all joints. Jobbie jobbed. (If you prefer, you can lightly screw the same Celotex to the timber instead, and then properly screw separate p'board over it - no real difference, and your choice).

Thickness? Whatever you can get away with, whilst not compromising on room space loss too much. I'd suggest you want at least 2" of Celotex-type insulation minimum, and 3 or even 4 would be better, tho' nothing like 'twice' as good.

For the ceiling, if the timbers are really just 3" deep, then all you can fit in between the rafters will be 1" insulation in order to leave the 2" void. In this case, I'd be looking to overboard with a further 2" Celo if possible.

For the walls, if you can get 2 or 3" of Celotex neatly and tightly fitted between the timbers, then you could get away with only one further inch of insulation before the p'board layer.

Seal - draughtproof - the insulation against your room floor - you don't want any annoying coldies coming through.

That section of flat roof - I'd also screw insulation + p'board on to that, or the one-piece insulated pasteboard. Tbh, since you have goner to all this superb effort, I'd be taking down that existing board too, checking how it is behind it, and trying to first add insul board between the joists as before.

The chimney wall is a party wall? In which case insulation isn't so important, but I'd still use insulated p'board here, even if just one inch's worth, simply foam-adhered to the brick. The brick will need to be dust free for this, so best primed using Everbuild Stabiliser (406?) or SBR first.

Superb response, thank you so much!

ThisIsWeird

You're welcome.

The thickness of insulation you end up going for is entirely up to you. Obviously the more the better, but it follows the law of diminishing returns...

I tend to have a pragmatic approach where the resulting loss of internal space can have a large bearing on my decision. The room I am in now is only 8' wide (used to be an attached garage) and has only a single-brick skin external wall - so was cold. I added just 1" of Jablite ('expanded polystyrene') in between the 2x1 battens screwed to the wall, over-clad with just a further inch of Thermal Laminate Board (insulated p'board), a higher insulation value product similar to Celotex. So, 2" in total, one of which is just polystyrene. It's as cosy as a cosy thing, and is dead easy to heat - the output from my ol' 37" plasma TV is enough to keep it warm.

(In retrospect, I wish I'd simply adhered 2" of insulated board to the wall - far less hassle, more effective, and a lot faster. Drilling and screwing in battens was a pita, and also leaves cold head screws under the 1" of top insulation! Personally, I'd avoid battening whenever you can as it just adds a lot of work for no benefit that I can see. I think the only thing FB and I differ on :-)  )

FreeBear

ThisIsWeird said: Personally, I'd avoid battening whenever you can as it just adds a lot of work for no benefit that I can see. I think the only thing FB and I differ on :-)  )

If you use multiple layers of Celotex/Kingspan (the foil faced stuff), joints can/should be taped. This gives you several barriers to moisture which will limit the scope for interstitial condensation. Cold spots forming over the screw heads is not an issue - Done right, there will always be at least 25mm of insulation above/below a screw. If you are using insulated plasterboard, mechanical fixings should always be used, so you'd still end up with cold bridges where the fixings are..

Using battens, you can add extra support for things like radiators & curtain rails. Also easier to fix back boxes & conduit as you build up the layers. Yes, more work, but it does have a few advantages.

stuart45

Those rafters look more than 3x2. The hangers could be 3x2.

casper_gutman

Are you sure this is a converted loft space and not an original "habitable space" of the house as first built? It would be unusual to say the least to provide a fireplace in a loft space!

FreeBear

casper_gutman said:

Are you sure this is a converted loft space and not an original "habitable space" of the house as first built? It would be unusual to say the least to provide a fireplace in a loft space!

Looks as if there might have been a lath & plaster ceiling in there originally. Servants quarters perhaps ?

CashStrappedTeen89

casper_gutman said:

Are you sure this is a converted loft space and not an original "habitable space" of the house as first built? It would be unusual to say the least to provide a fireplace in a loft space!

It may well of been. I took the advise further up and removed the flat roof. Glad I did as the frame has come off the wall either end and there was 4, yes 4 different types of plaster before I got to the roof.



unfortunately the frame was built over the top of old plaster to so I’m going to have to completely take it down to remove it all and asses my next steps. I refuse to bodge this so may be time for a small load to get a professional in and I’ll just do what I can do effectively.

ThisIsWeird

CashStrappedTeen89 said:

casper_gutman said:

Are you sure this is a converted loft space and not an original "habitable space" of the house as first built? It would be unusual to say the least to provide a fireplace in a loft space!

It may well of been. I took the advise further up and removed the flat roof. Glad I did as the frame has come off the wall either end and there was 4, yes 4 different types of plaster before I got to the roof.



unfortunately the frame was built over the top of old plaster to so I’m going to have to completely take it down to remove it all and asses my next steps. I refuse to bodge this so may be time for a small load to get a professional in and I’ll just do what I can do effectively.

Wow! You've done the bulk of the work, tho', and what's left should be the rewarding part.

Close-up pics of what you mean about the 'frame coming off the wall at both ends'? The horizontal joists look as tho' they're attached to the 'purlin' on the left sloping  roof, and the wall to the right? If which case it should be supported well enough - but I'm not a builder.

That flat ceiling joists also appear to be a more recent addition as metal joist hangers are used to hold them.

IF the flat ceiling joists are well enough attached, then the same insulating procedure should see it sorted - snugly fitting rigid insul sheet between the joists, gaps filled with foam, ditto against the end walls - fill any gaps - and then overboard with another layer to make a full envelope. Should be transformative.

Keep the pics going - it'll be good to follow your progress :-)