Garage conversion insulation how to attach to walls

[masterbanana]

Hi all, I’m converting my garage into room space. I’ve laid a DPM on the floor lapped up about 30cm with DPM tape attaching it to the wall and sealing the gap. I now need to attach PIR boards to the walls to insulate.

Can I attach these directly to the wall using e.g. foam gun grab adhesive, or should I build a stud wall and push fit the PIR and if so does it need an air gap?

Thanks!

 

[Notch7]

there are 2 ways to do it.

If you want to attach direct to the wall you probably need a DPM all the way up

the best way is to temporary fix the boards in place then fit 50 x 25mm battens vertically and screw these on -the battens can create a void for the services and something to fix plasterboard etc to. you can fit 75 x 25 battens where you have radiators if you want

 

[camerart]

Hi,
I've just done this, with building control.

I misunderstood some instructions from them and laid a damp course up the walls, then battens. They made me take off the wall damp course explaining that if there is any damp coming from the house to the wall it will condense instead of evaporating through the wall.

It's all be passed and I'm waiting for a plaster.
C

 

[freddiemercurystwin]

@camerart On all the wall or just the separating wall?

 

[camerart]

@camerart On all the wall or just the separating wall?

Hi F,
Only the external walls. This is a house not garage.
C.

 

[freddiemercurystwin]

OK that is a bit different, I'm guessing the OP's garage has walls that may be vulnerable to rain penetration but did you not have a vapour barrier or foil backed plasterboard then? If so then no moisture can get through.

 

[Ivor Windybottom]

Hi,
I've just done this, with building control.

I misunderstood some instructions from them and laid a damp course up the walls, then battens. They made me take off the wall damp course explaining that if there is any damp coming from the house to the wall it will condense instead of evaporating through the wall.

It's all be passed and I'm waiting for a plaster.
C

You've said what you didn't do.

What DID you do?

 

[robinbanks]

Can I attach these directly to the wall using e.g. foam gun grab adhesive, or should I build a stud wall and push fit the PIR and if so does it need an air gap?

I've done both with equal success, using expanding foam to glue PIR sheets to the walls, then battens with very long screws (long SDS bit, big rawl plugs) through, plasterboard mounted on the battens, wires and pipes in the void formed by the battens

If you go the stud route, consider making panels of studs with OSB attached to one side and membraned, then stood up, insulate between and over the studs, then fit a VCL and battens, wires and pipes in the void formed by the battens, buy plastboard mount back boxes athat aren't so deep you need to hack holes in the VCL (25mm battens + 12 mm plasteboard = 37mm back boxes)

 

[camerart]

You've said what you didn't do.
Hi I,
I did what the O.P. wants to do, that is, fit insulation board to the walls, remembering that they are moisture proof.
C

What DID you do?

 

[masterbanana]

Wow, so many replies thanks! @robinbanks if I go down the glue PIR directly to the wall and then batten on top (my preferred route), do I need a VCL at any point? Or just taping the PIR boards is enough? I currently have single skin red brick with a DPC installed level with the floor. I’m concerned if moisture gets behind the PIR boards - where would it go? The DPM I’ve installed is sealed to the wall with tape so presumably any moisture that comes through the brick would be trapped … or am I overthinking this?

 

[noseall]

Wow, so many replies thanks! @robinbanks if I go down the glue PIR directly to the wall and then batten on top (my preferred route), do I need a VCL at any point? Or just taping the PIR boards is enough? I currently have single skin red brick with a DPC installed level with the floor. I’m concerned if moisture gets behind the PIR boards - where would it go? The DPM I’ve installed is sealed to the wall with tape so presumably any moisture that comes through the brick would be trapped … or am I overthinking this?

Robustly tape (and foam bigger gaps) all joints and all layers of PIR and use Duplex boards etc.
Incidentally, I trapped DPM against the (single skin garage) walls, then trapped PIR against that with some studding, which was then also filled with PIR and taped.

trap.jpg

• noseall
• 26 May 2024

 

[Ivor Windybottom]

That looks like a very good method. Space-efficient and nice and solid, as the whole lot gets some rigidity from the wall, which you lose if you built an independent frame. I don't like the idea of having wood resting against the potentially damp outer wall, this solves that problem.

Does it need a vapour barrier under the plasterboard, to stop moisture migrating from the room condensing within the wood?

 

[noseall]

That looks like a very good method. Space-efficient and nice and solid, as the whole lot gets some rigidity from the wall, which you lose if you built an independent frame. I don't like the idea of having wood resting against the potentially damp outer wall, this solves that problem.

Does it need a vapour barrier under the plasterboard, to stop moisture migrating from the room condensing within the wood?

I covered it all in 18mm ply before plaster boarding

 

[Ivor Windybottom]

I believe you should have a waterproof membrane somewhere between the interior and insulation, to stop moisture from the air in the room condensing into water when it hits the colder environment behind the plasterboard.

Plasterboard and ply will not stop moisture. The coldest area behind it will be the timber. I realise it's sitting on Celotex, but it's still coldER than the surrounding celotex between the battens.

Alternatively you could put wide foil tape over the battens to straddle the adjoining celotex and form a single foil membrane. Probably the neatest way actually.

Obviously whatever membrane you use does get punctured by the screws through it. But the screws plug their own holes and it's still 99.99% sealed.

The aim is to create an impremeable membrane between the cavity and the warm, potentially humid interior of the room.

 

[robinbanks]

any moisture that comes through the brick would be trapped

It isn't going to bother the brick, and the insulation doesn't take on water ..

Does it need a vapour barrier under the plasterboard, to stop moisture migrating from the room condensing within the wood?

The wood is on the room side of the insulation. Not even sure how you'd achieve condensation there without a very carefully engineered screwup

colder environment behind the plasterboard.

If the environment behind the plasterboard is colder, something is wrong with the insulation detail

The coldest area behind it will be the timber

This is just rambling illogical nonsense. A piece of wood placed inside a polystyrene box that is filled with air at 21*C, will see that wood equalise at a temperature of 21*C. It won't automatically be colder "because it timber". It doesn't have a magical power to continuously absorb and destroy heat energy such that it remains colder than its surroundings

Alternatively you could put wide foil tape over the battens

Wouldn't make the slightest difference

Obviously whatever membrane you use does get punctured by the screws through it. But the screws plug their own holes

The screws tightly clamp a huge washer of wood over any hole, and the screw is gripped all round by the insulation it was driven through. There isn't an air route there

impremeable membrane between the cavity

There isn't a cavity. A hundred mil of kingspan with taped joints is an impermeable membrane