insulating a room, part 2

[JimK]

so - under what circumstances would the dew point move out of a solid wall into such a cavity behind insulated plastervboard on the inside of the wall ?

Cheers JimK

[The Natural Philosopher]

When the insulation is so good that the outer brick wall is very cold

Now normally that doesn't matter, because the wall will have been vapour proofed with foil tape so moisture wont arrive there from inside, BUT the problem is that it may arrive via soaked brickwork from outside.

Opinions are divided as to whether its beter to stop it getting in by waterproofing, or let it out by not waterproofing.

Ultimately the best approach is to put some form of DPM on the inside of the wall before insulating.

That way water can evaporate outwards, and any condensation is on the far side of the membrane.

[JimK]

mmm... wouldn't any appreciable amount of water in the wall already be condensed?

if the wall is "so cold" due to the insulated lining how would such a damp wall get enough heat to evaporate moisture to allow it to condense again????

if the wind/sun dries the outer side of the wall then surely it will wick the moisture back out the way it crept in?

Cheers JimK

[The Natural Philosopher]

yes..but so what..it still results in a wet wall.

yes, that's the idea.

[js.b1]

Yes, but there is an issue of time re moisture getting back out.

Conventional house bricks are somewhat porous.

- A vented cavity uses an air gap to stop moisture tracking across, instead moisture runs down the rear of the inner leaf and is eventually drawn out by warmer weather or in extremes weep vents. In very cold then warm weather it is possible for moisture in the cavity to condense on the briefly colder inner leaf - however moisture is soon drawn out by virtue of convection within the cavity from solar gain, vents etc. The outer leaf is only 1 brick thick.

- A solid double brick wall has no air gap to stop moisture tracking across, instead the wall remains somewhat damp. This is particularly true with wet verges or old roofs where the felt drop into the gutter has rotted allowing in particular prevailing wind to blow moisture back at the wall (splashback) or even into soffits (runback).

- A solid double brick wall with air-gap formed by dot-n-dab insulation will stop moisture tracking across, but under certain situations the inside of the insulation will be the coldest surface and below the dew point so creating condensation. That condensation must be drawn back out by virtue of convection within that cavity from solar gain - but separated by *2 bricks* not just 1 as with a vented cavity. Hence moisture can both build up & hang around that bit longer than a cavity wall. That can result in condensation causing floorboard cupping to dry rot in joists, floorboards & battens. Hence timber battens tend to be treated.

It comes down to the roof/wall/exposure parameters.

From experience of a relative's house which had cement board (not PB), then vapour barrier, then battens inerspersed with rockwool, on a solid double brick wall with wet verge and prevailing wind driven rain, the battens rotted from the rear and the wall & insulation were found to be running in condensation. Some people stick a vapour barrier behind battens to basically force any condensation straight back into brickwork.

[JimK]

mmm so if i filled a cavity wall cavity with insulation.......killing the gap AND the convection, what then for the dreaded moisture?

JimK

[js.b1]

The moisture can not cross the (waterproof) insulation. Done.

[JimK]

pah! not even by wicking through ?

mmm perhaps to your satisfaction ;>)

JimK

[js.b1]

No, waterproof insulation is either closed-cell (PIR) or extruded (XPS) or silicone coated (eg, glasswool cavity bats).

:-))

Personally I am fussy about damp, it does not agree with me. A whiff of outer leaf cavity brickwork with mould on it and I am curled up feeling really pretty ill. Same goes for lofts and such like, no idea how builders go on with it.

Below ground =3D proper tanking membrane, o-ring plugs, pump. Known damp =3D SBR/Primer + Mapei Keraflex + Marmox (eXtruded-PS) + Polyurethane the joints.

Marmox is much more expensive than PIR and provides less insulation - but it is designed for wet room floors & showers so gets my vote for known damp areas.

[JimK]

mmmm so what is in a cavity wall retrofitted with insulation?

JimK

[Tim Streater]

Rockwool. Which is wool you get from shearing trolls instead of sheep.

[JimK]

so is rockwool (from trolls or othewise) "waterproof" and incapable of transferring moisture or vapour from one side to the other?

Cheers JimK

never stop learning

[Tim Streater]

Well, I frigging well hope so! This 30s house has a 2" cavity and ties in very good shape. The advice is to get rockwool pumped in (they drill a trillion holes in the outer brick). I've enquired about just this point, and the consensus seems to be that rockwool doesn't absorb moisture. But I await comments from those here in the know.

[stuart noble]

Rockwool can hold many times its weight of water, hence its use as a growing media in hydroponics. Other insulating products, like vermiculite, behave similarly. Rather like a J cloth that holds water in the cell structure without actually absorbing it.

[Tim Streater]

In which case it would break the damp barrier, wouldn't it ?

[JimK]

fx clink /fx (penny under gravity...)

JimK

[js.b1]

Rockwool put into lofts is not water resistance treated. Rockwool put into cavity walls MUST be water resistance treated for this reason.

If you put plain rockwool in a cavity wall you will have saturated inner leaf as it actively wicks water from the outer to inner leaf, within a matter of 2-3 years.

Personally I would only have polybead, but it costs the installers (much) more.

[stuart noble]

I don't know how you would treat Rockwool to make it waterproof without turning it into a solid slab. Certainly the bats aren't remotely water resistant

[JimK]

treated with.....? any refs?

JimK

[js.b1]

If you ask a BCO whether you can put loft insulation in a cavity, they will say no - indeed some will check anything suspicious.

- Mineral wool loft insulation is not treated.

- Cavity wall blown in insulation is chopped & treated with an inert water repellant.

The cavity bats might be sufficiently compressed so as to not wick moisture by capillary action and are thus not treated, or they may have a surface inert water repellant, or just rely on an airgap via an insulation spacer.