Insulating against integral garage.

28 Feb 2017
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United Kingdom
I've just had an extension constructed whereby we have an integral garage with a bedroom above and utility to the rear. Garage is unheated, door is non insulated and has an external cavity wall. Partition wall to rear of garage into utility is single skin blockwork with door opening.

Looking at building regs, it suggests I full fill the 200mm ceiling voids with wool. From another job, i've got a couple of 100mm insulation boards spare and wondered what the best way would be to use the boards to prevent the room above suffering cold.

1. Boards on their own should be up against the floor. Should I use some wool alongside it to get a better job? If so, should I put this between floor and PIR board?

2. The blockwork partition between garage and utility needs insulating. I can see a potential cold bridge already as the wall is between joists and pretty much touches the chipboard above it. Building regs says 100mm insulation boards against the wall. If I was to put 50mm either side, would this achieve the same as 100mm on the habitation side? I was hoping that by insulating both sides, then any cold bridge would be reduced upstairs.
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100mm boards on the underside of the floor, and then 100mm or more of quilt in the rest of the floor void. Make sure any joists are sealed if they go in to walls. Double layer 12.5mm plasterboard and intumescent seal to the perimeter.

For the stud wall, 100mm between studs and 25mm up one face. There is no cold bridge in practical terms.
Not being funny but if you have already had this built haven't you contacted building control regarding building regulations approval :?:
Thanks for replies. Regs has approved a full fill of the floor void. With 100mm boards and 100mm wool then it'll remain full fill.

Woody, if I can just ask, when you say seal the joists, what with?

Also the stud you mention is a single blockwork partition so it's a solid wall. They stated 100mm on the face of it. Just wasn't sure if it could be split between garage and habitation sides.
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Woody, if I can just ask, when you say seal the joists, what with?
Think he said :-
Double layer 12.5mm plasterboard and intumescent seal to the perimeter.
I would also add 5mm skim coat to double layer of 12.5mm pl'bd and skim to give full half hour fire resistance over garage.(that's what building control usually ask for over integral garage )

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