How can I get rid of moisture between the vapor barrier and the insulation in a basement wall?


How can I get rid of moisture between the vapor barrier and the insulation in a basement?

David Mitchell Published: Aug. 14, 2018, 6:32 p.m.Last updated: Aug. 1, 2020, 3:44 p.m.

It's a fairly new house (about a year old).  The concrete basement foundation walls are framed (wood), insulated, sealed with a plastic vapor barrier, but not drywalled.  

I am led to believe that, during construction, the builder didn't immediately seal around the basement windows before he installed the insulation in the basement.  Subsequently it probably rained and moisture got in through the unsealed basement windows.  Now, I can see the condensation on the inside of the vapor barrier i.e. the side closest to the insulation.  The vapor barrier is dry to the touch from inside the basement.

Responses (15)

LJ H July 11, 2020, 11:01 a.m. Reply

Hi there.  I found mold behind my baseboards and have removed bottom sections if drywall to gauge the extent of the problem.  In the process, I learned that there is vapour barrier on both sides of the insulation, trapping moisture of course.  I know there should only be vapour barrier on one side (in my view, the outside). Do I need to remove all the drywall to remove all the inside layer of vapour barrier, or is removing the bottom inside layer of vapour barrier effective enough to get air flow? I want to fill the most effective thing, not just the easiest.  Thanks! 

Mike Reynolds July 15, 2020, 12:28 p.m. Reply

You are right, there should not be two vapour barriers in a wall system, particularly a basement. Standard basement construction is usually done all wrong, with the vapour barrier sandwiching wood and insulation in with the wet concrete so it is unrealistic to expect anything but a rotten wall in the future.

If you actually have a vapour barrier separating the concrete and the stud wall then that is a pretty good thing. Read more here about how to build basements so they don’t go moldy. In a perfect world, yes, you would not have that interior vapour barrier, rather just drywall over the insulation and let the wall dry to the interior.

That is really just an over view based on limited info, but you would probably be best to remove the drywall and the interior poly vapour barrier then reapply drywall. With luck and careful disassembly you may be able to reuse some or most of the drywall.  

Khaled Jaber
Khaled Jaber July 17, 2020, 10:20 a.m. Reply

Hello. I am a recent homeowner of a brand new build (single home, 2800sqft, unfinished basement)  in Ottawa, Canada. Last weekend I noticed the problem being described on this board. water build up behind the vapour barrier (is there a way to post pictures here??). The moisture is all over the place and apparently it is the same problem for all my immediate neighbours who have homes from the same builder. The builder says this is 'normal' for new builds and would take 1-2 years for the issue to go away and that no damage will happen because of this. They advised me to get a dehumidifier.  Firstly,  it does not look normal at all. The moisture build up is so much that it's dripping and fiberglass insulation is damp.  Secondly, having a dehumidifier run in the summer is a good idea in general, but I think this is a separate issue as the moisture is coming from the OUTSIDE and getting stuck on the barrier. So the dehumidifier solution is a red herring in my opinion.  RH at home is controlled between 48%-53%.

I see a black covering on the foundation walls when I move the fiberglass, isn't this supposed to keep moisture out ? If so, then that leads me to believe that there is moisture stuck in there at the time the house was built. I heard of 'wet construction', is this related and can someone explain what it is .  

So there are a couple of unknown variables. If there is moisture stuck between the foundation and vapour barrier, how much and how long will it take to dissipate? Is this going to be a reoccurring issue? 

Steps I took to remedy the problem: I made cuts in the vapour barrier and placed a fan to dry out the moisture. Got Tucktape to patch up the cuts and then wait for the next hot day to see if moisture manifests again.

Any advice from experts would be much appreciated!!


Marc B
Marc B July 20, 2020, 9:27 p.m. Reply
Khaled I am also in a new home in Ottawa and have the same issue as you do. My builder said it was because code requires the vapor barrier be sealed all the ya with no room to breathe which in his Opinion defeats the purpose and recommends I remove some staples at the top and bottom to allow it to breathe. Clearly the hot weather and the cold AC on inside doesn’t help either and because the houses are new they need to dry out (concrete wood etc). What were you told to do?
Khaled Jaber
Khaled Jaber July 22, 2020, 12:54 p.m. Reply
I got recommendations from Tarion and the builder to either remove the staples or make vertical slits to allow moisture out. This to me is not normal homeowner maintenance. Did they mention to you when will the concrete/wood will dry out? I would like to finish my basement at some point but don't want to do it knowing this will still be an issue.
Mike Reynolds July 24, 2020, 11:52 a.m. Reply

Hi Khaled and Marc, 

Sounds like both of you have insulated basement walls that have yet to be finished with drywall. If so, count yourselves lucky. You were able to see the moisture, those that have had drywall cannot see the moisture forming so they have no idea that the exact same thing may be going on, and won't until after the new home warranty has expired. That, in a nutshell, is why almost all basements smell moldy - the vapour barrier. 

Telling you to pop out staples and cut slits in it is an admission that vapour barriers in basements don't prevent moisture from getting in walls, they prevent moisture from getting out.

Vapour barriers first became part of above grade walls many decades ago, and the purpose was to prevent interior moisture ( from cooking, breathing, showering etc) from migrating through walls in winter can causing moisture damage. Allowing, and encouraging that to happen in basements is probably the singles biggest failure in home design and construction I can think of. 

As for your question as to how long it will take to dry out - if they put a waterproof membrane on top fo the footing before building the foudation wall, and they put an exterior water proof membrane on the outside (both of which are highly unlikely), then it will take about 5 years to dry out. If they didn't put a footing membrane in particular, it will never dry out. Concrete is porous, so a footing deep in the ground will absorb moisture forever and continue to deposit it in your walls the same way a sponge sitting in a bowl of water will always be wet on the top. 

You can read more here about why basements are moldy 

And you can see best practices for durable basement construction here.  In that page you will see an image of the  footing membrane I mentioned where it describes a "capillary break".  

If I bought a new house that had insulated basement walls and an exposed vapour barrier, the first thing I would do would be take it all down so walls could dry and put in a dehumidifier. Problem solved. 

Mike Strype
Mike Strype Aug. 16, 2020, 9:03 a.m. Reply

Quick question, if you remove all the poly and then drywall, won't the condensation just build up on the inside of the drywall instead of the poly?  Not sure which situation would be worse. Anyone having any luck with this situation? My house is 60 years old. Definitely not new concrete drying out. It only affects the walls with direct sun exposure on very hot days. Thanks!

Mike Reynolds Aug. 17, 2020, 12:26 p.m. Reply

Hi Mike, 

The drywall won't stop the moisture like a vapor barrier, it will move through it. The drywall will absorb the moisture, but  like any materials in a home, the drywall will assume the same relative humidity levels as the basement. So if you keep a dehumidifier running, it keeps the air dry. Humidity will always seek to create 'equilibrium', meaning the moisture will go where it is less dry. So if the air is dry then the drywall is able to 'dry' into the air. 

Basement construction set off on the wrong foot with vapor barriers and a lack of protection from ground moisture, so while this isn't the 'ideal' basement, its really the only way to fix it. A dehumidifier is a basement's best friend! 

Noaman Khan
Noaman Khan Sept. 5, 2020, 6:52 p.m. Reply

Hi, I am in Kitchener Ontario and had same problem in my 6 months old house basement in early summer. In my case I was seeing water behind back and left side walls near windows. My builder rep told me it's happening as I was running ERV in summer on low setting and suggested to keep it off or run occasionnaly in summers. Anyway in my case after I cut slits roughly 3" in x shapes every 6 inches or so around all moisture areas and kept ERV off while a dehudifier on for couple of months, it all went away and now I don't see any water vapours for over a month.   

I try to run dehumidifier for few hours every few days and hoping it can keep my basement dry.

btw my house is walkout so not sure if that make any difference. So at least what I learn is that make sure you are not running ERV all the time during summer time.



Noaman Khan
Noaman Khan Sept. 5, 2020, 6:54 p.m. Reply

Hi, I forgot to mention I have sealed back my vapour barrier with tuck tape after I noticed no more water behind it

Mike Reynolds Sept. 9, 2020, 8:55 a.m. Reply

Noaman, I don't see how the ERV has anything to do with it. I may be missing something in how you relayed what the contractor told you but I don't see the connection.  So, feel free to elaborate, but - if you had moisture behind the poly vapour barrier and you cut holes to let the moisture out and ran a dehumidifier, to me that sounds like what solved the problem. An ERV exchanges exterior air with interior air, it would have no direct effect on a buildup of humidity behind the vapour barrier. 

By cutting a bunch of holes in your vapour barrier you are letting moisture though, which is testament to the fact that the vapour barrier is not helping matters in a basement. It is not impossible that you had some other one-time source of moisture that you allowed to escape, but given that you sealed it all back up, keep an eye on it as you may find it builds up again. 

Kanina Wright
Kanina Wright Oct. 13, 2020, 10:58 p.m. Reply

We had a home built 3 years ago. It has a fully finished basement with 9' ceilings.  We found signs of moisture on our carpet /baseboard in the form of mold. We opened up the walls and found the concrete foundation walls and insulation were soaked. Water was literally running down the walls. No signs of humidity anywhere else in the home. Warranty is denying anything stating we keep our humidity levels too high (we are consistently between 40%-50% based on data we pulled from thermostat. Looking at a possible $40k- $50k bill to rip out and re-do. We are told we need to reduce humidity to 30% in winter months and replace batt insulation with spray foam. We are devastated. This does not seem right. Envelope engineer says there is significant air circulation behind the vapor barrier when furnace fan turned on. Isn't the vapor barrier there to stop warm air from inside home reaching cold air in the wall cavity? Any thoughts. We are desperate for some help. Can send pics /report. 

Mike Reynolds Oct. 15, 2020, 4:08 p.m. Reply

There isn’t much we can do for you directly to get between you and the company in terms of warranty unfortunately. That is devastating I’m sure, very sorry to hear about your situation, and it's far from the first time we've heard about this happening either. How we can help is by laying out the basic principles of building science for you hoping it helps somehow, meaning:

Concrete is very wet when poured. So new basements need a considerable time to dry out. Then as concrete is porous, if it's without moisture protection from the soil or if that protection is compromised (more below) it will continue to absorb moisture and stay wet forever. That’s just physics. Humidity will want to go where it's dry. This is the basic principle of the second law of thermodynamics. Therefore, if a concrete foundation is wet, it will want to dry to the interior of a basement since it can't dry to the exterior as that is wet dirt and/or has a damp proof membrane on there. If on that journey the moisture encounters a vapor barrier while doing so, it will be stopped and saturate any materials in its path or trapped in the middle. It's like putting soup in a tupperware - it's got nowhere to go!

So, if you have a concrete foundation in the ground that is unprotected from absorbing groundwater and moisture or is protected in a manner that's compromised, you will have wet concrete. If you have a stud wall and insulation against wet concrete and cover it with a poly vapor barrier, the lumber or steel stud wall and the insulation in there will get wet. And organic materials or flimsy and thinly galvanised metal studs that stay wet will get moldy and rot. Full stop.

The suggestion that air with moisture content in the area of 40-50% Relative Humidity in a basement will somehow get past a vapor barrier and saturate a concrete wall makes no sense to me - and points to deficiencies in the manner that vapor/air barrier is sealed at best. This level of relative humidity is in any case considered acceptable for residential living spaces in summer - and is only just out of scope for winter. Is the home fitted with an HRV or ERV ventilation?

If the concrete wall is entirely and flawlessly protected from ground water – including a footing membrane, waterproof exterior wall membrane and proper drainage, and left to dry to the interior for many years before interior walls were finished,  you could possibly argue that it wasn’t the source of moisture, but, if that was the case, then any moisture that passed through would likely be absorbed by this bone-dry concrete wall rather than running down it.

The consistent opinion of respected building scientists is that poly vapor barriers in a basement placed behind drywall are a recipe for disaster and encourage mold. You can read all about moldy basements here having said that, there is something that can be tried to potentially avoid the "rip everything out and start again route". Let's call it the "Rip out some bits and accelerate drying to save work and money" route.

Firstly, making a tour of the exterior, ensure that all around the home the damp proofing on the basement wall concrete can be seen above the ground level. A major cause of basement humidity in older properties is incorrectly placed fill or badly graded exterior hard surfaces which channel water down the unprotected sections of basement wall by arriving higher than the damp proofing. If there's no apparent exterior waterproofing (something like this basement waterproofing is ideal) then ask the developer to show you what they used and take it from there - I've seen this missed or companies still using simple bitumen paint which we consider suboptimal!

Next - if the above looks in order, on balance of probability the humidity you're seeing is simply the residual humidity trapped in the wall - as mentioned above in other postings like Khaled's. In this case, by removing sections of drywall every say 6' and slitting/opening the poly vapor barrier around the basement in a manner that can be carefully taped back up in the future, and running a correctly sized dehumidifier (maybe into a permanent drain to save endless emptying) - in a year or so the perimeter basement walls will dry to the point the problem stops. The mold will die if the relative humidity is low enough - and remember mold spores are always present about us - in a dormant or active state. Not saying for a moment this is ideal - but if this is the cause of humidity, and you don't have the patience to wait for the concrete to dry out before refinishing the basement walls, then the eco-friendly spray foam at your cost route may be the only one available - even though it only traps the moisture in the concrete rather than lets it dissipate - and may cause a potential freezing problem if in a very cold climate! It's far from ideal - but even the developer isn't really at fault as they probably only did as they were told to be code compliant (but yeah, ok, they could have done better if they were educated to the problem and care!)

I really hope this helps!