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

D
David Mitchell
Updated: Aug. 1, 2020

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 (51)

LJ H
LJ H 3 years ago

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 3 years ago

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 3 years ago
Marc B
Marc B 3 years ago

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 3 years ago

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 3 years ago

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 3 years ago

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 3 years ago

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 3 years ago
Noaman Khan
Noaman Khan 3 years ago

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 3 years ago

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 3 years ago
Mike Reynolds 3 years ago

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! 

Teresa Cuming
Teresa Cuming 2 years ago
Mike Reynolds 2 years ago

Hi Teresa, pull the vapour barrier off and leave it off. The source of moisture is the concrete, not the air, so you don't need to protect the wood from relative humidity in the air, that's not the risky source of humidity. 

It takes many years for the huge volume of water in concrete to fully evaporate, and your wood studs even come wet as well, so vapour barriers should not be put on basement walls. It's loaded with moisture behind there and it has no where else to go. As you keep seeing, it will retain moisture inside with the wood studs and insulation, and that will lead to rot and mold.  The reason being - concrete is pourous, and even if you have the black tar spray on the sides or even a waterproof membrane, your footing is almost certainly sitting unprotected in wet ground and it easily wicks moisture up into the entire concrete wall. It will stay wet forever and keep depositing moisture inside your walls if you keep the vapour barrier on. It simply isn't needed. Let the moisture escape and run a dehumidifier, problem solved. 

Installing vapour barriers on basement walls is in our opinion,  the biggest mistake make in home construction, and it happens every day. Don't listen to any contractor that wants to put another one up. Read more here, and pass this on to anyone that tries to convince you that you need one, they are simply ill-informed. Why basements are moldy and how to fix it. Good luck! 

Teresa Cuming
Teresa Cuming 1 year ago
V B
V B 3 years ago

Hello, I live in Ontario, Canada.  My home is about 30 years old. My basement was recently framed to be finished.  They left some space between the studs and the concrete foundation wall (so the wood is not touching the concrete), and they laid down some plastic on the floor so the studs don't touch the concrete floor.  My contractor has already put pink fiberglass insulation between the studs, but it has not been drywalled.  He plans on putting a plastic vapour barrie on top of the insulation, then drywalling.  What would you recommend we do at this point to prevent issues?

Mike Reynolds 3 years ago

Hi V B,

I would stop them from doing the vapour barrier. Your concrete walls are almost certainly not properly protected from ground moisture and therefore wet, and wood is an organic material, so if you seal them together with plastic you'll have only one result - mold. This is probably the biggest mistake in home construction and the biggest flaw in the National Building Code, that we are 'required' let alone even allowed to install one. 

If you need to install a vapour barrier to appease a building inspector then I would suggest you look into Certainteed Membrain, which is a breathable alternative to 6-mil poly vapour barriers. It looks the same to an inspector but at least it will let some of the moisture out. Here is our page on how to renovate a basement so it won't develop mold in case you didn't see it in the links above. 

I
Iain B 3 years ago
Mike Reynolds 3 years ago

Hi Iain,  the unfortunate reality is that if you have it happening in one area it's most likely happening elsewhere in your basement, so yeah, I'd check. For sure leave the vapor barrier off, that's really what caused the problem. If you're stripping it to the bare concrete I would make sure to separate the wood stud wall. either add 2 inches of rigid EPS insulation, or if you don't have enough space to thicken the wall with more insulation I'd put a poly barrier back up, but put it against the concrete. That way it will separate the wood and insulation from the source of moisture, they way yours and most basements is done - with the vapor barrier sandwhiching the wood in with the moisture is only ever going to lead to rot.  

J
J Fredrickson 3 years ago

We have a full basement with a treated wood foundation in Minnesota. Between the treated wood studs is fiberglass insulation. We are looking to finish the basement with drywall. What should we use for a vapor barrier? And, would the vapor barrier be on the interior side of the insulation?

J
J Fredrickson 3 years ago

Just as a follow up, do you recommend a different type of insulation than fiberglass? Maybe mineral wool?

Mike Reynolds 3 years ago
P
Paul S 2 years ago
Hugh Smith
Hugh Smith 2 years ago
Mike Reynolds 2 years ago

Hi Hugh, about your question - "will I risk freezing and cracking the wall in winter if moisture cannot escape except to the outside above ground-level?

Even if it doesn't *seem* wet in the ground, it is still in all cases, comparatively wet, unless you are in a climate which recieves virtually no rainfall and I don't think that's the case where you live. Letting moisture evaporate inwards will not actually dry the wall much at all, since that moisture will just be replaced. So don't worry that you are damaging your home by preventing moisture from entering through your foundation walls. I think painting on some sort of waterproof coating will work fine. 

Spray foam would also work very well. We don't love it as as building solution due to the fact that any material that has been spray foamed will eventually end up in a land fill, but at least it is much more sustainable now due to the switch to less harmful blowing agents. If you go that route I would check with the company you choose to make sure they have made the switch to climate friendly blowing agents. 

Hugh Smith
Hugh Smith 2 years ago
Mike Reynolds 2 years ago

Glad it helped Hugh. Just because you aren't seeing droplets form on the tinfoil doesn't mean it isn't wet and evaporating. Unless the foundation footing has been entirely encapsulated in a waterproof membrane, which almost never happens, then the concrete is without a doubt wet enough to allow moisture to evaporate inwards. The reading on your dehumidifier is the real answer, not the tinfoil.

A dehumififier doesn't consume a lot of energy, in fact the less moisture in the air the less energy is required for heating and cooling, so keep it running and don't worry that you are wasting energy. It is not only saving your basement from getting moldy, it is most likely saving you money or at least paying for itself by removing moisture from the air that you don't have to pay to heat. 

Hugh Smith
Hugh Smith 2 years ago

I had, in fact, been concerned about the energy usage of the constantly running dehumidifier, but had not thought of the idea that less humid air makes heating easier. So thanks for that perspective. I think I'm still going to go ahead and try to lessen the moisture infiltration by applying either spray foam or a paint-on moisture barrier on the interior of the cement walls. It looks like the paint-on solutions are either a latex sealant or a thick rubberized material like "Liquid Rubber". Do you happen to know if there is a particular product that is recommended for this purpose? I'm sometimes a little skeptical about the durability of latex paint products.

alice dodge
alice dodge 2 years ago
Mike Reynolds 2 years ago

HI Alice, 

How old is the house? It is fairly common to see moisture behind vapor barriers in fairly new homes, as they are built with wet lumber then wrapped in plastic so its no surprise that it takes a long time for moisture to dissipate.

In your case, I assume this wet section of vapor barrier you are talking about is close to the damaged wall section? It would make sense that you are seing signs of moisture there if nearby framing members are saturated and rotting. You need a vapor barrier in cold climates like Vermont, so I don't think removing it is the answer. Will it dry out during your repair? When you replace the soaked and rotting lumber I would hope that would cut off the source of moisture. One additional recommendation perhaps, is to get kiln dried lumber for that section, or find someone that can do a small amount for you, that way you are starting off with something bone-dry rather than quite wet, which is how most lumber deliveries arrive. I'm only putting that out there because I don't know the extent of the damage or just how wet it is, but that is one solution. 

Building up against the bottom of a hill is less than ideal due to the risk of bulk water running down the hill and causing problems. It is tough to really visualize, but from what I understand of your situation I would say it is most important to keep dirt, ground cover, and any water well away. Make sure there is at least 8 inches from any organic materials to the bottom of your siding, and if you can, dig a small ditch at the back as an added safety measure. 

alice dodge
alice dodge 2 years ago

Yes! That makes sense. Yes, definitely planning on digging out some of the grade and putting in some drainage stone. It's a 1960s house, without a lot of insulation, just regular fiberglass. I do think the moisture was coming from where the wood was wet from rotting - hopefully it will dry out now!

Mike Reynolds 2 years ago

Excellent, hope it works out Alice! 

Maria A
Maria A 2 years ago
Mike Reynolds 2 years ago

Hi Maria, thanks for the kind words, glad you are finding it helpful! By ' the NE' I assume you mean the North East and if not then probably Nebraska, both of which are cold so the answer applies -

On walls:

Blocks are very porous, and the ground is pretty much always ‘wet’ in comparison to the humidity levels you would want in building materials and homes, so it is best to keep that moisture out as best as possible. Great if your studs are in good shape, but if you were able I would try to slip a 6-mil poly vapor barrier behind the framed wall against the blocks. That will help keep your wood and insulation dry. Mineral wool can handle some moisture but dry is always better with home construction. Not sealing wood against the concrete with a vapor barrier will help a lot, but even better is to keep that moisture in the blocks and in the ground by separating it with a vapor barrier where it SHOULD be, right against the concrete. 

On floors: 

Cork is an insulator, but at the thickness of flooring it won’t add much at all. So if you have the head room and the budget I’d want to add some insulation. And yes, it will always wick moisture from the ground, so I would be as concerned about the longevity of the adhesive as I was about the cork flooring itself. If you bought them both from the same company find out if there is a warranty, and ask specifically about the adhesive and if it will remain adhered if the concrete is not perfectly dry. I would lean towards putting down a poly barrier, some sort of wood subfloor, and flooring on top.

For your reference – cork is between R3 and R4 per inch, but wood is only about R1 per inch, so engineered flooring with a cork top layer may only be about R1 total so you will still experience cold feet and a lot of heat loss. Cork tiles about 3/16ths thick would offer even less. I would want a floor in a cold climate to be at least R12 (or more) for happy feet and energy efficiency. 

On asbestos: 

We cannot speak to asbestos safety, so I would follow the guidance of industry professionals. Yes I would seal the floor if I suspected it had asbestos that wasn’t being removed, if it becomes waterproofed in the process that’s great, keeping moisture down is best.

Cindy Davidson
Cindy Davidson 2 years ago
Mike Reynolds 2 years ago

Hi Cindy, happy to help, but not sure what the exact question is. Are you asking about how to do drainage outside, or what to do with the inside? I'm wondering what stage you are at and what the next decision you need to make is. If you have the file open with the company for that long I understand it must be frustating, but it sounds like they have accepted responsibility, that would imply they will be financially responsible for repairs. If it were me and I found moisture behind the vapor barrier in basement walls I would want to strip all the walls and reinsulate it properly. Let us know what the report says and how we can help. We can't do much for getting in the mix between a home owner and the company that built the home, the best we can do is give you unbiased opinions as to the best way to proceed with repairs. 

Cindy Davidson
Cindy Davidson 2 years ago
Mike Reynolds 2 years ago

Hi Cindy,

Without seeing it I can only offer you general advice, though I am surprised none of these pros thought much of wet insulation. It is possible that the water is wicking up from the footing, but also possible that it is saturated at the bottom due to it draining down.

It boils down to this – houses should be built in a way that walls are dry; if the walls are wet inside, something wasn’t done right, full stop. Often times that is simply the inclusion of a vapour barrier behind the drywall on basement walls, which desite being common practice should never be done as it is nothing but a disaster waiting to happen.

So the short story on basements –

Walls cannot dry to the exterior because they are buried in dirt, which is wet. So they NEED to be able to dry to the interior, and a vapour barrier prevents that. But it is required by most building codes in cold climates, so unfortunately builders are often able to pass the buck on that one when they get moldy.

From what I understand about your basement I would likely do the following – remove the drywall, insulation and vapor barrier. Let the walls dry out nicely with a dehumidifier.

See if you can get a vapour barrier BEHIND the stud wall. That will keep the moisture in the concrete and keep the wood and insulation dry. Re-install insulation and drywall, do NOT put a vapour barrier behind the drywall. If your builder tries to talk you into it, refuse. Sealing insulation and wood against a wet concrete wall with a sheet of plastic should never have been allowed to happen, let alone be required, as it is the perfect environment to grow mold, and I don’t think you want to do that any more. If you are forced to put a vapour barrier on your basement walls by a building inspector, use a breathable vapour barrier instead. This will appease inspectors but allow moisture to escape.

Think about a maybe sump pump and battery backup to prevent flooridg, also grade the exterior way from the house, and extend any downspouts so they drain far from the home. We are a bit limited in what we can say as these other proffesionals have the advantage of seeing it, but at the least the info here should give you enough knowledge to question them. Insulation should not be wet, and that should at the very least be acknowledged and investigated. I hope that helps at least a bit at least!

Cindy Davidson
Cindy Davidson 2 years ago
Mike Reynolds 2 years ago

Cindy that is a very kind and thoughtful reply. Very happy to help and I hope this gets resolved so you can enjoy your home again. If there are other issues that pop up by all means pop a question on our discussion board. Good luck! 

K
ken steele 2 years ago
Mike Reynolds 2 years ago

Hi Cindy, The conclusions about hydrostatic pressure sound like it may be the problem. It would not be unheard of for a building company to push ahead when it might not be wise to, so it is possible they knew there was an issue. I think at this point I may be past the point of being able to offer any useful advice given that this situation is becoming more complicated.

I can give general info on how to build a durable basement (like above), but when it comes to doing some buildilng forensics to determine the source of moisture in basement walls I think you're in good hands with a geotechnical engineer onsite that can see the actual problem. I'm not trying to leave you hanging but I think at this point from a distance I just don't think I have any wisdom to offer beyond what you team can. Hope you get it resolved!

K
ken steele 2 years ago

Rick, thank you so much for your assistance!

Yes indeed, I think we have enough information to make the builder squirm. Our consultants also recommended spray foam, not insulation.

Rick keep well and thank you for your public service, Blessings on You!

William Hackett
William Hackett 1 year ago

Similar situation here in Central Canada - Winnipeg.  This past spring I noticed the carpet in our finished basement was wet in one corner.  I removed the drywall, poly vapour barrier and insulation to find a thick layer of frost/ice on the corner poured concrete walls.  Have let it dry out with a dehumidifer.  But need to know how to properly refinish the wall. Is the moisture coming in from outside or is it warm air condensing on the cold concrete wall?  Was plannin on using rockwool between the studs.  Should I use a vapour barrier ontop of the rockwook and studs between the drywall? Or no vapour barrier at all?  Should the wall be painted with a rubber membrane first?  Any ideas?

William Hackett
William Hackett 1 year ago

Oh, failed to mention that this is not new construction.  The house is about 70 years old.

Philip Templeton
Philip Templeton 1 year ago
J
Josh White 1 year ago
C
Cody VanWyck 4 weeks ago

Hi Mike! I have a house built in the 60s. The basement is a walkout and was already framed with 2x2s fastened right to the block wall with a poly vapor barrier between the studs and block. I am going to add drywall but want to know what I should due to properly insulate without having to redo framing. Should I remove poly vapor barrier and install rigid foam board?