Insulation in cavity of gable walls

The Barrier

I'm looking for advice on whether to insulate the cavities of the two gables of my wall plate dormer new-build. I have used 80mm in a 120mm cavity on the first floor and up to nine feet in the gables so far. My builder however says that this is all I need. He says that if I insulate the rafters well (200 mm of high density rigid board) i wouldn't need to insulate the remainder of the gable up to the point of the roof. We are just about to start the roof and I would like to know if I should insist on the insulation being carried up or is there anything to be gained from it.

gillad

the insulation only needs to go past the ceiling height in your dormer room and if its insulated to 9 feet then everything is fine.

beyondpassive

The inner leaf is a major cold bridge, insulating up the gable will reduce the massive heat loss you will get as the inner leaf conducts heat into the unheated attic and cavity voids. This is just one of a number of inner leaf cold bridges. Your perimeter inner leaf sits on an uninsulated rising wall. This will reduce the performance of your wall elements by 15-25%. Any chimneys will do likewise. The junction at eaves and roof is another cold bridge.

Roof insulation boards are not effective for insulating on the slope as they can not be cut to form a continuous insulating layer at the wallplate ( which also sits on an uninsulated cavity closer block.

Your builder is not wrong, he is building to the minimum allowed standard. This is the approach we have taken for the last 18 years, as speculator standard was deemed good enough for lifetime homes. This standard of building is not ideal for one off's as due to the large surface area relative to volume, heat losses are many multiples of semidetached and terraced houses.

The most important area to insulate and airtighten are the sloping areas of roof where you have a room in the roof, these areas are exposed to the highest summer solar gain, heat loss due to exposure to a cloudless sky and heat loss due to wind and stack effect pressures.

In you case I'f you are using PIR board between the rafters, I'd seal between the boards, your insulation is only as good as the weakest point. Line inside the raters with additional 60mm thick board to remove the cold bridging thru timber and line with OSB with plasterboard over, then dryline the gable blockwork.