insulating attic room under slate roof?

[pauljsgreen]

i have used the search but there doesn't seem to be a definitive answer.

i have an attic room (by original design) in a victorian house.

the roof was leaking so we had to take down all the lathe and plaster leaving the rafters exposed. the leak has now been repaired and i have to make good the room.

it had no insulation whatsoever between the plaster and the outside roof.

i am left with, from the outside inwards, slates nailed onto planks about a foot wide and half inch thick nailed onto the rafters which are about 10 inches by 3 inches.

the space between the rafters is a bit haphazard because of the age and shape of the roof.

i need to (economically) insulate this area and plasterboard over it screwing through to the rafters.

what i can't get clear is where the insulation gap should be if i need one under these circumstances. do i pack the roof with glass wool, put a visquine (sp?) barrier across the rafters and sheet over that?

b & q has a subsidised 3 roll pack for £10 covering 5.4 sq m.

the slate roof is watertight, but certainly not wind tight which makes me unclear about the need for a gap.


any possible help appreciated.

many thanks
paul

 

[gday2uk]

Didn't search very far then //www.diynot.com/pages/in/in002.php See 'Sheet Insulation'

Stay away from mineral wool type insulation, the reason it's cheap and B&Q are subsidising it is because it's crap. Google 'Kingspan' and 'Celotex' as two brands of decent insulation. After that Google 'insulating rafters' and spend a few hours reading. Any more questions then ask them here.
The only thing I don't know about is what happens when like you there is no roofing felt present. This work is probably subject to building regs as well so your best bet would be to get local BI in to tell you what to use - job done

 

[jeds]

The gap should be on the cold side of the insulation. Normally it would be 50mm but if you have no undertile felt then 25mm would be reasonable. I would advise as much insulation between the rafters as you can get in but leaving a 25mm gap and another 40 or 50mm continuous layer beneath the rafters. Use PUR foam type insulation - mineral wool is not suitable for this type of application.

 

[mattylad]

When I had my similar roof, re tiled (with underfelt) we put 50mm kingspan in on top of the plasterboard that was already there.
It really improved the room.

I used the offcuts to also insulate the vertical walls in the eaves.

In your position, perhaps you are unable to do much about the underfelt, however if there is not enough room for the kingspan + air gap, you can always add battens to the underside of the rafters to increase the depth which you can then screw the plasterboard to.

Lowers the ceiling by a little, but hardly noticeable.

I wonder if adding underfelt pinned either side in strips (perhaps with lengths of beading etc) might provide some assistance in keeping the wind & rain away also any future leaks not damaging the ceiling?

 

[mace]

if you would like to get it at least close to current standards you should maintain a minimum of 50mm airgap between the insulation and the battens then at least 100mm of something like kingspan kooltherm between the rafters placed flush with the inside face of the rafters, then 30mm on the underside of the rafters to counter the effect of thermal bridging through the rafters, and then the plasterboards.

quilt of any kind will be massivley cheaper but certainly not suitable in any way.

good luck!

mark

 

[pauljsgreen]

thanks so much to everyone for the replies.

sorry about the delay in getting back, but you gave me a bunch of new tasks - mainly measuring, setting up a spreadsheet and planning the room next to it to get some economies of scale since the insulation's going to be so much more expensive than i thought.

the search i undertook was only in the forums unfortunately. the link from gday2uk was great.

the rafters are 160mm so looks like i've got enough room to get 100mm in and still have a 50-60mm airgap.

what's the minimum i can get away with on the edge of the rafters behind the plasterboard? i assume that's to stop the "thermal bridging"? really want to minimise the reduction in room size/headroom.

since i've accepted this change of direction i'm going to do the two rooms together.

this is a fairly intact victorian semi and the first room's insulation was convenient as there was a leaky roof necessitating the removal of all the plaster and lathe on the walls on the roof side.

the other room is a slightly different shape with no apparent roof problems.

is the accepted practice just to rip off all the original lathe and plaster to get access to install the rigid boards.

i have no problem doing this, but it just seems a shame and pretty invasive.

many thanks again
paul

 

[jeds]

If you can get 100mm between then 30mm beneath will bring you up to the current standard and would be sufficient to avoid thermal bridging problems.

I understand wanting to preserve lath and plaster but it's not really feasible in this situation. If you try inserting the insulation through gaps at, say, 1m intervals You will remove so much lath there won't be much left to save - and then how would you fix the 30mm layer? If you want the higher standard of insulation the lath and plaster has to go.

 

[pauljsgreen]

....If you want the higher standard of insulation the lath and plaster has to go.

ok, many thanks, i'll get on with it!


paul

 

[sharknose]

Would it be ok to just have a 30mm gap between the kingspan pir and the under side of the roof battons plus the extra bit of space (10mm or so) between the under side of the battons and the lime mortar underneath the slate. I dont have any felt. just the lime mortar which fills in the gaps between the slates

 

[jeds]

Would it be ok to just have a 30mm gap between the kingspan pir and the under side of the roof battons plus the extra bit of space...

Doesn't strictly follow the regulation but in reality will probably do the job. Having no under-tile felt increases ventilation - which helps.

 

[geraint]

thanks so much to everyone for the replies.

sorry about the delay in getting back, but you gave me a bunch of new tasks - mainly measuring, setting up a spreadsheet and planning the room next to it to get some economies of scale since the insulation's going to be so much more expensive than i thought.

the search i undertook was only in the forums unfortunately. the link from gday2uk was great.

the rafters are 160mm so looks like i've got enough room to get 100mm in and still have a 50-60mm airgap.

what's the minimum i can get away with on the edge of the rafters behind the plasterboard? i assume that's to stop the "thermal bridging"? really want to minimise the reduction in room size/headroom.

since i've accepted this change of direction i'm going to do the two rooms together.

this is a fairly intact victorian semi and the first room's insulation was convenient as there was a leaky roof necessitating the removal of all the plaster and lathe on the walls on the roof side.

the other room is a slightly different shape with no apparent roof problems.

is the accepted practice just to rip off all the original lathe and plaster to get access to install the rigid boards.

i have no problem doing this, but it just seems a shame and pretty invasive.

many thanks again
paul

if you have 160mm you are laughing... 80mm cellotex between the rafters will do what you want to achieve. or full fill with rockwool.
then board with duplex plasterboard. simple. hopefully end of......