Reducing condensation problems

[Penners]

I'd welcome anyone's views on the following, please.

Mrs P has a small studio (she's an artist, and a pretty good one), which is actually a lean-to extension to our Georgian brick cottage. It has three exterior, single-skin, 9-inch, brick walls.

The studio has no central heating, but it has a wall-mounted electric convection heater, which is kept on at a low level throughout the winter months. Despite this, the room suffers from humidity problems, so we also have a fairly good quality dehumidifier in there.

In spite of all these precautions, the room has developed black mould spots on the walls behind bits of furniture and in the "dead-air" corners, caused no doubt by condensation. This is bad news for Mrs P, who suffers from an asthmatic reaction to mould.

In order to try and resolve these problems, I plan to line the interior walls with insulated plasterboard, and fit the room with a humidistat-controlled extractor fan, in addition to the convection heater. As I mentioned above, the room is already very small (2.2m x 1.7m), so I want to minimise the loss of space that this interior lining will inevitably cause. Do you think I could get away with installing the insulation-backed plasterboard by gluing it to the brick walls, or do you think it's essential that I batten the walls first, in order to create an air-gap between the bricks and the plasterboard's insulation backing?

Incidentally, the ceiling is quite well insulated, with about 250mm of rockwool between the plasterboard and the sarking felt. The roof is single pitch pantiles.

Any thoughts and/or advice gratefully received.

 

[Zebra]

I can't see the benefit in leaving a gap. There will be a dew point within the structure, and if moisture can penetrate through the inner wall surface and gather inside the wall, then it will do so on the first surface it meets which is cold enough for it to do so, probably the inner surface of the brick. In fact, by insulating the walls, you're bringing the dew point in, because the brick walls themselves will be colder than they would be uninsulated.
If there is a dew point within the wall and this is behind the insulation, then insterstitial condensation can form, and mould can still form within the wall structure. If there is a gap and the gap is ventilated, then all well and good, the moisture is carried away, but if the gap isn't ventilated then I don't see the value in having it.
One solution is a vapour control layer on the inside of the wall structure. This doesn't necessarily have to be totally impermeable, but it does have to be airtight. Any gaps, and you get the warm air finding its way into the cold structure, and condensing there. The plaster itself does provide a degree of protection, but there are still electrical backboxes to consider.
The other solution is a totally breathable wall structure, so that although moisture can condense in the wall, as soon as it warms up it evaporates away again. However, you won't get as much insulating-value per cm of thickness as you would with rigid foam.
I know most of this is overlooked by most modern builders, and is probably a little over the top in most cases. I just know about the importance of airtightness because of my own flat roof which went mouldy above the insulation because there was insufficient ventilation, and also at work I've many times felt the dripping underside of roofing felt where a hole has been cut in the plasterboard to install downlighters.
Sorry, I'm rambling now. For what it's worth, I'd say yes, stick your plasterboard straight onto the brick (if this is acceptable from a conservation point of view), and pay attention to airtightness.

 

[Feltwell]

Thoughts from the Feltwell camp......

So, at the moment, what is presumably happening is that moisture in the air is condensing on the surface of the walls as, being single skin, they are cold. Insulating should reduce this effect, but - being relatively thin single skin walls - the bricks *may* also be slightly damp on the inside due to moisture coming through from the outside due to rain etc. At the moment this can evaporate inside, especially as the space is heated.

I don't think it's a good idea at all to glue insulated plasterboard directly to the brick - I think you need a gap, to deal with any external moisture penetration. I'd batten the outside wall using treated roofing battens, with decent gaps in them to allow airflow between the "cells", then put your insulated plasterboard onto that. The cavity would perhaps be even better with a couple of airbricks venting to the outside, but then again modern houses have no venting into their cavity and they are OK with that.

Yes, you'll lose a inch or so each way in the room, but is that really so much of a problem? See - just what you want from PP - consistency :wink: Making a gap shouldn't cause any problems, even if it did end up being unnecessary.

You could even fix your battens, then staple roofing membrane across them before plasterboarding - probably OTT though, as unlike a roof there should be no danger of driven rain / snow penetrating the wall.

Would the extractor fan work? My thought is that they are designed for bathroom use and presumably cut in at a reasonably high humidity level. There's also the danger of it pulling warm moist air in from the house, unless the door is very well sealed. Why not start without, with your dehumidifier in place, and see how it goes - it wouldn't be difficult to retrofit one, especially if you put a power supply in whilst doing your plasterboarding, you can always just leave an unused fused spur in place.

 

[Penners]

Thanks Zebra and Feltwell. Your opposing views are a useful contribution to the thought process (and I'm not being sarky - they really are)!

Feltwell - yes, I imagine there is an element of moisture penetration through the bricks, but probably not a great deal, given the 9-inch thickness. However, if the inside face of the wall is bonded to an impermeable insulation material then wouldn't that tend to stop the penetration in its tracks and encourage it to migrate outwards again?

On the other hand, if there is a ventilated air-gap between the insulation and the cold bricks, won't this encourage warm interior air to strike the cold brickwork and cause condensation in that gap?

At present I tend towards Zebra's argument that glued-on insulation will tend to prevent condensation by keep the cold bricks and the warm air apart. But I'm open to arguments to the contrary!

 

[DJH]

I've had success with foil backed insulated plasterboard fixed directly to damp walls using good old expanding foam ( :shock: ) The foam grips well to both the foil and a damp surface after all, it forms part of make up the sticking plasterboard to the foil and foam loves a damp surface/atmosphere to cure nicely. Easy and quick to do giving time to straighten each board as you go.

I've also cut off the insulation from the bottom of the plasterboard in a series of sharks tooth or zig zag pattern about 8" high at each point and fixed grills/vents on the skirting to aid dispersal of any evaporation.

Twelve years on and no problem so far...did I just say that?

Doug

 

[Feltwell]

I confess I'd missed the 9 inch part of the first mail, just read single skin and assumed 4 1/2".

My only (unscientific!) other thought is that all new mass-built houses are built down to a price, whilst having to satisfy building regs. If they could get away with single skin brickwork with insulated plasterboard bonded directly to it then surely they would? There has to be a reason why they bother with facing bricks, cavity (often filled with insulation), blockwork, dot and dab plasterboard.

Single skin brickwork with insulated plasterboard bonded directly to it would surely be much cheaper - a big labour saving, which is where they see the benefit rather than materials. I can't help but think that if they could get away with that then they would, they'd get the necessary U-value just by increasing the thickness of the insulation bonded to the plasterboard, but there must be a good reason why they can't do this. Modern timber frames still have a cavity and brick facing, although the brick is a facing only and has no structural purpose.

 

[Lime]

I know this is a very non-period-property solution but I'd be tempted to use 2mm polystyrene glued directly on the bricks using a good wallpaper adhesive.
It is simple and effective, cheap and quick to put up and able to be removed leaving minimal damage.
http://www.gowallpaper.co.uk/saarpor-heatsaver-insulation-polystyrene-2mm.html

 

[mattice]

If you go down the route of increasing ventilation you could look into a positive pressure vent and/or an integrated heat recovery unit rather than an extractor. I remember finding some single room/flat solutions for 2-300 that seem to be generally well received for reducing condensation and should be easy to install.

 

[Zebra]

Even if there were rain penetrating the brickwork from outside, then having it circulating around inside a tiny cavity, rotting your battens, is little better than having it sit there until the warm weather takes it away again the same way it came in. So if the cavity is ventilated, all well and good, but if it isn't then there's no point in having it.
I don't think ventilating the cavity to the inside is a good idea, as you'll allow even more moisture into the gap.

 

[Feltwell]

I don't think ventilating the cavity to the inside is a good idea, as you'll allow even more moisture into the gap.

I'm with you on that - ventilating to the outside would be a much more sensible option but I guess again means the sealing of the new internal insulation will need to be very good.

 

[Zebra]

Yes, and ideally, this "vaour control layer" would be as near as possible to the inside, to prevent moisture from getting into the wall structure. There are all sorts of tapes and membranes on the market which help with vapour control, airtightness and with sealing up gaps. I've use a couple of rolls of "Tescon tape" to seal up tricky gaps and ensure the continuity of the airtightness layer. But I've also improvised where necessary - frame sealant, silicone....
It's particularly important where you have a delicate structure to protect - a timber frame for example, but this situation seems less vulnerable. Is it soft porous bricks with Lime pointing Penners, or new hard bricks with cement pointing?

 

[Feltwell]

I've use a couple of rolls of "Tescon tape"

Is that the tape that overstates its profits then tries to stick itself desperately over the gaps and hopes that no one notices? :wink:

 

[Flyfisher]

While sharing some of Feltwell's concerns about a humidistat-controlled fan working effectively, if it did would there be much need to add an insulating layer to the brickwork anyway? If the fan effectively removes humidity through ventilation then there may be too little to condense on the cool brick walls.

Insulating AND extracting at the same time may well work, but it won't be known which or if both are necessary. Would it be a fairly simple job to installed a suitable extractor fan first and see what happens? Sounds like less work (and cost) that a complete lining/insulation job, and wouldn't preclude that later should extraction alone not be sufficient.

 

[Penners]

Many thanks for those further suggestions, everyone.

Doug - When you stuck your insulated plasterboard to the wall with expanding polystyrene foam, did the expansion of the foam not push the board away from the wall over a period of an hour or two. causing misalignment of the boards and unwanted gaps behind? In places where I have used that foam, I've always been amazed by just how much it expands and "overflows" the gap. Or did you find that the mass of the board was sufficient to resist being pushed?

Lime - your suggestion did pass briefly through my mind when I was thinking over the problem, but I've always been rather sceptical of the insulation capability of a 2mm thickness of foam. Do you have any experience of the material? Does it really insulate a cold wall sufficiently to eliminate condensation? A "period" solution is not important in this case - the house isn't listed and the structure in question was added to it within the last 20 years (as a potting shed!).

Feltwell - I really don't know whether "downmarket" builders would be allowed under the Building Regs to build in the way that you suggest. I can only imagine (perhaps naively!) that the regulations are not blind to other factors besides insulation values, and seek to ensure that new builds have some kind of minimum longevity, too. But I'm just musing...

Mattice - I'm not familiar with the technology that you've suggested. Please could you point me to a website with a typical unit?

Zebra - the structure is relatively modern - about 20 years old - and was built with moderately hard facing bricks/cement mortar. I get what you say about doing everything possible to isolate the exterior brick wall from the interior warm air. The fixing method that seems to be advocated by the insulated board manufacturers is a continuous bead of silicone sealant around all four edges of the board, supplemented by a grid of dabs of sealant across the fixing area. Fire regulations require two mechanical fixings halfway up the board.

Did you use Tescon tape on the interior face of a butt joint between boards? Does it skim OK with plaster?

Is that the tape that overstates its profits then tries to stick itself desperately over the gaps and hopes that no one notices?

:lol: :lol: :lol:

 

[Penners]

Would it be a fairly simple job to installed a suitable extractor fan first and see what happens? Sounds like less work (and cost) that a complete lining/insulation job, and wouldn't preclude that later should extraction alone not be sufficient.

Yes, that would certainly be the simplest/quickest solution, FF. I suppose I'm just a bit sceptical about whether it would be sufficiently effective without some kind of further condensation prevention on the walls. As I mentioned above, I have noticed that the black mould is forming particularly in the corners of the room and behind bits of furniture, from which I assume that the air in those locations is not being moved sufficiently for it to pass through the dehumidifier. So I wonder whether a humidistat-controlled extractor would have any better success.

But I take your point that further work could be done later if the humidistat fan failed to provide a complete solution.

Dilemma, dilemma....!

 

[Lime]

Many thanks for those further suggestions, everyone.
...........
Lime - your suggestion did pass briefly through my mind when I was thinking over the problem, but I've always been rather sceptical of the insulation capability of a 2mm thickness of foam. Do you have any experience of the material? Does it really insulate a cold wall sufficiently to eliminate condensation? A "period" solution is not important in this case - the house isn't listed and the structure in question was added to it within the last 20 years (as a potting shed!).
.............

I can confirm that in my experience the tiny 2mm of polystyrene does prevent condensation from forming.
I have not used it recently but in the 60s, when I owned a large house let out into flats, condensation was a problem on the top floor ceilings.
Particularly where the plaster and lath ceiling was directly attached to the rafters.
Several of the flats had gas cookers and the condensation created would pour off the ceiling in rivulets.
2mm of polystyrene completely eliminated the problem.

 

[Penners]

That's interesting to note - many thanks. It's set me wondering whether a slightly thicker version of that wallcovering might be available for even greater efficacy - I shall enlist Mr Google's help in investigating!

 

[DJH]

No Penners, I didn't find that happening. You only need a sting, or line of foam, applied to both the sheet and wall in a random pattern and hey presto, job done. It's very efficient. I have, in the past, on very uneven walls with voids behind marked the boards before sticking them to the wall and, when the foam has cured. drilled a small hole and pushed the nozzle of the gun through and made good any voids that way.

You could, of course, use the proprietary mushroom head plastic annular ring shank nails which are fixed by hammering through 8 mm holes drilled through the board where needed. The heads are driven just into the face of the board without breaking the facing paper the downside of which is that they then need stopping in. Another downside is that if the wall is very uneven then the fixings can distort the board matching the unevenness. Plastic nails don't rust and don't form a cold bridge between the cold wall and warm face of the finished surface of the board.

It's important that a foil backed board is used to maintain a vapour/damp barrier against the cold/damp wall.

Happy New Year,

Doug

 

[DJH]

Two of the rooms in our house had that polystyrene foam on the outside walls with a wall paper covering on top. It all came off in one piece and hadn't stuck to the walls at all well. As Lime says a good quality adhesive would be required but I'm not sure what that would be especially on a damp surface rather than a dry lath and plaster as Lime describes.

Cow gum perhaps?

Doug

 

[Penners]

Cow Gum?! I haven't seen that since I worked in a design studio in the 1960s!! I imagine it's suffered a calamitous drop in demand since the advent of Spray Mount.

Thanks for your thoughts and advice, Doug.