Might this be the best way of fitting an isulation board like Celotex GA4000 :
(nabbed from
Also, I'm restricted to 6cm as I'd like to leave a gap between the tiles and the insulation. Any tips on the best price/performance for that thickness? I will use board-faced insulation on the inside, but I'd like to keep these reasonably thin - say 6cm.
Thanks, Rob
Have you got a breather felt under the tiles or a traditional one? Generally with a breather felt you can have a very small or no gap behind the insulation. With traditional felt they normally suggest leaving 50mm gap, and fitting ridge and eves vents (or vent tiles).
When I did my loft conversion, I nailed some battens in place toward the back of the rafters (i.e. nearest the tiles) to stop the boards pushing too far back, cut them for a snug fit, and foamed any gaps. They did not need anything else to fix them. I arranged for the board surface to be flush with the underside of the rafter. Then I fitted another layer of
30mm foam under the rafters, tacked in place with the occasional PB screw, foil taped the joins, and finally fitted normal plasterboard. (it was slightly more effort than using a foam backed board, but I could also get the insulation massively cheaper on its own.Sounds more than adequate. I have not looked at the specs in the current building regs, but when I did mine I "required" 50 in the gaps, and 20 under. I actually fitted 30 under since that is what I could get. I seem to recall they have raised the spec a bit since but not massively for lofts. However look at the bigger picture and see what the state of insulation is elsewhere, also how much headroom etc you can afford to lose.
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When I did mine, Building Control required 100mm between the rafters and 25mm over. I battened the rafters for extra depth.
Cheers Richard
On Tuesday 21 May 2013 16:32 geraldthehamster wrote in uk.d-i-y:
Bit Nazi - my BCO seemed taken aback that I was proposing to achieve 100mm over 80% with a mixture of between and under. I think he would have been happy with less.
Virtually impossible to "friction fit" insulation between joists. Cut undersize & fill the gaps with foam.
Yup I knew they had increased it... there was also a slight easement for loft conversions IIRC (or was in 2004 anyway) compared with what would normally be required for a roof.
On Tuesday 21 May 2013 18:26 harry wrote in uk.d-i-y:
from
Nope - even I can manage it (unless the space is awkward).
The secret is to cut a slight angle so the smaller side goes in first then becomes tight.
But foaming is OK too - just costs more...
There is: nothing. Just 115 year old slates. So maybe I could go further towards the 12cm joist depth. No vents at all at the moment. The tiles are likely to need renewing in the next 10-15 years, so I'm just trying to design in something that'll work at roof renewal time.
Thanks for that - looks to be a good system. I've not got a fair bit to do. Take your point on the plasterboard - using celotex it works out at about £20 per 2.4 x 1.2 sheet extra.
I could give the BCO a call - but is it really necessary to follow their guide?
Thanks for the guidance, useful.
Rob
In which case you can pretty much fill the depth. When you reroof you can use a breather membrane then.
I can usually get 50mm PIR boards at around £14 for a 8x4 sheet. Still it depends on how much you are doing. If its only a few sheets the PB+PIR bonded together combo is easier to work with.
Depends on what you are doing really. If you are doing a full conversion on a building notice or full plans submission, then you will need to follow their guidance to be able to get a completion cert at the end. If however you are just improving the insulation for the sake of it, then its up to you. (technically, changing any thermal element these days comes under building control, but I expect that is another "rule" that is paid little attention unless there is a real reason one needs to!)
Yes, quite right. However, this area is attic rooms.
What I don't follow is how the roof void gets ventilation beyond the swirling about through the slates. Filling with insulation would block any flow of air between the eaves and the roof apex area.
The answer there would be some vents then - a number of the houses round here have them, but none of the 20 or so newly roofed houses have any that I can see.
Good, thanks for that. I'd meant to say there is lot - maybe 10 8x4 sheets. But if it works out easier/neater that £200 may be worth the hit.
It's just refurbishing really. Taking out the old lath/plaster so I thought I might as well do this while I'm there.
Thanks, Rob
What is SUPPOSED to happen is that there is a gap between the insulation and the sarking (if you have such) and that gap should extend to soffit vents. So air circulates above the insulation in one set of vents and out the other. If there is no gap between the insulation and the ridge board, then normally you would use breathable sarking and/or some kid of ridge vent that spans both sides of the the ridge.
As you can appreciate you MUST totally airproof the warm side of the joists and the insulation board. With celotex you use a foil sticky backed tape. If you dont seal the gaps cold air can circulate behind the plasterboard and the insulation is useless in windy conditions.
you could also use a mastic like expanding foam or decorators caulk.
The main need for ventilation is to ensure the roof timbers don't get wet or more importantly, stay wet if they do. So the air coming through the gaps in the slates will typically ensure that the back of each timber can dry. A little bit of extra "space" however is not a bad thing.
You can save vents for when you get a membrane - its surprising how much air flows through tiles / slates when they are unbacked. (an experiment with a smoke match is quite informative)
Yup makes sense.
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