Passive or Vapour vent / Internal insulation

[greenfox]

Well folks, you've certainly left a gal a little confused

I have to confess I was a little worried about using wall insulation after my first post; I'd done some more reading and it seemed to be quite risky (mould developing in or on the insulation, or the issue moving elsewhere). I'm going to be doing all I can to reduce moisture and increase ventilation in the whole flat so I was erring on the side of cleaning the area well, treating with anti-mould paint and possibly including vents in the plinths.

JohnD - would that be your preferred approach if this were your flat?

I've also decided on kitchen units with an 80mm service void, rather than ones that fit snug to the wall.

I have only had the place a couple of months so haven't been through a winter yet. The wall behind the units has always felt dry, it just looks awful, the wooden units and worktop are black at the back and it all smells very musty. The worst-affected area is where the two external walls join. They're facing NNW (worst of the two) and WSW - the space outside is open and it can get very windy. I'm assuming (hoping) that the people who lived here before just weren't vigilant about ventilation. There is currently a pretty rubbish kitchen extractor but there wasn't one in the bathroom when I moved in.

 

[mointainwalker]

Lets all beat JohnD up

Take 200 lines Deluks, saying :

"I must not split my infinitives "

Greenfox

I'm not sure what you are reading but I cannot envisage any circumstances in which mould will appear on ( and certainly not in ) insulation as it will always be warmer than surrounding materials. I suspect you must be reading about mould due to moist air penetrating insuffiently -sealed insulation and growing in the cracks between individual panels.

You may have been overwhelmed with information but you appear to have forgotten things from your previous post. If you prevent moisture- laden air from coming into contact with the cold wall then it cannot condense.

Sacrifice some of your 80 mm to either dot and dab a piece of insulation backed plasterboard onto the wall or make yourself a small stud wall > The latter is much more work but described hundreds of times on here - do a search.

I get the inpression that the wall in question isn't huge so buying one sheet of pb ( 2500 x 1200 ) would probably cover everything in one go and this would then solve the problem of moist-air hitting wall : insulation backed pb being impermeable


Your wall will still be cold - in fact colder as it is not getting even less warmth from the kitchen - but you will be shielded from this by the insulation.

End.

 

[JohnD]

Lets all beat JohnD up

Take 200 lines Deluks, saying :

"I must not split my infinitives "

That's not an infinitive.

Anyway, in English, it is entirely permissible to occasionally split an infinitive. In some other languages, including Latin, infinitives are single words, which is (probably) why people trained in the grammar of those languages thought it must be in some way "wrong" to carelessly do it.

Attempting to slavishly follow grammarians rules can lead to ugly sentence construction, which is something up with which I will not put.

...I'm going to be doing all I can to reduce moisture and increase ventilation in the whole flat so I was erring on the side of cleaning the area well, treating with anti-mould paint and possibly including vents in the plinths.

JohnD - would that be your preferred approach if this were your flat?

If it was me, I think I would
(1) increase ventilation of the whole kitchen to reduce humidity (extractor fan and trickle vents) and forbid the draping of wet washing around the house
(2) if still necessary, introduce ventilation to the exterior behind the cabinets (air from outside is generally colder, and therefore containbs less moisture, than internal air). This could be done by drilling a couple of 18mm holes through the wall. Depending on wind direction, this would tend to make the cabinets colder (which may be OK for food storage) and possibly introduce draughts. However there might already be some draughts e.g. throgh a suspended floor or a plumbing duct. In which case just cut off warm moist airflow from the room into the behind-cabinet space.
(3) if still necessary, apply an insulation layer with an impermeable layer on the room side, to the wall. This will prevent warm, damp air reachiong the cold wall. The aluminium quilts might be a suitable choice. It is very important to have no gaps in the wall covering, including at skirting board or corners, and to overlap it onto internal walls at the ends.

 

[greenfox]

Hi, thanks very much for the replies - bit of a delay as I couldn't get online for long enough to post yesterday...

I suspect you must be reading about mould due to moist air penetrating insuffiently-sealed insulation and growing in the cracks between individual panels.

Yes, that's exactly the sort of thing I meant - I'm worried about getting the seal right all the way around, including to the floor. It's a concrete floor - so assume that's also pretty cold in the space behind the units. One wall is 3.1m long and the other is 2.3m - the units run the length of both, so I'd have to insulate both of them for the whole length. The shorter one has a load of metal service pipes behind, some of which I'm sure are defunct, plus the sink and mains water feed to the flat etc. I'd have to cut the panels to work around these and ensure 100% seal round everything. I can understand in principle that insulating the wall would help but concerned if I don't get it exactly right I might just end up with a much smaller cold patch than before. So a smaller space for water to condense --> less effective evaporation --> more mould
Also I've ordered my units now and don't want to lose the space as it's going to be snug.

If you prevent moisture-laden air from coming into contact with the cold wall then it cannot condense.

This is of course the salient point - and making the air less moist in the first place is the obvious start. With the two extractors as mentioned previously and also a vent on the new front door when I get it. (There is no wall between this door and the kitchen area.) Plus leaving vent holes on all windows open.

Now to address the specific question about whether to introduce external air to the space behind the units:

Woody said it might make the condensation worse
Deluks suggests introducing heating in the area plus the addition of plinth vents
JohnD suggests 2 x 18mm vent holes to the external wall but cut off warm, moist room air (assuming there are no existing draughts, which there aren't). How would you cut air supply off, do you mean just don't have vents in plinth?

I know the very simple aim is not to have the moist and warm air from the kitchen come into contact with the wall behind the units - but I just don't think it will be possible to seal it 100% - the units themselves aren't airtight for a start.
If I introduced a small vent (or 2 x 18mm holes) in the external wall behind the units AND vents in the plinths, would that not provide enough airflow to prevent condensation, given that I'll have a front door vent and vent holes on the windows in the rest of the flat? Or will it just allow too much warm moist air in - and hence worsen the condensation?!

(1) increase ventilation of the whole kitchen..
(2) if still necessary, introduce ventilation to the exterior behind the cabinets
(3) if still necessary, apply an insulation layer...

It's the "if still necessary" bit that bothers me; once the kitchen is in, access will be impossible and I'm not going to want to rip it out.
However, if I did all 3 points in your post, this would equal my own third option from my OP! Ok I suggested an actual vent instead of holes but the principle is the same: both insulate and ventilate the area. (Also when I said insulate so the "wall doesn't get as cold" I was referring to it from the perspective of the warm moist air. I understand how the actual wall itself will be colder but we wouldn't care about that as it would be behind the boundary of warm indoor air and insulation layer.)
I've pretty much dismissed the insulation idea for the reasons above, I just need to sort out whether to include an external vent (and what size) and if I do, whether to also vent in the plinth.

These steps are decided:

1. remove all traces of the black marks, and clean with bleach and anti-mould cleaner
2. paint entire walls (plaster) and floor (concrete) below worktop level with anti-mould paint
3. fit (and use vigilantly!) kitchen and bathroom extractors
4. fit a decent vent to the front door and slide the existing window vent holes to permanently open (will look at retro fitting bigger vents to the windows too)
5. open windows where possible and reduce moisture-generating behaviour (no wet laundry, cooking without pan lids etc)

Thanks again for the help - I feel like I'm making progress on understanding something that despite having simple laws of physics governing the factors, is actually a pretty difficult problem to solve in a real-life setting!