Insulating eaves dwarf walls - rockwool or insulation board?

[Trevelyan]

Hi all,
I have a 1980's loft conversion which has about 100mm fibreglass insulation between the studs on the dwarf walls to the eaves. I would like to increase this (on a budget!), I have been thinking about either trying to lay rockwool at 90 degrees to the studs (i.e. left/right, but not convinced about how to get it close/tight enough to provide benefit) or to install insulation board on the back of the studs (to also prevent cold bridging on the studs).

Would the rockwool option work? If I go insulation board I am then concerned about damp - the dwarf walls have no vapour membrane of any kind and the eaves themselves are well ventilated, what type of board should I use to avoid getting condensation in the dwarf walls?

Or third option, do I need to remove the existing fibreglass insulation and put insulation board, covered with some kind of vapour membrane, between the studs against the plasterboard? (Will this still get me damp in the studs?)

Cheers,
Trev

 

[cdbe]

When I did my loft conversion some years ago the designer specified dritherm 32 cavity batts for the dwarf wall. They are a high performance batt and quite solid so they would stand up/stack tight behind your wall with little securing. B&Q have these on sale at the moment at a bargain price (I think they're clearing out the 75mm thickness as it's a difficult size for modern buildings) - you won't get more bang for your bucks - I've just bought a load for my extension as I'm using a 75mm cavity.

 

[Trevelyan]

Thanks, certainly looks like a cost effective route. I guess my only question is about air tightness - will they be effective just stood behind the studs, or would I be better going for some kind of board which can be physically fixed/sealed?

 

[JohnD]

We keep seeing shocking fires where foamed plastic insulation board has burned. I wouldn't now have it in or on my house.

Mineral wool insulation doesn't burn or emit toxic vapour. However it does need to be thicker for the same level of insulation. It is cheaper, though, and does not have to be cut with such accuracy because it can be stuffed into a tight gap.

 

[tvrjohn]

When I did my loft conversion some years ago the designer specified dritherm 32 cavity batts for the dwarf wall. They are a high performance batt and quite solid so they would stand up/stack tight behind your wall with little securing. B&Q have these on sale at the moment at a bargain price (I think they're clearing out the 75mm thickness as it's a difficult size for modern buildings) - you won't get more bang for your bucks - I've just bought a load for my extension as I'm using a 75mm cavity.

Hi CDBE thanks for writing about the insulation slabs, have now purchased 10 packs from B & Q a bargain.
Was considering retaining them with a vapour barrier type sheet at the back in the cold / vented part of the loft space ? and plasterboard on the front

 

[cdbe]

I've actually found some old pictures of mine:

Looks like 9mm OSB at the back, no pictures of the front but probably foil backed plasterboard. With hindsight there is clearly a potential issue with cold bridging where the timber studs are so I'd probably put some strips (or sheets depending on my budget) of 25mm celotex on the front of the studs to prevent this - if you completely covered with celotex and taped the joints you wouldn't need foiled plasterboard.

 

[tvrjohn]

Hi cdbe
thanks for adding the images as it has clarified my concern with vapour barrier, so better to use OSB ok that seems a better solution.

Regarding the cold bridging on the vertical studding, I had not considered that, is that really an issue to address ?
Maybe I should read up on cold bridging !

thanks again

 

[foxhole]

Cold bridging should not be an issue , You can add insulation or insulated plasterboard to front .
Worth checking social media for insulation bargains , have some 100mm thermal board local to me at just £20 a sheet on Facebook .