Insulation Board or Mineral Wool

Philip Barber

Philip Barber

Joined
24 Oct 2016
Messages
5
Reaction score
0
Country
United Kingdom
Hi All,

i am just looking for some guidance on insulation of the rafters in my loft. I have done some reading around and people recommend insulation boards as they are thinner for a high level of insulation.

The only downside I have noticed with our rafters is whoever built them way-back-when (house was initially built in the early 1900's) managed to build wonky rafters. My dad (DIY 'expert' in his own mind) thinks i should shove 25mm mineral wool down the rafters and board over it; rafters are only 3 inches deep and I believe doing the 25mm would leave the 50mm air gap that is recommended for condensation purposes.

Lastly, I did think about getting thermal-backed plasterboard, give up the two inches of space, and leave the 3 inches as the air buffer. It does also seem that you can only get the thermal-backed stuff in a tapered edge which I hear isn't too good if you have to cut it into funny shapes to fit the wonky spaces.

Any thoughts on the above would be much appreciated. If you need any more details, just ask and ill try and get details.
 
Problem with adding to the floor is that it wont insulate the loft itself; want to make the loft warm as well. As for the other question, there is roofing felt currently there.
 
Then wedge all the felt laps open with some plastic or timber to give ventilation, and you can insulate upto the felt leaving something like a 25mm gap. Ensure there is a gap in the felt right at the ridge too.
 
You can fix your PIR boards to the underside of the rafters- either solvent-free Gripfill or stupidly long plasterboard screws with penny washers will do it- longest screws I've seen are 100mm which sort of limits you to 70mm PIR with 12.5mm plasterboard
Then tape the joins in the PIR and screw your plasterboard up (with more stupidly long screws) having remembered to mark on the PIR where the rafters are :)
 
You need 135mm of PIR if you're going to get up to building regs. This would be 100mm PIR in between the joists, and 35mm insulated plasterboard over the top of them. You've only got 75mm joists, so you need to batten them out first. You could go for a 25mm airgap under the tiles, but 50mm would be better. With a 50mm airgap, you then need a 75mm batten on the underside of the rafters. You can cut and wedge the insulation into the top of the battens, or place a 35x25 batten down the side of the rafters to make sure you keep the insulation away from the tiles. The you add the 35mm backed plasterboard across the joists.

And having done all this you wouldn't need to add any more insulation under the floorboards.
 
newboy said:
Up to 200mm readily available

http://www.screwfix.com/c/screws-nails-fixings/woodscrews/cat840066
Click to expand...

Not black phosphated ones suitable for plasterboard. Using ordinary wood screws on plasterboard is a recipe for disaster: the alkalinity of the board will eat the screw in the presence of moisture (most human acivities generate Moises) and the 200 mm turboscrews to which you refer are 20p each, whereas plasterboard screws are less than a penny each.. you'd spend more screwing the boards up than you would on the boards and skim
 
cjard said:
Not black phosphated ones suitable for plasterboard. Using ordinary wood screws on plasterboard is a recipe for disaster: the alkalinity of the board will eat the screw in the presence of moisture
Click to expand...
So that's the difference between drywall screws and regular screws! I always wondered.
 
You must log in or register to reply here.
Back
Top