ᐅ Insulating a Garage Roof: Correct Order of Installation

I have now had three contractors come to seal our garage roof, and I have received three different opinions and price quotes.

For a 6x6 meter (20x20 feet) garage, the prices range from 1400 to 3800 euros.

I just want a watertight garage that doesn’t develop mold inside.

What is the correct method for sealing a garage roof?

Are two layers of bitumen enough?

Is a layer of insulation required in between?

Is a vapor barrier needed?

How was it done in your case?

So, the garage roof should be very well sealed – I’m not a fan of bitumen. I used EPDM membrane without a single seam – just one continuous surface. It’s not cheap (the material including adhesive costs just over 1000 euros), but it’s watertight and, most importantly, any damage spots are immediately visible.

You don’t need to install insulation underneath it (i.e., above-roof insulation), but whether you need insulation below is up to you. I stuffed 200mm (8 inches) mineral wool between the rafters.

You’ll also have to install a vapor barrier, because especially in summer there is a lot of moisture that gets into the insulation and then into the wood. I installed a climate membrane simply because it compensates for minor sealing defects. And it can’t be perfectly airtight anyway, since I still have a gap between the brickwork and the concrete lintel of the door (though a trim will cover that) and at the back the insulation goes directly against the brickwork – is that airtight? Well, I can’t do it any other way.

I installed the insulation and vapor barrier myself.

If I had a garage with a roof truss, I would probably do it the way you did, but I have a cast concrete ceiling.

I saw this membrane or a similar one on YouTube and I suspect it might be the best solution, but the cost is quite high.

You can definitely make it waterproof with bitumen, I think, but inside it should not get wet due to condensation.

However, it will get wet inside unless you provide forced ventilation.

If the garage is unheated and well sealed, I don't see why the interior would get damp from condensation...?!?!? Of course, if you drive in with a soaking wet car during summer at 35°C (95°F), it will get a bit steamy inside, but enough to eventually cause mold? Hmm...

If it is cool inside, you will quickly experience problems with condensation in the summer.