How to best insulate eaves/ceiling space

benson1980

Hi all,

Before I start plasterboarding up our converted loft room, I wanted to upgrade the insulation which is pretty minimal/non existent above our first floor ceilings. 

I've taken some photos of what I've got. The second photo shows the space above the first floor ceilings with no insulation, and which has very limited access (as can be seen from first photo). The last photo is one from outside- the dormer being our first floor and the veluxes being where our loft room is.

Any suggestions? I was on the process of pushing as much roll type insulation as I could into the gaps with a pole, or similar, before realising this would probably be a mistake due to cutting off this ventilation. Am I best off just pushing it partially through into the gap across the ceiling (there's a span of around 1.6m of ceiling space past the horizontal timber which the cables are attached to), and accepting there will be a bit of a compromise?

Hopefully this all makes sense and any advice much appreciated.

ThisIsWeird

By far the best solution, I believe, is to line it all with rigid insulation or Thermal Laminate Board (ie rigid insulation already bonded to p'board) on the inside.

The stuffed insulation in the sloping rafters in the first photo is potentially asking for trouble. Two issues - the major one - is the way it'll dramatically reduce ventilation, which is pretty essential to keep things dry and rot free. The second is that the soft insulation will trap any water that does get past the tiles/slates and hold it there - to potentially cause rot. You could say - it's a rotten idea!

tumbleweed

The second pic shows the awkward narrow gap and difficult access down the sloping wall. Again, block that at your most likely peril.

So, what I'd suggest is:

1) For the sloping rafters, tightly cut and fit rigid insulation to a thickness that still leaves around 50mm gap between it and the roofing felt. Tap it in firmly until flush with the outside edge of the rafters, and spray-fill any remaining gaps with foam. Then, either overboard the whole caboodle with TLB, or first a layer of rigid insulation, taped along its edges, and then p'board.

2) For the ashlar wall (is that what it's called?), you can again tightly fit as much rigid insulation as you like, ideally to its full depth, in which case you can then overboard with fairly thin TLB. Or, fill in between with the presumably same 2" of rigid as the rafters, in which case line with at least 2" more on the inside. (These figures will not/are unlikely to conform with current build regs, but is what I 'personally' would consider the optimum compromise betwixt insulation value and loss of internal space.)

3) For the sloping void with tight access, I'd suggest forget trying to insulate that actual gap, but focus instead on insulating it from the inside of the room. That means either lining with rigid as before, and then p'boarding, or going straight to TLB. Again, as much as you 'can' that makes sense.

benson1980

Yes fair point regarding the wool type insulation. This was already in situ and to be honest I was debating what to do with it. I think it's been like that for the best part of 20 years without issue (installed by previous homeowner). I do have a grands worth of insulated plasterboard to board from the inside so it partly came down to cost and trying to work with what I had already.

What also swayed me in part was that rockwool advertise their products to be installed between rafters so it is all a little confusing.

also- working around all the various battens/noggins with rigid board looked like a bit of a nightmare.

That said if advice on here is to remove it and go for rigid board I would rather do it properly.

So if I'm understanding the other part correctly, potentially just leave that ceiling void? We've already decorated upstairs and just had all the ceilings skimmed so the options from doing it from downstairs on the first floor isn't an option for us right now.

ThisIsWeird

benson1980 said:

Yes fair point regarding the wool type insulation. This was already in situ and to be honest I was debating what to do with it. I think it's been like that for the best part of 20 years without issue (installed by previous homeowner). I do have a grands worth of insulated plasterboard to board from the inside so it partly came down to cost and trying to work with what I had already.

What also swayed me in part was that rockwool advertise their products to be installed between rafters so it is all a little confusing. Between 'rafters' or between 'joists'? The latter, yes, but the fomer, probably not.

also- working around all the various battens/noggins with rigid board looked like a bit of a nightmare. Yes, cutting rigid insulation board to fit snuggly may seem like a 'mare, but it gets easier. A large knife should do this accurately and without much mess, but a squeeze of exp foam can make up for a ton of errors...

That said if advice on here is to remove it and go for rigid board I would rather do it properly. Personally, I would not be relaxed about leaving the ventilation through-flow cut off.

So if I'm understanding the other part correctly, potentially just leave that ceiling void? We've already decorated upstairs and just had all the ceilings skimmed so the options from doing it from downstairs on the first floor isn't an option for us right now. Not at all ideal to leave it empty, as this will now be the most obviously cold parts of your ceiling. But, neither would I stuff it with rockwool. A dilemma.

I'm guessing this loft conversion is not being done under BC oversight :-)

benson1980

No definitely not building regs compliant.

The only other thing I can think of doing for that ceiling void is opening up some additional access (cut floorboards of loft room) and slide rigid insulation between joists up to just shy of the ventilation. That also seems like a lot of work, to insulate what would be less than quarter of our first floor ceiling space perhaps...

I kinda wish when we were reskimming upstairs that we considered stripping and putting the insulated plasterboard up first, but didn't realise there'd be no insulation up there....