Positioning of Insulation

dj01

Will be starting a new build soon enough, house is a dormer bungalow. I'm trying to find out what common practice is relating to the part of the pitched roof that would just cover storage attic space. I was planning to just insulate only the part of the pitched roof that runs over the rooms, i.e. not insulate the pitched part of the roof that goes from the partition walls out to the external wall, if that makes it any clearer?

Anyone any experience of this?

Thanks.

Aeneas

Our builder did not insulate the part from the partition (ie the bedroom walls)to the outer wall. One result was that wind and air movement from the ventilation inlets flowed under the floors and entered the bedroom with considerable heat loss, particularly on windy days. Luckily we had left small doors into the roof cavity for storage so that I was able to put insulation in to prevent wind blowing under the floors. It might also be a good idea to get your builder to put extra insulation behind your bedroom walls. Upstairs, dormers are in effect timber frame houses with nothing between you and the elements except the plaster on the walls and the slates on the roof.

De_man

on the pitched part of the roof, softboard (see viking house threads)
on the internal pitched part of the roof i used "rafterloc" as from experience rockwool compresses over time and doesn't leave enough of an airgap for roof ventilation.

joists 300mm rockwool, with 3/4" ply just resting on top (for the moment) ceilings upstairs 38mm kingspan TW52 insulated slabs, (what a job to put up) downstairs ceilings normal lafarge foil back slabs

all external walls "platinum" aeroboard sheets in the cavity, all external walls on the inside 38mm kingspan insulated slabs fully bonded and mushrooms

overall very expensive but i hope to save in the long run

hope this helps

dj01

Thanks for your suggestions. Aeneas, do you mean the builder didn't insulate between the floor joists outside the partition walls, or the pitched roof area out past partitions, or both?

The house will be timber frame, so I'm planning on insulating it well. On the (inner) external framework I'll be putting in 80mm Kingspan TW55 rigid insulation between the studs and then closing with K18 42mm insulated plasterboard. I had planned insulating the complete first floor right out to the external walls to prevent what Aeneas had experienced with drafts, but just wasn't sure on the pitched area outside the partition walls.

De_man, when you say 'what a job to put up' in regard to the Kingspan TW52, do you mean it's messy and slow with having to cut the insulation to size, and fitting it to smaller area's etc?

Aeneas

The builder insulated neither the floor joists to the outside wall nor the pitched roof. When insulating the floor joists make sure that the insulation comes above the joists to make a wind proof seal against the partition. If your upstairs internal walls are well insulated I would see no great advantage in insulating the pitched roof element - the floor insulation will insulate the rooms below as well as preventing wind and air movement upstairs. Also the cavity under the pitched roof will be permanently ventilated.

De_man

dj01 wrote:

De_man, when you say 'what a job to put up' in regard to the Kingspan TW52, do you mean it's messy and slow with having to cut the insulation to size, and fitting it to smaller area's etc?

no, not at all its very easy to cut and to fit, it's just a bit heavier to lift when compared to the normal slabs, say with a normal slab i can manage them with two of us, for the upstairs i had to draft in one extra

dj01

thanks for the advice, very helpful!