Garage Conversion - Complete Novice

I spent a lot of last year researching whats needed for this very job and am currently midway through doing the work, so hopefully can help

For the walls I'm using 50mm polystyrene in between timber posts which will then be covered by DPM and finished with OSB. I've secured the posts to the floor and concrete wall using angle brackets and concrete screws - the posts are 47mm deep (accommodate 50mm poly just fine), but actually come out as far as 60mm from the wall, due to a concrete slope between wall floor. This suits me as I wanted a bit of an air gap between the wall and the poly anyway. I have a colleague who used Grip Fill (the green one, don't get the solvent free crap) to secure timber and PIR to the walls directly, but my concrete sectional garage walls are very uneven and I prefer fixings to adhesives (although having spent the weekend drilling into a concrete floor with a cordless combi drill, I'm questioning my life choices!)

On the roof, due to space constraints (and not wanting to remove the bitumen corrugate I fitted last year in order to do it properly), I'm putting 50mm polystyrene in between the purlins, with 25mm Celotex (PIR insulation same as Kingspan) secured to the underside. This should be covered by OSB or similar too, but I can't afford to lose the height as I want to fit a personnel door in the side and won't have the clearance.

You absolutely must have a membrane covering every wall and ceiling. If you're using PIR insulation, the silver foil is the membrane. Buy some aluminium tape to cover joins, screws and the inevitable holes you'll put in the stuff while handling it.You can buy proper breathable membranes, but I bought too much 1200 gauge DPM for disposing of the asbestos roof, so I'm planning to use that for my walls - plus it's a lot cheaper (Toolstation were great when I shopped around for DPM). The important thing is to stop any moisture being able to get through to your insulation, so DPM on the warm side.

PS my Celotex delivery had some Xtratherm sheets in too and both are foil coated on both sides. I assume Kingspan will be the same.





(Excuse the botch job - my theory is sound.. my skill in actually implementing it less so!)

I'll try and help if you've got any other questions