Damp barrier

[Animal]

Some insulation has a fair bit of damage to the foil, so need to add damp blocking on the paint layer. Is shellac impermeable enough?

[Andy Burns]

Is the damage *so* bad you can't patch-up the foil?

[Animal]

no possibility, no. The foil is behind insulation, plasterboard and plaster now.

got plenty of that.

[John Rumm]

Foil tape would be the obvious fix... what is the application?

[Fredxx]

More detail required.

Damp has a habit of making any impermeable layer flake away.

Without further detail pointing to a solution, my limited experience is to let the wall breathe.

[Animal]

I don't know what details you want. It's insulation with a damaged vb, I'm looking to paint something on to minimise damp entering the insulation & condensing. Letting it breathe is totally not the solution.

[Fredxx]

Without any details simply paint an impermeable layer where the vapour barrier should be.

Don't paint an impermeable barrier that can simply be "pushed off" through the pressure of water. That is why vapour barriers are on the reverse side of plasterboard.

There really is no simple solution. Replace any board where the foil is damaged, with foil backed plasterboard.

[RJH]

Is the foil really that critical? Surely the insulation board is pretty water tight?

[Animal]

I thought that someone might understand how VBs on insulation work & know what paint film can improve a damaged VB. It's been done with shellac. If that proves not sufficient, recoating is always possible. I didn't find any flake paint, the usual go-to for things like this.

[Andy Burns]

if you can't apply foil tape per earlier suggestions, because it's

<quote>behind insulation, plasterboard and plaster<endquote>

then how come you can re-apply shellac?

[Animal]

That makes Rodney look smart.

[RJH]

So your point is that, say, Celotex without foil is water permeable?

[The Natural Philosopher]

they wouldnt foil coat it if it were not

[Andy Burns]

Feel free to explain my stupidity in precise details, because from here you seem to have done a shit job of describing your problem.

[John Rumm]

This from the one asking vague questions without any context whatsoever!

Neglecting to mention this is already installed, and not damage on a board waiting to be installed.

Shellac at best is only semi impermeable to moisture long term. Great as a sanding sealer, good as an intermediate separation layer, and when built up with an oil finish into French polish. It is however not that flexible, and so would not be suitable applied to flexible services like foam. (assuming that is indeed where you have applied it - we don't know unless you tell us)

[Animal]

I said it's foiled insulation (ie board) with a later paint layer, which must therefore be on pb, plaster or render. Not sure what's not clear.

Surely it's obvious that unused board with vb damage can have foil applied? Hint: I'm needing to apply this on the paint layer.

"so need to add damp blocking on the paint layer"

Guess I should spend the time & money to find & use the standard ali flake paint. Had I known there was damage earlier this would have been easier.

[Andrew]

No it isn't. It is 'closed cell' foam and is (or should be) gas impermeable because all the tiny bubbles are filled with something like ?argon to improve its thermal characteristics.

The foil is only there to reflect long wave radiation, or that's how I understood its function.

Cheapo expanded polystyrene is open-cell and must be protected from water ingress. When Building Regs started mandating under floor slab insulation, many a clueless builder put the slabs of EPS on top of the DPC and then poured wet concrete over it. The water and fines from the concrete then migrated into the open cell construction rendering it thermally useless.

Extruded Polystyene (originally sold with the pink panther logo) is closed cell construction, same as 'Celotex'.

If you want to apply an additional vapour barrier to the wall that is being insulated, then a coat of bitumen paint on the wall is quite effective unless this wall is being tanked and subject to hydrostatic pressure, in which case you need specialist advice.

[Animal]

The insulation in question is mostly polystyrene foam, almost certainly expanded. In principle I could bitumen paint it, but not much would stick on top & it would thus look terrible.

Plastics are generally water vapour permeable to varying degrees, closed cell construction does not mean no VB required.

[John J]

If you know all this why are you asking vague questions?

[Animal]

Surely if you read the thread you know what was asked and why.

I see uk.d-i-y is a good bit smaller than when I used to be here. Maybe replies of the sort in this thread are one reason why.