Solid wall insulation

[oddjobber]

Hello all,
I have a very old basement flat with cold solid brick outer walls of which one wall along one side of the property has been cement tanked before plastering because the bottom half is underground. The outer walls, especially the tanked one, get quite damp with condensation and affected by mould growth in the winter months, so I have to use a dehumidifier most of the time to keep it under control. All the walls are ok in the summer and there does not appear to be condenstaion issues with internal partition walls even in winter.

Question: I want to insulate the inside surfaces of the outer walls with foil-backed 2" celotex boards, with joints taped for a vapour seal, but I'm not sure how to fix it to the walls, bearing in mind I was told I'm not supposed to penetrate the tanked walls with any fixings. Also If I use wooden studwork I'm not sure if it would get damp and eventually rot away behond the celotex. Any ideas on fixing the celotex to the tanked walls would be much appreciated. Cheers folks.

 

[noseall]

You could try foam fixing them.

But then how do you fix the plasterboards to the celotex.

You could consider using insulated plasterboards.

 

[Perry525]

Mould need food, water and a temperature above 40F to grow.
If you clean the wall with soap and water, that will remove the food, mould does not grow on clean concrete.
Is the damp coming through the wall?
Or is it the result of water vapour from the air, condensing on a cold surface?
If it is merely condensation, then fixing insulation board to the wall with a drywall plaster will do the trick.
Using polystyrene that is at least 3 inches thick will ensure the heat and water vapour are kept in the room.
Buy a hygrometer to check the humidity in the room.
Keep in mind that the water vapour in the air will usually stay suspended in the air until the temperature drops, keeping the room temperature steady will help avoid condensation, it will them probably only form on your windows.

 

[noseall]

If it is merely condensation, then fixing insulation board to the wall with a drywall plaster will do the trick.

....not if he does this....

I want to insulate the inside surfaces of the outer walls with foil-backed 2" celotex boards

 

[Perry525]

If it is merely condensation, then fixing insulation board to the wall with a drywall plaster will do the trick.

....not if he does this....

I want to insulate the inside surfaces of the outer walls with foil-backed 2" celotex boards

==================
Perfectly true!
Not proposed for foil backed, only polystyrene backed.

 

[oddjobber]

Thanks guys. Yes I believe it is condensation causing the mould because the cement tanking is only a few years old and seems in good condition. I hadn't thought of the polystyrene backed boards with plaster board facing because I was told they are not as insulative as celotex. Also I was told that moisture can penetrate polystyrene over time, whereas it can't get through the foil backed celotex(is that correct?), so would i also need a vapour barrier of some sort? (but I don't know what that would be)

 

[Perry525]

Polystyrene is the generic name for all sorts of closed cell insulations.
The proprietary names usually add an inert gas that leaks away over time, they usually give the expected insulation value in 15 years time.
With basic polystyrene you get what you pay for, a closed cell insulation that is 98% air and 2% plastic it will never change its insulation properties it will be as good in 50 years as it is today.
Dow brought Styrofoam to market 51 years ago, Styrofoam installed then is in perfect condition and working today.
I installed polystyrene 44 years ago that is still as good as new.
It is used for buoyancy in boats and jetties all over the world, at three inches thick there is no possibility of condensation forming due to temperature difference.

 

[mointainwalker]

@perry

The proprietary names usually add an inert gas that leaks away over time, they usually give the expected insulation value in 15 years time

Care to give exaples of brands and added insulating gases as well as any brands that give values based on 15 year aging ?

I don't believe there are any .

 

[oddjobber]

Thanks perry, polystyrene backed plasterboard gives me a good alternative to celotex and easier to fix to the walls.

 

[ajrobb]

http://www.british-gypsum.com/produ...es/gyproc_thermal/gyproc_thermaline_plus.aspx

This is closed foam for moisture resistance.

 

[Perry525]

look at Spacetherm.
http://www.proctorgroup.com/products/thermal-insulation/spacetherm

 

[Perry525]

And Oxford mail
http://oisd.brookes.ac.uk/lowcarbonbuilding/resources/OxfordMail27Aug2011.pdf

 

[oddjobber]

Thanks guys, that is all useful stuff, especially the links. I will probably use the gyproc plystyrene backed wall boards on outside wall where space is not an issue, and the wall lining stuff on another outside wall which doesn't get quite as cold or damp. I am also now thinking of doing the floor too, thanks to third link about the victorian house that was done up as a trial.
Excellent info. Thanks guys!

 

[edphill]

Hi OP,

Looks like a lot of experienced people have already given their professional views, but I'll add my two penneth in case it's any help.

As the basement has already been tanked, and if it was a good job (i.e. no moisture coming through the tanking), then it could well be a condensation issue.

If you aren't sure if the tanking could have failed, you could always paint a new tanking membrane over the existing cement tanking. You can get a paint from Kingfisher called Damp Guard which we used in our cellar.

If you think the tanking is good and the problem is definitely caused by condensation, have you thought about ventilation? Oftentimes in cellars the damp problem is caused by poor ventilation - have you got extractor fans / heat exchange fans with humidistats installed? You can get some good, almost silent ones which could help significantly with the condensation issue.

Hope this helps.

 

[oddjobber]

Thanks edphill, yes I had been thinking of combatting the air moisture issue. I use dehimidifiers at the moment (in winter) but I like the idea of silent heat exchanger fans etc. Its something I want to look at. I have also been looking at moisture barrier paints mould resistant paints for the inside surfaces too. Expensive but probably worth it. Thanks for the tips. I feel like I'm approaching the job well armed with some good knowledge now!