Insulation board, batten and moisture.

Postby P J Lenny » Fri Oct 15, 2010 1:20 pm

I have two solid block (1950's) exterior (north facing and west facing) walls in a bungalow bedroom. These are very cold walls and often any attempt to warm the room results in condensation. The walls are plastered, semi gloss painted, in good condition and with no signs of damp.
This solution has been recommended:
25mm battens with K18 insulation boards fixed to them (k18: 25mm+12.5mm plasterboard)
My concern: Because there is a 25mm air gap between the insulation board and the wall, is this an invitation for moisture to get trapped in there and cause problems down the road?
Are there any precautions to take with this solution?
:idea: :?:
P J Lenny
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Simply Build It

Postby P J Lenny » Mon Oct 18, 2010 9:48 pm

P J Lenny wrote:25mm battens with K18 insulation boards fixed to them (k18: 25mm+12.5mm plasterboard)
:idea: :?:

Sorry that should read 50mm+12.5mm plasterboard
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Postby 30yearsinthegame » Sat Oct 23, 2010 6:26 am


before you batten out and board please make sure you try this first>

1 if you have air vents make sure they are clear.
2 if no air vents are installed , install them.
3 double glazing with air vents.

condensation arises from warm air meeting cold air. ie on windows and cold walls. warm air is heaver than cold air and tends to form condensation above waist height. poor venetlation is the problem. when warming the romm at night try to open the window just a tad, this will allow for the warm air to pass, i know you will lose heat but this is a problem in this country with cold wintery nights. i have vents on my tripple glazing and still sometimes get some, my fault really because my heating is always on.

if you batten out you can not use k18 because you will be left with a gap. you will have to treat it like a cavity and use kingspan tw50 and upwards this has a foil backed coating both sides, exspensive but the best stuff. do not use polyboard as we call them because if there was any damp it would collect mould.

think very carefully about battening out by the time you went around the room you would lose a few sqm.

remember when battening , if you do, to wash the walls down with suger soap and treat the walls with an anti fungal paint. you can buy the paint ready made or buy an add mix.

the big one... remember to put up the battens with a membrane behind, use a brick and block dpc which is cheap and chearfull and will do job fine, put the batten to the wall make it level and drill your pillot holes, only when that is done you can put your dpcup and batten, try not to go too mad with packs and wedges as this will leave gaps past your insulation, if it was my job i would straighten out the walls with muck first.

to be honest what i would charge you for straightening walls, fixing dpc and batten, insulation and board and skim, you could fit new windows air vents and carpets .

hope you get this point, if you do batten do it right , first time.

hope this helps , really wish you luck, come back and let us know how you got on.
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Postby P J Lenny » Mon Oct 25, 2010 10:41 am

30yearsinthegame wrote: If you batten out you can not use k18 because you will be left with a gap. you will have to treat it like a cavity and use kingspan tw50 and upwards this has a foil backed coating both sides

Thanks, that's some great input. The Techs at Kingspan, when asked, seem to think that the 62.5mm K18 + gap is ok (though they have not been installing it in conditions like mine, as you have.) So you feel the battens should be have this TW50 between them so that the gap is closed. Do you mean that the K18 goes over the battens+TW50?
P J Lenny
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Postby 30yearsinthegame » Tue Oct 26, 2010 6:48 pm


if you use the tw50 you will not need k18! ki8 i believe is made of polystirene . if it gets wet mould will form, then begin to smell, the mould could turn into fungi if the mould canot breath.

tw 50 has a insulated film both sides. water will not be able to form through the insulation, how ever you walls do need to breath, i forgot to mention this last time! install air vents on external walls or use small air inlets. builders use then sometimes where the sccafolding has been, they are around 3 " depth and 4 " long and pop in your perp joint (i believe im no brickie) this will allow for your walls to breath. if you cant install them because you have no cavity then you will need to install air vents. this can be done under floor boards to prevent them sticking out your wall. A builder told me to not install the air vents unless you see a problem and could be the way to go, i guess it all depends on whats under the floor boards.

sorry i cant help a great deal, its hard to say with out being there.

keep us up dated.

kingspan would'nt really have a clue in situations of repair, because their products have not been designed for this. I dout you spoke to tech guys because if you did they should of known about this, but hey you know now > in all fareness to them tw50 costs alot more.....

good luck hope this helps
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Postby trevorcarrott » Wed Dec 15, 2010 4:17 pm

I'm confused as to why TW50 has been suggested instead of K18? KingSpan's PDF clearly shows TW50 with vapour barrier on 2 facing sides being used within a cavity wall. If this was used for a solid wall, then one side of the VB surely would not be required as it should be either the dry lining or thermal board with lining? The cavity created is then between the second VB and the fixed battens on the solid wall inner face. Whereas, K18 PDF has one VB and is fixed to battens, looking like it should be. Alternatively, through my confusion, if the TW50 was fixed straight onto the wall with plaster/adhesive dabs and the battens fixed through the TW50 into the wall and then lined - I could understand, but it's recommended that DPM is backed on the battens, which suggests that they are in fact fixed directly onto the solid wall. In addition, if this was so, we know that solid walls are penetrated by rain and cold from outside, which would come into contact with the VB of the TW50. Where would that moisture go? There's no cavity - it's further in. I've also read that dabs on an external wall are not a good idea, which Celotex seem to advocate, as the outside dampness will slowly eat away the dabs.

Thanks for the info on ventilating the space within the wall. I 've looked up recent info, which suggests that movement of air through the wall is 17x more effective in reducing relative humidity from 65% than just having materials, which are 'breathable'. It sounds such an obvious thing, but I haven't found any post/website that details the inclusion of ventilation within the insulated wall? All, I've found are 'words' to the effect that the space must be breathable.

Does anyone know of any recent grant initiatives for helping people insulate solid walls who are not on benefits. There seems to be lots of talk, but no real hard evidence of actual help with funding?
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Postby Perry525 » Mon Dec 20, 2010 2:12 pm

Hot always move to cold.

Warm wet/humid air always condenses on a cold surface.

Breath on a cold window or mirror and you get a patch of condensation as our breath is one hundred percent saturated. You get the same result by placing your hand on a cold surface.

This is the situation with your walls, the water vapour in the moist air condenses on the walls.

This is perfectly normal. In most situations walls are plastered and papered, the water vapour in the air settles on the walls and is unnoticed, in time it makes its way through the walls to the cold outside and blows away.

With most rooms the window is the coldest spot and condensation forms on the window, when the curtains are drawn, making the recess the coldest spot, open the curtains and the passing warm air, warms and drys the window during the day.

Contrary to what is written elsewhere.

Adding holes in the walls, leaving windows open, merely gives your expensive heat and water vapour the chance to disappear into the sky and be replaced by colder drier air from outside, that you then have to pay to warm up.

Then the process starts all over again.

The best and cheapest way to proceed is to keep in mind that you will continue breathing and producing water vapour and that water vapour can either be expelled to the outside along with your warm air, not a good idea... too expensive.

Or you can buy and use a dehumidifier.

The dehumidifier produces a cold surface that attracts the water vapour in the room and collects it.

Your warm air stays indoors.

For the sake of your health, opening the window a bit for about ten minutes to change the air, as controlled ventilation will do the trick.

Having holes in the walls is wrong.
The hole lets your warm air out and the cold air in.
It cannot be controlled and depends on the passing wind to suck the air out, in exposed conditions a hole can strip the heat from a home in a very short time and add a fortune to your heating costs.

All homes built from 2016 will have to comply with the new EU regulations = Passive House level 6. These homes will be built as air tight boxes, with controlled mechanical ventilation.......... to reduce the cost of heating etc.
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Postby trevorcarrott » Tue Dec 21, 2010 9:15 pm

Thanks for your reply - you have a nice way of putting things!

In a nutshell you're saying don't ventilate the space through or behind the new wall? Instead, open windows or use the dehumidifier or use a mechanical air vent at the expense of losing some of your heat.

I have 4 rooms adjoining the solid wall, which would mean having 4 dehumidifiers and the hassle of switching on/off every day - seems impractical and expensive. Opening windows generally occurs at weekends, but less so during work days.

I'm worried that moisture from the elements (outside) will penetrate the solid wall, hit the vapour barrier of the insulation in the new wall and then have nowhere to go. We know that then means the possibility of mould growth. I understand that water vapour from inside the room will not condense as much because the insulated surface will be much warmer - so that just leaves the issue of what's happening the other side of the wall. Our friend in previous posts has suggested venting the wall and even venting down through the suspended floor. I understand where he's coming from in that cold/warm air from under the house would be flushed through behind the new wall, expelling the air and moisture out. Effectively, this is occurring behind the new wall and so you would think that there shouldn't be much heat loss from the actual room itself. Whilst this is logical, it could only apply to rooms downstairs. I couldn't apply this theory/practice to upstairs' rooms. Of course, cold/warm air could find its way in from outside the solid wall, but this wouldn't be so bad as 1) warm air would dry out any lingering moisture and 2) cold air (being heavy) would drop to the ground and disappear down under the house.

I have spent time researching mechanical aids for ventilation and there are a number of products similar to what's used in bathrooms and kitchens, which work at very low wattage levels and are programmed like a humidistat, only coming on when there's moisture in the air. This kind of device would normally be used to withdraw moist air from the actual room and not just from the void behind the new insulated wall. Any advice?

Am I being overly concerned with what might happen behind the wall? Are moisture levels coming through the solid wall likely to be so small that it's hardly worth worrying about? Indeed, when you view Celotex and KingSpan, there's no mention of ventilating the void between the new insulation and solid wall. But then there's lots of literature encouraging people as they add more insulation to think about adding/improving ventilation? It really is a minefield with so many conflicting views!
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