Garage Conversion - Insulation

[TorqueVR]

I will very shortly be helping my son convert his attached garage to an extra habitable room. We're quite capable to do it but need advice about insulation. How much will we need under the floor and on the party wall next to the neighbours garage?

[smokey mow]

What method are you planning to use to insulate it and with what specific products? Dot and dab, insulation between studs, timber floating floor, masonry screed, insulation between timber joists? for the same thickness each will give you different U-values.

For the party wall you will need to consider sound insulation as well as thermal.

Are you confident that you have the required technical skills, knowledge and competence of the building regulations to act as principal designer?

[TorqueVR]

If we dot and dab I can't see how to insulate unless we fix insulation board first, and don't know if dot and dab will ahere, so we'd probably batten out and fit insulation between. With regards to the floor we could either lay insulation and a screed over the concrete which would be easier to marry up to the ground floor in the house, or do an insulated timber floor.
If we knew how thick the floor and wall insulation needs to be we can then decide. Sort of chicken and egg situation.
Alternatively we could do what many homeowners do - ignore the regulations, don't inform the building inspector and do do what we want or think is right!!

[andya7]

As a refurbishment... as opposed to an extension;

Upgrading a solid uninsulated floor to living accommodation will require a U Value of 0.25
- 70mm of Celotex GA4000 will give you a value of 0.23 with a ~75mm screed
- 60mm of Celotex GA4000 will give you a value of 0.25 with a timber board (i.e. not screeded)

You can play around with the figures to match up the level in the house; increase insulation/decrease screed

Upgrading a 215mm (?) solid wall will require a U Value of 0.30 so the following are options;
- Celotex PL3000 60+12.5 insulated plasterboard on 25x50mm timber battens = 0.30
- Celotex PL3000 65+12.5 insulated plasterboard on adhesive = 0.29
(joints to be taped and sealed at the perimeter to create a vapour control layer or install separate vapour control layer behind insulated plasterboard)

Other options are available, but the above should give you a starting point... values courtesy of the software I use

[Little Lofty]

The floor can be built up to the required height with pir with a floating 22mm chipboard floor on top, I recently used 90mm as we had to make up 110mm, luckily the existing concrete garage floor was level. The easiest way to insulate the wall is to build a stud wall and insulate with PIR, you may have to use sound-bloc plasterboard if its party wall.
If you are doing this on building notice, building control will expect you to know the regs, as you will be principle designer and principle contractor, some LABC may not let you diy this as you have to be deemed competent. They will no longer provide solutions, you are expected to build it so that it complies. You could get plans and spec drawn up and go the full plans route, labc are not keen on building notices.