glass fibre rolls between roof joists Information, help, tips and advice on cavity walls, ceilings and lofts etc....

[andypitts]

Planning to insulate and line a largish floor-boarded loft with flat roof.
Currently walls are lined with just vertical wooden boards in front of the eaves spacing behind them - or a neighbours brick dividing wall.
The roof currently is just deep 10" joists up to the wood lining below the roofing felt.
I want good but cheap insulation material and because access is tight - via a narrow steep staircase, I plan to line walls and roof with long lengths of tongue and groove / shiplap rather than lots of cutting plasterboard down to a size small enough to get sections up the staircase.
Questions;
Is basic fibreglass roll good enough as an insulator for this, held behind the lengths of tongue & groove?
I note when using fibreglass rolls between floor joists they now recommend 270 mm thickness - but product still seems to be sold as standard maximum 200 mm thick. Will a single 200 layer be adequate? - it will fit on top of the tongue & groove between the roof joists ok but with v little space left for ventilation - is this an issue?
And it will have to be compressed down to say 80mm thickness when pinned between batons behind the tongue & groove for the vertical wall sections - will this impair the insulation properties plus is ventilation an issue again?
Any ideas appreciated - cheers.

[stoneyboy]

andypitts,
If you are going to insulate between the 10" roof timbers you must ensure adequate ventilation above the insulation or you will serious damp problems. Mineral fibre is OK for this purpose.
For the wall sections fit the mineral fibre between the stud framing but make sure ventilation is provided behind the insulation.
end

[eljaybee]

Have you thought about using a rigid board insulation - polyurethane or polystyrene - polyurethane with a layer of silver foil to both sides is better and you get a better thermal preformance with less thickness compared to fibreglass.

[andypitts]

thanks eljaybee
was coming round to this thought particularly for vertical sections to give a gap for insulation (although does the foil mean this not needed?)
Any recommendations of supplies of the polyurethane blocks ?
cheers

[eljaybee]

The gap is always required for ventilation purposes unless there is a breathable felt on the roof (or that the roof has been formed as a warm deck rather than the normal cold deck)

Suppliers - most of the main builders merchs will have what you need, even most of the DIY sheds have it. The top manufacturers are Kingspan and Celotex