insulation of small loft space with narrow joists & raft

[hroodbwai]

hi, apologies for covering well trodden ground, but I can't find a direct answer in previous posts.

I've recently moved into a new build house that has a single ply flat roof supported by king post trusses. There's not much storage space, so we wanted to board part of the loft. I've got some loft boards to screw over the joists, but I'm a bit stuck as to what to do with the insulation.

Unfortunately the joist and rafter spaces are only 70mm deep.
At present there's 270mm of glass-wool laid over and for the time being I've placed some of the glasswool insulation between the joists. Obviously this isn't going to be sufficient thickness and it's going to be compressed once boarded. I'd looked into using space board on top but it's required thickness would reduce the height to an unusable level (I'm already struggling to crawl around as it is and the trusses limit accessibility).

I was thinking of laying down PIR board like Celotex 70mm between joists and then counter-battening to place another 30mm across to reach the required 100mm. however I wasn't sure if you can then lay chipboard straight on top or you need 50mm ventilation space, which would again reduce the useable space to the point of negating any worth to the project.

If not possible, is it an option to split the 100mm between rafters and joists by adding 30mm battens to each and 50mm board insulation. If done where would I have to put the vapour control layer?

The other option I wondered about was using a multi-layer foil product like thinsulex to top up the insulation, but I'm not sure whether I'd need to put it in a layer over 25mm batten across the joists to provide the required air gap and then put the chipboard on top or whether it's better to staple it to the rafters. most of what I've read says that foil by itself rarely lives up to it's billed R rating, with only hybrid systems being certified and that it needs an air gap to perform well. However I've not found anything that actually describes how to properly combine them. I'd have thought that given the glasswool insulation is by definition filled with air it should be ok to layer the foil over it directly.

Also one thing that confuses me is that because the trusses are present throughout, whatever solution I go for, surely there'll always be a thermal bridge present and there's no way to get a completely tight vapour barrier.

any help/suggestions much appreciated!

Julian

 

[PrenticeBoyofDerry]

You don't need ventilation under the loft panels, so you can lay timbers perpendicular to your existing joists to rise the height levels and fully fill with insulations. You must ventilate the loft space though, this can be done a number of ways, normally by leaving a ventilation gap at the eaves of the roof, to allow air circulation in and out of the loft.
It is ideal that you have vapour check barrier, this should be above the ceiling board and below the insulation, check to see what type of ceiling board you have if they are foil backed your already on to a winner.
The recommended insulation levels for mineral/rock wool is 270mm within loft spaces.

 

[hroodbwai]

thanks!

ended up installing a vapour barrier over existing plasterboard and joists, battening to required height and using 100mm PIR board. Managed to get from local builders merchants at knockdown price by getting them to match prices found online.

Only regret was that board was too thick to cut straight through with knife. Ended up having to score both sides, break along the lines and then trim down to size with a heavily sharpened kitchen knife. If had to do again, would definitely either go for thinner size and layer up or use an appropriate saw.