Housing - Renovation: tanking and insulating

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- CommentAuthorrsk1
- CommentTimeJan 29th 2019
The back wall of my kitchen is a retaining wall. I currently have it exposed brick / render and am about to insulate and plasterboard it, 50mm PIR / PUR. I'm thinking I should tank the wall: it feels pretty dry but there is soil / rubble / who knows what behind it up to 2m height and we have had quite a dry summer. However, cementitious tanking slurries need to have only breathable materials on top of them. Because i'm insulating i'll be having a vcl on the inside, ie. non breathable.

Is there a way around the problem? Is it really a problem? Are tanking and non breathable insulation compatible?
- CommentAuthorJeff B
- CommentTimeJan 30th 2019
Is it possible to dig out behind the kitchen wall and build a new retaining wall? I only ask because we did such a thing in a friend's house several years ago (same circumstances as yours) and it put an immediate end to his damp problems.
- CommentAuthorgyrogear
- CommentTimeJan 31st 2019 edited
Not the same problem, but I insulated my crawlspace walls with 100mm XPS.

However, they were not wet on the inside !

(I imagine that the outer face of the 5 rows of blocks (which are 2 meters underground) were tanked because I see that tanking was applied to the crosswall root...).

The point of my contribution is, even if you tank, even if you dig out behind the wall as suggested by Jeff B, you could *still* get wetness seeking in through (or under) the foundation, just like I find in my case.

In my case it's "only a crawlspace" so I have not gone to town on it, but (for other reasons) I poured a limecrete slab over the floor. I still find that the damp migrates horizontally under (or through) my slab, becos I see it in my access gallery which is quite some deeper than the floor.

gg
- CommentAuthorNick Parsons
- CommentTimeJan 31st 2019 edited
I have a similar, not-fully-resolved issue with my very slow 'tanking' (not tanking really - 'bubble' drainage membrane with perimeter drainage channels) project. The supplier's old guidelines used to say this:

''THERMAL INSULATION
8.1 The application of xxxxxx
membranes as part of the process to
provide a dry decorative surface is very
broad and very often there is the temptation to
install internal thermal insulation directly behind the
wall finish, in front of the membrane, as part of the
dry wall system. Irrespective of the construction or
system being employed this practice has the
potential to cause condensation behind the
insulation and if a membrane has been used it is
likely to form on the surface of the membrane. If
insulation is to be installed in conjunction with an
xxxxxx membrane system it should be directly
behind the wall finish and not more than 12mm
thick and even then the possibility of condensation
forming should not be ruled out.
You are asked to note that this is common to all
internally applied thermal insulation and is inherent
to building construction and water vapour
movement generally. This is not just relevant when
using xxxxxx membrane systems.''

(I am not certain they say any different now. I just can't find any installation instructions on their current site).

This seemed counter-intuitive to me. To refurbish a basement these days and not to insulate seemed a real retrograde step. I spoke to the tech. dept. and their solution was (a) to use a VCL (and we all know how those are always 100% effective, don't we? ) and (b) (in my basement case) to de-couple the wall and floor membrane at the room perimeter, above the drainage channel, so that if any condensation does occur on the membrane, it runs down (albeit on the room side) the cavity formed, in my case, by 30mm steel u-section studs and into the drainage channel and thence out to drainage. It's not brilliant (I'd rather not accept that condensation - particularly in a (moist) kitchen - might be running behind my wall surface, but it's pragmatic).

You could use the drainage membrane instead of tanking, and chop out a channel at the edge of your floor where it meets the rear (retaining) wall. That could drain to the outside through the side wall.

The wall lay-up would be: drainage membrane, metal studs, 50 Pu mechanically fixed to studs and taped at all perimeters and joints, and 9mm or 12.5mm plasterboard fixed with low-expansion foam. Make that a wall you don't try to fix anything to!!

Alternatively:

Take heart from the fact that the wall does feel fairly dry, lime parge it, then wall lay-up as I'd suggested but omitting the membrane. Then if you do get interstitial condensation (on the old wall surface) you are not relying on it running down an impermeable surface, but it could, to some extent, be 'managed' by the lime andd the mass if the wall (of course you don't know whether that wall might have been bitumen-treated previously on the *outside*).
- CommentAuthorgoodevans
- CommentTimeFeb 1st 2019
If its not too late - I would suggest insulate with say eps on the outside and don't insulate on the inside. At the same time the 'drainage' outside of the wall could be checked/improved giving additional assurance and/or the wall could be tanked on the outside surface - a better location for tanking.

In this way the wall will be warmer, therefore drier, with a reduced chance of condensation. Insulating on the inside, means the wall behind will be be even colder than at present potentially causing condensation (which would be interpreted as rising damp or water penetration).
- CommentAuthorgyrogear
- CommentTimeFeb 2nd 2019
Posted By: rsk1The back wall of my kitchen is a retaining wall.

Is there any smell of damp or moisture in the kitchen ?
Is the ground outside actually wet ?
Is it protected from wet (such as by roof overhang?
Does the ground surface slope away from the wall and is there a surface drain etc.?

If "no" to the first two, and "yes" for the second two, I'd personally go ahead and insulate/decorate the inside.

I can't see how "condensation" could occur under the ground (except in a pipe, cavity etc.).

You could also consider contacting a water-diviner to set your mind at ease !

gg
- CommentAuthorrsk1
- CommentTimeFeb 4th 2019
<blockquote><cite>Posted By: gyrogear</cite>

Is there any smell of damp or moisture in the kitchen ?
Is the ground outside actually wet ?

</blockquote>

I would say the kitchen feels a bit damp, but not massively so. The ground outside is covered in concrete so don't know what's under there apart from 10m of uphill sloping garden. Limited digging I have done doesn't indicate a external bitumen coating.

The wall has been covered in 15mm XPS backed plastrboard for 20 years and didn't feel damp when i pulled it off so i am inclined to ignore it, but wanted to know what best practice would be as now is my only chance before i cover it up for another few decades!
- CommentAuthorrsk1
- CommentTimeFeb 4th 2019
<blockquote><cite>Posted By: Nick Parsons</cite>
Take heart from the fact that the wall does feel fairly dry, lime parge it, then wall lay-up as I'd suggested but omitting the membrane. Then if you do get interstitial condensation (on the old wall surface) you are not relying on it running down an impermeable surface, but it could, to some extent, be 'managed' by the lime andd the mass if the wall (of course you don't know whether that wall might have been bitumen-treated previously on the *outside*).</blockquote>

How would the lime and wall 'manage' the moisture? Would they not just stay/get increasingly wet?

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