Bay Window insulation

[frank999]

I am insulating a small single skin brick bay window, creating stud inside with friction fitted PIR between studs, plaster boarded over.

50 or 100 mm PIR ?, I'm simply try to stop air condensing on the bay wall and stop damp stains in winter.

Should I have an air gap between PIR and existing existing bay structure, say 50mm ?

Some suggest using combined plasterboard with PIR backing - assume you cant buy that with built in vapour barrier, and vapour barrier then has to go somewhere ? using combined that doesn't make sense.
By using separate boards you can staple vapour barrier to studs directly over the PIR, then separate plasterbaord over top of that (as separate boards) - as normally done.

Any thoughts or guidance please.

Thanks

 

[John D v2.0]

If you're purely stopping condensation then even 20mm of polystyrene stick to the wall would be ample. The trick is doing it without leaving any thermal bridges though as the smaller area without insulation, the wetter it'll get.
Basically you need to but up to the window frames and basically leave no original wall uncovered by a good layer.

 

[frank999]

Hi John, thanks for that straight forward answer.
I'll go with 50mm PIR ans 50mm air gap, should you include a vapour barrier still, I always thought you should be stopping moisture from the room penetrating any air gap behind a 'wall'.

 

[John D v2.0]

Yes definitely stop damp air from the room getting to the cold side, it's hard to do that so avoid using too much insulation then you'll reduce the risk of water building up.

 

[frank999]

Will create vapour barrier between plasterboard and PIR.

"it's hard to do that so avoid using too much insulation then you'll reduce the risk of water building up."

... guess you cant plug every gap or make it hermetically sealed from the inside, although you can try.
I thought the thicker the insulation the better though, if you manage to create a good vapour seal between inside and outside - and the heat is restricted as much as possible from radiating through the insulation then you will reduce condensation in the void.

If condensed moisture is going to be present in the void, should some sort of weep vent not be installed ?

 

[John D v2.0]

Indeed you can't perfectly seal, but you can improve things. As with most things to do with moisture, you either want it to be able to dry faster than it gets wet on average, or you want to have a drained cavity to outside (ventilated too if there is timber in there).
The thicker the insulation, the worse if the permeability is lower towards the outside.
Example: the wall and insulation have equal value, but the wall is impermeable and the insulation is wool.
Indoor temp 20c, outdoor 0c.
The surface of the wall will be 10c, so you might get condensation.
New example, the insulation is 9x as good as it was.
The surface of the wall is now 2c, so you're pretty much guaranteed to get condensation.

Improve the vapour control layer and you reduce the risk.

You can take sections through the wall and draw graphs of humidity against temperature if you're really inclined - sorry about the size

 

[noseall]

Here's one we did earlier....

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Bow2

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Bow4

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Bow6

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We fitted 75mm betwixt studs, foamed and taped joints including any gaps in the studs or where air could get up through the floor.
We battened the plug back boxes off the studs so that we could then fix another 50mm across the studs. these were also taped. We then fixed foil back boards and skimmed.

 

[frank999]

This vid explains quite well:

Thanks John - I called Celotex, but they have no charts for heat loss through their PIR, a chart would have been interesting to do.
Will need to look at your diagram a bit more - still don't understand that by increasing the insulation thickness (and therefore reducing the temp. on the face of the PIR) you get more condensation - assuming a sound VCL.
Does this not help keep the temp in the air gap lower, and outward face of the PIR lower - I thought condensation occurred when a colder moisture laden body of air comes into contact with a warmer surface (or air front - hence we then get rain).

Noseall - Celotex suggested the additional 50mm layer over the top - as you have done, so you have gone for two layers of PIR (75 and 50mm), and tried as best to form an impervious Vapour Layer.
I will assume yoo only have 25mm gap ?
You have no air venting for this void ?

I have plaster over the brick work, guess that should be stripped off as it would just act as a sponge, drawing moisture in.

 

[John D v2.0]

Assuming a perfect vapour seal on the inside, there won't be a problem, but that's a big assumption.
And condensation occurs when warmer air hits a surface colder than it's dew point. So your bathroom window when you have a hot shower for example.

 

[frank999]

Condensation is a'condensing, and wall paper will be a'gettin damp - finally getting round to doing this job.

I am thinking of leaving the plaster on the walls, (we have stripped the wall paper though).

I am concerned about moisture condensing on the foil back of the PIR in the 25mm air void (we are fitting 75mm PIR, foil taping, adding an additional VCL - in case of movement, then plaster board on top) any moisture could drip down onto the existing floor boards and could cause issues else where, I am thinking of running a line of foam along this junction (PIR and floorboards) to at least cover the wood and protect it, any moisture can gather and be held against the back of the PIR and wall (we will chip away 50-100mm plaster at base of wall to limit soaking in).

Vents will be installed through the single leaf brick barrel wall to try help clear any moisture that does gather, I am considering just small 35mm ones, as opposed to 70mm, simply because I do not wantt o disturb the brick work too much.

Any thoughts appreciated.