Garage conversion insulation how to attach to walls

[Ivor Windybottom]

Sigh. All points missed. Do you know what a vapour membrane is? Let's start with the basics.

 

[Ivor Windybottom]

It's an impermeable membrane that should be between the room and the insulation.

If there isn't one you'll get damp within the insulation.

Yes there's a cavity even in this case, it's between the plasterboard and wall and happens to be full of celotex but it still performs the same function as a cavity.

 

[Ivor Windybottom]

There's a transition of temperatures across the cavity - from room temperature on the inside, to outside temperature on the outside.

If you let that warm damp air from the room get into the insulation then it's likely to condense at some point, which will cause damp, rot and smells.

 

[Ivor Windybottom]

Plasterboard and ply are not impermeable.

 

[noseall]

It's an impermeable membrane that should be between the room and the insulation.

Eh?

Are you confusing vapour control with a DPM? It sounds like you are.

Vapour should not migrate from the warm side to the cold side - VCL - fitted behind the plasterboards via sealed PIR foil coated boards with foil tape etc.

Damp/rain/water should not permeate into the internal structure - DPM (impermeable membrane or visqueen)

 

[noseall]

There's a transition of temperatures across the cavity - from room temperature on the inside, to outside temperature on the outside.

If you let that warm damp air from the room get into the insulation then it's likely to condense at some point, which will cause damp, rot and smells.

Warm damp air can get to the internal face of the insulation, but not beyond it to the cold side.

 

[freddiemercurystwin]

Eh?

Are you confusing vapour control with a DPM? It sounds like you are.

Vapour should not migrate from the warm side to the cold side - VCL - fitted behind the plasterboards via sealed PIR foil coated boards with foil tape etc.

Damp/rain/water should not permeate into the internal structure - DPM (impermeable membrane or visqueen)

No he's not, a VCL is a not a vapour barrier they are not the same things. You fit a Vapour Barrier behind plasterboard, that is an impermeable membrane not a Vapour Control Layer which typically is not airtight. If you wanted you could swap a VB for a DPM fitted imediatly behind the PB in a wall situation they would achieve the same thing.

 

[noseall]

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

The wood is at room temp, due to the continuous layer of insulation, separating the wood and the garage wall.

 

[noseall]

No he's not, a VCL is a not a vapour barrier they are not the same things. You fit a Vapour Barrier behind plasterboard, that is an impermeable membrane not a Vapour Control Layer which typically is not airtight. If you wanted you could swap a VB for a DPM fitted imediatly behind the PB in a wall situation they would achieve the same thing.

With regards to my image I posted, the black polythene is acting as a DPM. The foil tape is dealing with the vapour etc. I'm a bit lost on Windy's explanations and terminology in respect to that.

 

[robinbanks]

If there isn't one you'll get damp within the insulation.

Not inside a sheet of aluminised kingspan you won't. WUFI can claim the dewpoint is in the middle of a big block of PIR all it wants; you aren't going to see water dripping out the bottom of it

a cavity .. happens to be full

No. Go and look up the definition of "cavity" in a decent dictionary

 

[Ivor Windybottom]

Sigh. Way too much armchair keyboard warrioring going on here.

If there isn't an impermeable membrane (whatever you call it) between the room and the insulation then condensation will accumulate within it.

If that block of insulation includes wood then it will rot, treated or not.

Most people don't like the look of polythene for internal decor, so it's commonly put behind the plasterboard. Or under the floorboards in the case of a floating floor. Basically as close to the interior of the room as you can reasonably get it.

Otherwise, if there is unventilated insulation between the room and the membrane then humidity from the room may condense within it, as the warm air from the room penetrates through the plasterboard, cools and its dew point is reached. It doesn't matter if it's not "cold", if it's coldER than the room then there's potential for condensation.

Foil-backed plasterboard is one solution. Or a polythene membrane. Or celotex IF it's a contiguous layer. If it's interspersed with timber then putting wide foil tape over the timber would be OK-ish too, if care is taken over it. Just as long as there's a single airtight layer of something directly behind the plasterboard.

 

[noseall]

If that block of insulation includes wood then it will rot, treated or not

Not if the insulation is detailed correctly, as in how I showed it in the image, ie. a continuous layer of insulation behind the studs. The timber with then be 'warm' as in the same temperature as the room.

 

[Ivor Windybottom]

I disagree. The timber will be colder than the room, it has a U-value and is effectively absorbent insulation. There's a potential for condensation to gather within it.

 

[Ivor Windybottom]

I do like your way of doing it. In fact I'll probably steal your idea when I convert my garage. It gives you a nice solid, well insulated and damp-proof internal wall. Without the issue of the wood being in contact with the potentially damp outside wall.

But I'll add something between the plasterboard and the wood. Probably foil tape, mainly as I messed up an order and have a massive box of 75mm wide stuff sitting around.

 

[noseall]

I disagree. The timber will be colder than the room

Nonsense. It's how ALL warm roofs are detailed. So a wall with a continuous layer of insulation across it, protecting the timber studs, is no different in that respect.