Condensation behind vapour barrier
Hi,
We have a 13 year old house in New Brunswick, Canada, with a concrete basement, partly above graded and partly below. The basement is partly finished: Half has just studding and fibreglass insulation, part of that has vapour barrier over the insulation, the other half is drywalled over that studding and insulation and vapour barrier. The concrete floor is exposed, the basement feels dry and does not have any mouldy smell to it at all. If you feel the concrete behind those areas with insulation but WITHOUT vapour barrier it is cold but bone dry. However, where there is vapour barrier, there seems to be condensation on the concrete foundation wall, particularly above grade. I noticed all this when I cut through from the main floor to widen our basement stairs. There I found a lot of moisture on top of the foundation wall and on the rim joist. I’m pretty sure this is not moisture penetration from outside – at that point the top of the concrete is 3 feet above grade and under a deck. Any thoughts? In general we get a lot of condensation in the main floor or our house, the windows (which are double glazed) are ususally covered in condensation in the mornings – we have an air exchanger but its pretty old. I wondered if moist air was getting behind the vapour barrier from the floor above – the surface of the finished basement wall is 8 inches in from the wall above.
All suggestions greatly appreciated —
James & Judith
All suggestions greatly appreciated ….
Replies
The moisture may be from the studs drying out; most studs here in Eastern Canada are not kiln dried and even if they are at 15%, there is a bit of drying left to be done. By putting the plastic up you're trapping some moisture behind it from the drying. If your studs were quite dry, there will not be much moisture to deal with. In thousands of cases people have put the plastic up and drywall on before this phenomenon has occurred but there has been no problem with the walls.
You should have some moisture protection of some sort against the concrete wall. See "Keeping the Heat In" at http://www.nrcan.ca or go to http://www.buildingscience.com and search for basement insulation. The second method is better but the other has been done many times without problems. Make sure you understand good airsealing techniques.
Thanks for that. We bought the house about 18 months ago and I think that wall was probably up for at least six months prior to that. Would the wood still be drying out after that time? We have in general found the previous owner's renovations to have been sub-standard so I wouldn't put it past him to have put the vapour barrier on straight away. Looking down on the wall from above I've notice the moisture is definitely dripping down from the top of the conrete foundation wall. There is also moisture on the rim-joist in-between where the floor joists join it.
I'd really doubt the studs are still drying after thirteen years
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Since you have condensation covered windows I suspect you have a general humidity problem. Solve that first.
Is there an air space in behind the batts or are they placed hard against the concrete wall? If there is a space, it'll allow convection currents to form bringing moisture up from the footing/wall/slab intersection where no moisture proofing was done at the wall pour. This moisture condenses at the inner surface of the cold concrete and in the rim joists. In Quebec, they have actually seen ice build-up in this area.
Is there any airtight vapour barrier work done in the rim joist areas or any where else?
Is the condensation you found all on a single wall like the north wall and it's dry on other walls.
Measure the relative humidities in the house. If it's above 40% day after day, time to upgrade the air exchanger. But continue with good airsealing on the basmemnt walls/joists. The air exchanger/HRV cannot save you from poor work.
ataboy!
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Any penetration in the TOP of the envelope ("loose" ceiling lights, unbaffled bath fan ducts, Plumbing vent chases, etc.) will allow warm air out and "pull" cool air in through any opening. The negative pressure can be considerable when the difference between outside and inside temps is high. Think of the house as a "hot air balloon". The more air you let out the top, the more cool air will replace it. You may well be pulling cold air under the sill/rim joist, causing condensation at the top rows of blocks.
At approx 30 degrees of difference, the humidity in the warm air will condense where it meets the cold air.
Concentrate on closing all the outlets in the top of the envelope and you may see a marked improvement in the condensation. I did it to all the terribly loose light cans, elec outlets, switch covers and tightened the window stripping in the ENTIRE house. RESULT: condensation in the full basement all but disappeared.
Of course, I could be all wet.
Troy Sprout
Square, Level & Plumb Renovations
Good laboratory case, since you have 2 wall assemblies in the same basement.
Vapor barrier on inside of basement wall is inviting trouble. Concrete is absorbant like a wet sponge. Moisture from 'the sponge' will evaporate into the cavity. Moisture can come from ground outside, can be wicked up from footing ("rising damp"), or from warm air condensing on cold concrete surface (requires some air leakage, or convective currents within cavity). With a vapor barrier, moisture can get in, but cannot get out. Below grade wall are not going to 'dry to the outside' like an above grade wall assembly does.
As else being equal, you are much better off without a vapor barrier in this case.
A better solution is to place a moisture barrier in contact with the concrete. Ideally over that you would have a continuous thermal break, such as foam board. Over that you can do whatever you want.
From what I've read on building science.com, they don't recommend a vapour barrier below grade. Foam board that is 1" thick or thinner is used directly on the concrete, the batt insulation (non-fibreglass) is used on top of that. We used the Roxul insulation, I think it's rock wool. We used a vapour barrier above the concrete.
Thanks to everyone for their input! There IS a gap between the studding/fibreglass and the concrete wall - it seems the previous owner did this deliberately, there are 2x4 spacers on the floor keeping footplates of the studs away from the wall. Everyone we've spoken to has suggested that we should have the insulation right up against the concrete. Ideally I think we'd strip the basement and start again. Someone at an a local hardware store recommended injecting an expanding insulating foam into the gap where there is drywall, and redoing the insulation and/or studs where we can get at easily.
But our remaining question would be, why did the previous owner do it like this? Is there ever any good reason for having a cavity between the finished wall and the foundation wall?
There are a lot of reasons but no good reason!
I think you've already discovered why it's not a good idea to build a stud wall directly against a concrete wall. If the studs were fastened to the concrete, all that condensation you've observed would by now have penetrated the wood studs and they would either be starting to rot or be badly mildewed. The concrete is impermeable enough to be acting as a second vapor barrier and it's on the wrong (i.e. cold) side of the wall. When you insulate the wall inside of the concrete wall, the inside surface temp. of the concrete wall drops below the dew point for the relative humidity of the inside air and you get condensation forming on the inside of the concrete wall. Probably the only way to effectively insulate a concrete basement wall in a cold humid climate would be to have an insulating contractor spray on foam from top to bottom including the bays between the floor joists at the top of the wall, then build your finished wall inside of that.
George:
There are a couple of wall systems that work in basements but.....you have to do good work for continued good performance without problems.
We have the BSC wall which is a sort of belt and suspender's approach allowing that many people will not do the job really required on the air leakage aspects of the wall to stop all warm, moist aIr leakage into the wall. (did the final deficiency inspection on an upscale house here today and even the professional insulator did what I would call a poor job on airsealing in the still visible joist areas in the unfinished basement.)
In cold Canada, the NRCan/CMHC wall has probably hundred's of thousands of applications with the large majority performing well. This is a cheaper wall with no second chance as above if you don't do the installation and airsealing well. It requires dry studs to be installed since there is a chance of partially sealing water between two sheets of plastic by using wet studs.
By the way, the concrete is not that impermeable that it acts as a vapour barrier. In the Canadian wall, a sheet of 6 mil poly goes against the concrete wall from the sill plate of the inner wall up to grade to stop moisture that is permeating the concrete from the exterior damp soil.
It's funny that I said the Canadian wall since the BSC wall is a recommendation of Joe Lstiburek, a Canadian and principal of BSC with head office in the Boston area.
Basement walls should not have a vapor barrier. The need to be able to allow moisture to travel through the wall, not trapped inside.You get out of life what you put into it......minus taxes.
Marv
Marv:
Even the BSC wall has some vapour barrier/retarder in it- the foam against the concrete. It reduces moisture diffusion from the exterior damp soils and provides a warm enough surface to stop warm interior air from condensing if it gets to that area of the wall.
The double vapour barrier wall will work but it has to be done well. The book that's been promoting it has been in print (revised-reprinted 3-4 times) for about 20-25 years now and no changes have been made on this technique. It's done by contractors so if problems were occurring, they'd be the first to be sued and then sue the authors- the Canadian gov't.
Edited 12/15/2005 10:42 am ET by experienced
There are lots of ways to do a basement wall. I believe that the any water vapor that comes through the cement wall should be allowed to pass through the wall too.You get out of life what you put into it......minus taxes.
Marv
When you put the six mil plastic on the wall, you essentially stop vapour movement since the concrete adjacent to plastic now comes to equilibrium with the outer wall surface over a period of time. In equilibrium, water is not moving.
The plastic on the wall also stops insulation in the wall from getting wet if there is ever a foundation wall leak. Run this piece of plastic down the wall and under the bottom plate and this water does virtually nothing to the wood framing, insulation and drywall. It may stain the carpet or baseboard trim though.