Rigid Insulation

peterc14

I know in the attic you are best to roll out insulation between and over joists.

However, say in an area like a porch or one story sun room where it is not that easy accessed could you use roll isulation over the joist and then use rigid insulation between the joists as this could be done easier working from the ground?

Would there be any problems with this idea?

soldsold

Yep, this is the way it is done.

A good spec is 200mm insulation on the flat between the joists, and another 200mm perpendicular across the joists. All the above is done in the attic, glass wool on the flat, fluffed up and tightly cut to fit with no gaps. Bulk it up a bit at the junction between the sloping roof and the flat ceiling.

For the sloping roof, cut solid boards of PIR or Polyurethane (Kingspan/ xtratherm/ quinn) leaving a 50mm gap between the boards and the felt, then use an insulated plasterboard under this sloping area. Eg if rafters are 125mm, use 75mm boards.

Dont use downlighters in the room unless you drop a service cavity below the airtight layer - I screwed 50mm x 50mm timber perpendicular to the rafters, then another 50mm x 50mm to the first one giving a 100mm depth service cavity which allows HRV box section, downlighters, and any other wiring to run inside the airtight layer. Downlighters punched through plasterboard into the attic will make for a lot of heat loss.

The 100mm service cavity will be insulated on the sloping section, but not on the flat section as its cheaper to thicken the insulation on the flat in the attic.

peterc14

Thanks soldsold... but see in a porch area where you are working from the ground up as opposed to in the attic looking down (if you know what i mean).... can I put the rigid insulation between the rafters instead of the roll as they will friction fit between the rafter and will be easier to fit or w do you have to use the rockwool roo type of insulation?

soldsold

Either roll out glass wool or solid boards tightly fitted will be fine. However I'm not seeing how you could roll out insulation over the joists if you cant roll it between the joists. If you post a pic or description of the joist thickness and current setup ie is the roof felt on, etc it will be easier to give a definite answer.

In any case, you will need to have twice the thickness of glass wool to give the same insulation as PU/ PIR boards in theory. But in reality the boards arent usually fitted tight enough so glass wool might be a better bet. Also you need a 50mm ventilation gap between insulation and non breathable roofing felt, so it all depends on the roof structure.

peterc14

soldsold... attached is my poor attempt at a drawing..

sydthebeat

insulate before felting

soldsold

I'd just use glass wool all over, its cheaper by about half for the same u value, will be faster to fit - you could run a few strings along the joists to support it if it starts to sag. You can also get this insulation in a bag I believe where its like a sleeping bag you roll out.

If you think its handier you can also use solid PIR boards as you had planned if you want, but should fill the joist to within an inch or two of being full so there is no gap between the two types of insulation.