Insulation of rafters

kmb

Hi All,

I put on a extension 15 + years ago over the attached garage to add a fourth bedroom.
This room is still colder than the rest of the house, bare in mind that it is in theory external to the main hose though linked by a internal door.
When we were putting in the insulation in the back sloping ceiling, it goes from 8 ft to about 4 feet, we only used loose attic type insulation and foiled backed plasterboard.
I need to rip this out and put the solid type insulation in I presume or is there a better insulation method?
Rafters are 4 x 2"
I rather not put 2" x 2" lathes to increase the insulation to 6" as it will make the sloping ceiling smaller to move around.
This area is a en suite just loo and sink.

Any advice welcome.

kieran

Lumen

Assuming you have regular non-breathable membrane over the rafters, you need to leave 50mm ventilation gap, which severely limits the depth of insultion you can use between the rafters. Consider phenolic or PIR insulation boards plus another layer over the top (insulated plasterboard) to prevent cold bridging, and sort out the air tightness.

kmb

Hi Lumen,
When the roof was done i don't remember any membrane been put in.
Lets presume that it is just the bare rafters,roofing felt and tiles.
What is the process from here?
Expanding foam if any recesses on outer walls?

https://www.kingspan.com/gb/en-gb/products/insulation/insulation-boards/therma/thermapitch-tp10

Insulated Plasterboard for Reveal 22mm (8x4 ft)

Siga Majpell 5 Air Tightness Membrane Interior 50x1.5Mt


So roofing felt, 50mm gap,thermapitch tp10, plasterboard.
Where does the membrane come into the equation?

Lumen

Membrane, felt, sarking. I don't know why the construction industry has so many words for the same thing. Anyway, if it's non-breathable you need the 50mm ventilation gap.

Plasterboard on ceiling will need to be insulated or else cold bridging and crappy u-values.

Run the numbers....

https://www.uvalue-calculator.co.uk/