Moisture weeping into building between DPC membranes

[Isitwinteralready]

Hi,

A recent extension on the back of my house is having some issues with moisture weeping into the building at floor level between two DPC membranes in the internal wall.

Whilst we're still investigating just what the cause can be with the builder, who I must say was superb throughout the build, I thought I'd raise my concerns here and hopefully get some ideas from the clearly experienced people on this forum.

We wanted to be able to back-fill the external side-wall with earth, to maintain an external floor-height for some paving stones, so the builder came up with the idea of raising a DPC up the cavity to keep moisture out if any were to ingress through the external brickwork. He said that the method used was standard practice and was something he'd done before without any issues many times.

I took photographs during the build for my own historical purposes, and I've drawn a picture here showing how he's carried out the work in this area :-

http://i.imgur.com/VqVMC.jpg

There has been a steady trickle of moisture along the area shown on the picture, definitely coming between the two membranes that can clearly be seen from inside the building. This has been ongoing for some time now. I first thought it might have just been water that had ingressed during the build, but it's shown no sign of stopping over the past two months and I'm not getting a bit concerned as to just what's causing it, and more importantly what we can do to sort it out.

After talking to the builder I raised my concerns as to why a 'trough' of DPC was made that crossed the cavity, where any moisture at all that was ever either to build up due to condensation, or get ingressed at all at any point due to pure water-ingress would now certainly be 'led' into the building by what I can only describe as a trough of DPC.

I asked why the cavity DPC was not turned the other way, so any moisture was led 'out' of the building and not 'in', or why the cavity DPC was not cut along the bottom section, so as not to bridge the cavity at all, and any moisture would simply drop into the cavity bottom. I'm not sure I got a good answer to those questions.

Has anyone got any views on what's gone on here, and the possibilities for the moisture ingress ? I'm beginning to think it's purely a condensation issue now to be honest, rather than any sort of internal leak above the DPC being 'caught' by the trough, as the moisture simply isn't enough given the amount of rain we've had lately. It seems to be more of a steady trickle rather than a 'stop/start' deluge.

I'd also like to hear of any possible solutions that I could suggest. I'm loathe to do anything from the inside of the building, purely due to cosmetic issues now we've decorated, but I could probably get into the cavity from the outside if required.

Could any possible solution, if it is condensation-based, be just getting into the cavity and cutting right across the bottom section of the DPC that's bridging the cavity ? I'm assuming any moisture would then drop straight down the external wall and into the cavity, rather than being 'led' into the building, but if the cavity-insulation is getting damp due to any condensation, could that in itself be an issue with damp crossing the insulation onto the internal wall, above the internal-wall DPC ?

Apologies for the long post, but I hope I've explained the situation, and I'd be very grateful indeed for any help or advice.

Cheers,

Isitwinteralready

 

[theoldun]

Before answering your problem and to determine cause , do you know the following?
Is external skin rendered block, or brickwork and if so make of bricks?
Is the cavity tray formed from 1200g visqueen poly or polymeric DPC?
Where is floor insulation?
Is slab finished with screed?
Be interested to see photos.
Regards oldun

 

[JohnD]

in your picture, the external ground level appears to be practically at the DPC. Is that true? Was it built like that or has the level been raised?

 

[Isitwinteralready]

Thanks for the reply, much appreciated.

Not sure I can answer all your questions without asking the builder, and that's not possible right now, but the external skin is brickwork, not sure of the type, but they're a mottled type like these :-

http://brickhunter.com/imagebank/preview/dark-multi-smooth-3.jpg

I'm really not sure about the DPC, other than knowing the sheets wrapping up the external walls and under the concrete floor are of the large sheet variety, and looks like a heavy-duty plastic sheet, and the flat slices along the wall-lines are from a roll, perhaps 150mm in width.

The concrete floor is insulated underneath, with 50mm kingspan, and slices of kingspan around the edges. These slices are covered in most places by a thin skim of cement from during the build, and this has been tracking the damp moisture away from the area between the DPC where the moisture is occurring. I should say that where the moisture is drawing into the building, it's only in one corner where he's wrapped the DPC sheet up the external wall for the purpose of allowing me to back-fill the earth against the outside wall, and it's not a problem anywhere else, so it does seem somehow specific to this particular area and aspect of the DPC construction.

The concrete slab is just sealed concrete at the moment, with a view to tiling the whole floor. We've not been able to progress anything really until I see this moisture stop, so it's a bit of a worry that's holding things up really.

Here's a picture showing the DPC sheeting going in. The rolled up bit is the bit that goes up the inside of the external wall. In the second picture you can see the external-wall DPC roll being kept down with a brick, although the one for the internal wall isn't rolled out yet. This went on top of the existing sheet and slab DPC you can see internally.

http://i.imgur.com/uI9Gn.jpg

The left hand picture only shows the concrete slab and the start of the internal wall. The external wall isn't there yet, but is visible on the right hand picture.

Your help and interest really is appreciated, thanks.

Isitwinteralready

 

[JohnD]

are those scabby bricks your house wall?

 

[Isitwinteralready]

Hi John,

The first picture I posted was a rough idea of what the DPC sheeting was supposed to allow me to do, to raise the external ground level higher than the first (lower) DPC and stop moisture ingress through that particular part of the wall.

The higher external DPC 'lip' is actually a lot higher than that, and I've not had earth up anywhere near it. I've back-filled a bit but took it all out again when I first noticed the moisture, so at the moment it's all clear externally down to the lower-level DPC, whilst I investigate and try to find a solution.

I should also add that in this particular corner, externally, there is an external air-brick in this corner wall, allowing air to be drawn into the under-house floor cavity. I thought I best point this out as I've thought it possible that damp or moisture is getting in there and running down into this DPC sheet/trough :-

http://i.imgur.com/AQMWy.jpg

I'm not sure just what's happening internally to that air-brick, but I'm pretty sure it's just a hole behind it into the cavity gap, with the air being picked up internally through a 100mm round vent into the under-floor area of the house. I wouldn't be concerned about this area if this were a 'closed system' drawing air through the air-brick 'just' into the 100mm vent pipe, but I think it's clear behind the vent brick into the cavity area, so it's been a bit of a concern that there's a 'chance' that the moisture is getting into the cavity area in this area, and then it's got nowhere else to go other than to be drawn into the building between the DPC membranes, as shown on my first diagram.

Thanks again to everyone for their help and interest, it's good to get these things explained to cover all the bases.

Cheers,

Isitwinteralready

 

[Isitwinteralready]

Hi John,

Yes, the older looking bricks are on the rear wall to the house, where the extension was built from.

They look worse due to the build I think, they're really not 'that' bad. :O)

Cheers,

Isitwinteralready

 

[noseall]

Are those perforated Carltons suitable for sub-dpc?

 

[Isitwinteralready]

Are those perforated Carltons suitable for sub-dpc?

Hi,

I've absolutely no idea, and this is the first time I've actually thought about it. You're right that they're perforated bricks though.

Is this just a general question, or do you think it could have some bearing on the moisture issue ?

Given that I've got what looks like a DPC 'envelope' in the cavity, with moisture getting 'inside' that envelope, and the moisture running into my extension between the two DPC membranes, I was more concerned with what's going on above and inside the DPC envelope to think too much about lower down, given that any moisture would then have to penetrate the lower 'trough' of the DPC sheeting, if it was coming from below, and that's why I've not been too concerned with that area, but do you think it's something to consider here ?

Thanks for your interest and help,

Cheers,

Isitwinteralready

 

[Isitwinteralready]

Back again after doing some digging over brick-data. It's amazing what you learn when you're given some pointers, so thanks for asking about the Carltons !

It seems that all bricks below DPC must be F2 graded for frost-resistance, and I guess some sort of low water-retention, is that correct ?

If that's right, and it does look as though I've got Carlton "Civic Multi" bricks, then this page makes it look as though the vast majority of Carlton bricks are F2 rated :-

http://www.carltonbrick.com/brick-data.html

Am I right in thinking that I can at least stop worrying too much about my brick-faces popping off with some bad frosts ?

Thanks again for all the help,

Cheers,

Isitwinteralready

 

[theoldun]

Could be a good stab there nose my friend, but will bet you three half a crowns that they are perforated Hanson Dark Multi Smooth. Are yer on? Any way, what ever one they are, they are both F2 rating for severe exposure , Carlton with absorption of 10% and Hanson 7%

Winter Wonderland. Technically, your builder was wrong with his DPC detail. You should never fall a cavity DPC down from outer skin to inner skin. A cavity tray should always fall from inner skin to outer skin. You should never use Polyethylene Visqueen sheeting for cavity tray. You should use Polymeric or similar DPC. How ever after saying that with a slight modification and high degree of workmanship, the detail could have been made to work.
First of we suspected penetration through bricks, but if the bricks are either of the two mentioned above, then this is very unlikely, providing the perps were not just tipped but fully jointed, then theoretically there should no ingress of water through to the back side of the brick and running down the cavity to the cavity tray and then in to inside. We never like to criticise other peoples work, but whilst the brickwork is presentable, there are features that show he was not a time served bricklayer and you could have your doubts as to the perps being filled especially with snapped quarter bond headers.
However as the way you describe the ingress, it is not down to condensation or dampness but to external ingress.
If the airbrick that you mentioned is the one in the corner next to the down pipe, then this should not let that amount of water in, how ever in a low driving rain it could be hitting the down pipe and bouncing in through the air brick, but realistically this is grasping at straws.
If the penetration is in either of the corners of the new walls and existing the favourite spot for ingress is between the internal corner where it is tied in to existing with brick starter ties. Ingress runs down the starter ties. Ensure that you have a good mastic seal in the internal corner from top to bottom.
What ever corner the water is getting in ,check the brickwork from top to bottom for any obvious defect, then remove 3 or four bricks below the level of the cavity tray and slit the tray to enable the water to run into the cavity and not to inside. Ensure your ground level is at least one course below the lower 100mm DPC.
Give it a spell to see what happens. If no change then come back and will tell you a way to plug it from inside.
Regards oldun

 

[Isitwinteralready]

Oldun,

Thanks very much for taking the time with your really insightful post, especially where it's educating a non-builder-type such as myself, it's always nice to be able to learn something along the way with these things.

I'm very glad that it looks like the bricks are ok for the job firstly, that's one worry I don't need to harbour, so thanks for that.

I'm still not too convinced about this being a full-on water-ingress issue, as there just doesn't seem to be any real 'flow' of water, and what little moisture there is doesn't seem to be affected by the weather at all, and we've had some real downpours up here since the build has finished, but of course I will endeavour to check all the angles you mention with regards to mastic in corners and looking for faults or issues in the external brick-work.

I've dug away the thin screed covering the internal foam edging around the concrete floor, and exposed the two lips of DPC inside the room, and can clearly see where the moisture is seeping in between the two lips, so this is most definitely moisture that's appearing 'inside' the DPC envelope in the cavity. I'm not sure how that can happen with ingressed water through the outside wall, as it would then still have to cross through that DPC envelope wall to find it's way inside the envelope and then into the room. Hence my thinking that this could be a condensation issue, perhaps being 'encouraged' by the open 'sucking' through the insulated cavity-space of the air passing through the air-brick, through the cavity-space (where it could be in an area 'open' to the DPC envelope), and then through the internal wall through a 100mm vent-pipe. If cold air would condense anywhere around that open part of the DPC envelope, then this would be a way for moisture to get 'in' to that envelope and then find it's way down to the bottom and hence into the room. I only mention this again due to you dismissing it without perhaps knowing my full thinking surrounding the details of the vent, so any thoughts would be appreciated.

As an interim measure, and just to see if things improve, I'm going to block up all the holes in the external air-vent tomorrow and see how the moisture is over the next couple of weeks. Hopefully that will be enough time for me to see if it affects anything, and now I've scraped the thin screed away from between the internal DPC and the concrete floor, I'm at least satisfied that any moisture is going to stay localised and not creep away along the floor inside the room.

Certainly the clear course of action is to remove some bricks and slit the bottom of the DPC envelope. I'm amazed that this was seen as a benefit, and I'm glad that you've raised the same concerns.

Many thanks again, I'm truly grateful for the time people take to help people like me with these issues. This is clearly a great board with some great expertise, but most importantly is the willingness to engage and help out with advice and guidance, and it's much appreciated.

Cheers,

Isitwinteralready

 

[JohnD]

I'd be very inclined to drill a couple of holes in the outside wall and see if water runs out.

 

[Isitwinteralready]

I'd be very inclined to drill a couple of holes in the outside wall and see if water runs out.

Hi John,

Do you mean just through the brickwork, perhaps under the DPC ?

What makes you think that the cavity is filled with water ? The moisture is very localised, with a run of about 350mm along the internal area of the two DPC membranes, and that's the only place where this extra DPC 'envelope' exists along that wall, so it seems to be to be directly related to that rather than a more generally water-filled cavity, or have I misunderstood you ?

If we're looking at a water-filled cavity here, wouldn't we expect the whole 6m length of wall to be displaying this moisture issue through the internal DPC line, rather than just in this localised area ?

Thanks again for your interest and help John, it's much appreciated.

Cheers,

Isitwinteralready

 

[noseall]

Old Codge coined it when he explained the dpc detail.
It should resemble this image with the low external end of the tray being at the same height as your elevated dpc.