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I’m opening up some of our loft area up to the rafters where we have a newly installed roof with Velux, so that we can get a bit of light on to our landing as there are no windows. This will form a mini atrium type affair.
I will need to insulate between the rafter where I have taken out the ceiling and intend to use some sort of board. On top of this there will be plaster board and a final skim. Any recommendations on what to use or not to use? What sort of thermal performance figures should I be looking at?
I’ve spoken to the roofer and he has already told me to leave a ventilation gap between the insulation and the new roof.
Whatever the builder tells you, cos you'll need building regs sign off if he's not FENSA registered, so you'll need to make sure that you have the correct U values for the insulation
Spoken to building control. They are not concerned as the Velux was fitted between existing rafters and the guy who did it is FENSA registered.
Basically I'm just boxing in a Velux, but the box is a lot bigger than the Velux.
Anybody know what sort of "U Value" I should be looking at?
Somehting equivalent to your loft insulation?
When I did an extension recently involving Velux windows the insulation had to go between the rafters and cover the inside of the rafters too. So 100mm plus 80mm from memory....expensive process!
The house is 200+ years old. The insulation largely consist of whatever has died up there.
I'm not sure I understnad, you are making what I might call a "light tunnel" through an unused and uninsulated loft-space correct? And there's **** all insulation above the ceiling? Then it's not going to make any difference Shirley?
Unless you use this opportunity to insulate above the ceiling?
If I've got that wrong then it's cos I don't get this: "between the rafter where I have taken out the ceiling". Or is there insulation under the new roof? In which case some thing with same R/U value as that?
There is some insulation above the ceiling but it isn't great. I'll be taking the opportunity to improve that too.
My concern is the insulation between the rafters, as the thickness of this will be determined by the depth of the rafters, taking into consideration the need for an air gap between the back of the insulation and the underside of the roof.
You are right, it is a light tunnel, but it is approx. 6ft x 6ft, with the Velux in the middle.
It looks like I can get anything from foam block to much fancier and more expensive sandwich type materials. Is a foam block up to the job if I cover it with plasterboard?
Use 75mm celotex and stick 250mm of Rockwool in the rest of the loft. Oh and don't forget to insulate the sides/walls of your light box.
Usual air gap is 50 mm so most insulation will go below (ceiling side) and across the rafters. Something celotex like. Just do the valve to work out the required thickness for the required U value. Part L will have details.
Great, thanks.
I'll be insulating the sides too.
...and another question.
I don't know how deep the rafters are yet. Assuming they are less than 125mm I'm going to have to fix this stuff to the underside of them. Do I just screw through it into the rafter?
How do I fix the plaster board? Longer screws though the insulation into the rafter or with an adhesive?
I love it when my missus gives me a "little project".
Alot of conversions we do we put 100mm between rafters and 50mm insulated plasterboard over the top. You'll need 100mm plasterboard screws for that as it's almost 75mm with the insulation and board altogether. If you want more between the rafters just use tile lats nailed to the rafters to gain another 25mm. Just make just you leave a gap behind so its not touching your roofing felt
Building regs for my loft conversions lately consist of 125 mm kingspan in the pitched roof. 75mm cut in between roof rafters (125mm deep, leaving 50mm gap above for air flow) with 62.5mm insulated plasterboard over the top of all rafters
You could increase or decrease the amount in between the rafters to keep to the minimum of 50mm airflow. It's important to have the insulated plasterboard sheets covering the underside as this stops you getting thermal bridging from the rafters.
Had an attic room insulated in an old house. Ended up with PU* foam backed plasterboard. Had we gon for rockwool we wouldn't have been able to stand up. I still don't think it met the required values but it massively improved things.
celotex, kingspan etc.