Follow up question re: basement subfloor (climate zone 7a)
A couple of weeks ago I asked a variety of questions pertaining to my livable space basement retrofit for a 1974 home in climate zone 7a. Recap: there is no exterior foundation or under slab insulation, etc. No water penetration issues into the interior but part of the basement has shown recurring efflorescence and some spalling in a few places.
All HVAC is in basement between floor joists. As crazy as it sounds, I wasn’t aware of dampers until recently so I have now adjusted these to better balance the heat throughout the house. I have also completely sealed all hot air and return air ducts, etc.
Too much heat was being directed into the unfinished basement so for the first time it’s now cooler down there and the slab even feels a little colder.
My question is, could the earlier warmer basement conditions could have been a reason for a lot of the recurring efflorescence? If by cooling the interior side of the slab, could this reduce the amount of vapor transmission?
If so, then maybe I’ll install the foam insulation directly on top of the slab and not bother with the dimpled membrane (whose purpose was to create an air gap).
Thank you.
Todd
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Replies
Todd,
Have you tried taping a square of polyethylene to the concrete for 24 or 48 hours, so see if moisture beads up under the poly? The test is described in this article: Fixing a Wet Basement.
-- Martin Holladay
Hi Martin, no, I haven't tried that; though, I'm aware of that test and have read that article before.
I never bothered trying it because since I've had the house for the past few years I've never once had any condensation appear anywhere in the basement. I also live in Alberta, so the air here is always relatively dry. Before I fixed the outside grading, as previously mentioned in another thread, after a couple huge rains resulting in a foot of water around half of the house, I never had any water penetrate into the interior except through one tiny hole. So I know my foundation is sound in that regard.
However, I'll try this test today and see what happens over the weekend.
Cheers
Todd
Todd,
If you perform this test, it's quite possible to get moisture under the poly even if you have never seen damp concrete. Evaporation occurs continuously, so the fact that the surface of the concrete is dry doesn't tell you much. Lots of moisture may still be entering your house this way, even if the concrete looks dry.
-- Martin Holladay
Hi Martin, yes, I'm aware of this but after much reading on your site and others I was under the belief that a little moisture like that is ok, even for a finished basement, as long as (1) water can't/ doesn't pool inside, and (2) it's as airtight as possible, so not allowing warm interior air to contact cold air on slab/ foundation. After all, my slab has no exterior protection so no matter what it will always be wicking some moisture from the soil and there will always be some level of vapor transmission.
I understand, however, that we're more talking about excessive underslab moisture and too much vapor transmission... I guess I'm just unclear if any level condensation on the poly is considered normal and acceptable, or if this means "no" you have a moisture issue that needs to be addressed before you go any further?
Btw, while I already did much reading on the subject of grading and redirecting surface water last year, I wasn't aware of the GBA site so I have been subsequently reading up more on the subject following this discussion. So I will post a question on this topic in a new thread so that it's more easily found by others searching the site.
Cheers
Todd
After a day, I have no moisture under the squares of poly I taped to several different places around my basement slab and on one wall. I also laid down a 2x8' panel of 3" thick XPS and placed a 25 lbs weight on top, and under a portion of this I have a taped ~12"x12" poly test patch. Under the poly there showed no moisture. However, under almost the entirety of the foam panel the concrete is visibly damp.
So thoughts? Recommendations?
Cheers
Todd
To add, I taped the 6 mil poly using tuck tape. I applied a couple of extending layers so the tape has ~4" or so of possible adhesion. I know there isn't a perfect airtight seal; nonetheless, even with a tiny bit of air being able to get under the poly, is that enough to ruin my test? The "sweat" on the slab under the 3" XPS may be more telling, however. I don't know what conclusion to come to. I haven't tested this, but I'd say with near 90% certainty on the other side of the basement there wouldn't have been any dampness under the foam.
I live in 5b, 90 year old house with slab basement. Slab is 3-4 inches at most, sitting on a sand base. Soil is very sandy. We excavated for sewer replacement in Aug and the soil was dry, dry, dry even 6-7ft down - hence the oaks and maples in the yard going after the sewer for 90 years.
In the past I have done the taped plastic test and had no moisture. This year, since Oct we have had record moisture. Snow on the ground (multiple feet) since Nov. Days on end of rain. It's suppose to drop 8" of snow today.
This winter I have clear signs of vapor drive. If a plastic bucket lid or tarp is set down and left for a couple days the concrete will be damp when lifted. The moisture evaporates before your eyes... but still. I bought accurite meters and the space itself (basement) sits at 35%, high is 38%. Summer is in the high 40's.
I have no outside water management and have been working on that. Gutters going up this spring (cape style house). I am fairly confident that with some water management I can lessen the outside moisture that makes its way down along the foundation. Even with all this water we have had, we don't have any bulk water issues (never have from what I can tell).
My concern was the dew point issue where the warm air (summer and heated air in winter) would condense on the cold concrete. I had a large closed cell foam backed area rug (12x16 I believe) down there on the bare concrete for years and that was never wet and when I rolled it up to get rid of it there was nothing but dry concrete (this was a couple years ago).
I believe Dana commented on a thread I had stating at the depth I am down, and the constant soil temps condensation probably isn't a concern. A portion is finished (tile right on concrete basement) and we have bath mats and rugs to keep toes warm and we have zero issue with dampness, mold, etc.
That said - and given I have head height concerns - our plan is to use this https://www.dmxairflow.com over the concrete and then a rigid core lvt plank tile. I am thinking it should work well.
Sean, I'm in climate zone 7a, so our winters can get to -35 and even colder; though, not an everyday occurrence, so I have to I have, for the most part, fixed the majority of the grading issues, but there are still a couple of other things I need to figure out.
My neighbor is ever so slightly higher than me and all of their surface water drains into the center of my yard... this water then has to move downhill and towards the street, and my house is in the middle of this, So I'm planning to top my existing sod with topsoil to raise my yard grade, but I still have to redirect that water somehow. It either has to go along the fenceline and out, or diagonally across my yard and out the other side. So you too must think about the external influences in order to make the best decision for the interior... that's what I'm trying to figure out for my place.
Regarding your situation, the question to ask is, can you put foam insulation directly onto the slab or do you need/ want an air barrier? I'm still trying to determine the best approach in my case. If you're leaning towards the latter, I highly recommend you save your money and instead of purchasing dmax I'd purchase an EXTERNAL foundation dimpled membrane. It's literally the exact same product for a fraction of the price. The salesperson will tell you otherwise and the packaging for the external membrane will prob say, "not for internal use." That's BS; it's all marketing... exact same product.
Moreover, if you do more research basement subfloors or, internal foundation walls, Martin, Peter Yost, Dana, and others all focus on establishing an effective "thermal bridge" to prevent condensation. I'm no expert... clearly, but if it were me, I'd first make sure that I've solved my external water issues and that my basement will remain dry, and then decide the next best steps. The dimpled membrane will give you an air gap but it doesn't provide any insulation. So while the rigid core lvt plank tiles may have some foam insulation engineered on them (?), is this enough... from what I've been learning, the answer is 'no.'
So, imo, we face somewhat similar decisions... but if I were you, dimpled membrane or not, I'd install at least 1" thick rigid foam and ~3/4" subfloor, and then whatever finished flooring product you choose. Fyi, there are closed cell, high density EPS foam panels that have suitable compression rates and ~R5 per inch instead of the typical R4. Climate change is happening, even if Trump says otherwise ;)
Outside of my recommendations, hopefully someone more knowledgeable can better assist you.
Cheers
Todd
Todd,
Any time you finish space that is below grade, you are assuming a risk of water damage. If you don't want to assume that risk, build an above-grade addition to your house.
There are ways to minimize the risk, and it sounds like you are doing that. With the type of evaporation from the concrete that you are describing -- some evaporation, but not a lot -- I think you can proceed with your finishing plans, as long as you accept the fact that there remains some risk.
If I were you, this would be my proposed stack-up, from the bottom up:
1. Existing slab.
2. Polyethylene.
3. Dimpled membrane (perhaps connected to a sump -- perhaps not).
4. Rigid foam.
5. Plywood subfloor.
Thanks, Martin.
What's the purpose of the poly sheet in this application? I'm assuming it's to add another layer of air tightness and moisture barrier, and not simply relying on sealing the seams of the dimpled membrane.
However, if the poly runs up the foundation wall 6" or so (be it up the concrete or up the foam on the wall), then the dimpled membrane... if I did put in a sump, how would it then connect to the dimpled membrane with the poly in place?
Todd
Todd,
Good point. If your basement includes a sump, skip the polyethylene.
-- Martin Holladay
Thanks Todd and Martin.
I have looked into the platon (I think it is) and perhaps some landscape fabric underneath to stop the clicking sound you can get. My space is small, so I don't really mind putting in something that costs a bit more if it works a bit better.
The standard dimpled mat, with the hundreds of small divots on the top? Makes no sense to me indoors. If you have a bulk water issue, washer hose goes, etc you got 100's of little puddles underneath your flooring? I'll need about $200 of the DMX airflow for what I am doing.
Regarding the foam, I left some xps down for a week and there is some dampness when I lift it up. Normally, if I would cover my floor with foam, does that moisture just sit there all the time? That's what has me worried about that situation. As is with the slab open whatever is there is evaporating, and I would think the dmx product would help with that. Again, humidity levels are low, even now. Not sure what to make of it.
Foundation wall build out is 2" foam, 2x4 wall with Roxul. I ducted the basement space but keep the dampers closed (I put dampers on 6 runs when I redid some duct work). It's about 65 down there in winter. The concrete itself is around 60 most of the time.
Sean,
Martin is far more qualified than I am to help you. However, from what I've been learning over the past while, nothing is going to save you in the even of a flood. If the water level gets higher than your subfloor then anything above is at risk. The dimpled flooring is only 8 mm high, so it only (1) gives you minimal protection from a little water than may pool on your slab, and (2) it provides an air pocket that allows for some evaporation of moisture emanating from your slab.
I have the same situation where a piece of XPS on the slab will develop some moisture underneath. A little bit isn't horrible, but none would be better. In a basement, one can't always expect the latter so what we do is all bout mitigating the risks of finishing a basement into a livable space. That's why it's important to fix all of your external issues before finishing your basement. If you haven't done that yet and have a little concern about it, then hold off for a bit until you're confident that you won't have any external water penetration when it rains a lot, etc.
As Martin said in his last post to me above, he recommends the dimpled membrane and then at least 1" of foam to create an effective thermal bridge to help prevent condensation from forming. If you're not doing a sump pump then you could also lay some poly on the slab first, and then the dimp membrane. The poly and the dimp membrane must both be properly sealed at all seams so that you can get it as close to 100% airtight as possible. Warm air meeting the cold slab is what forms condensation, and you can't really stop vapor transmission from the soil up and through the slab so you just have to prevent it from reaching your interior "finished" space. Let the vapor/ moisture evaporate into the wall assembly and safely dry to the interior.
XPS will work just fine BUT, it's 2-3 times the cost of somewhat comparable EPS foam and it's far less eco-friendly. Do some research and find a high density, closed cell EPS foam... it will be likely type 2 or 3, and may be used for under-slab insulation. If you can find one with a membrane or foil, then that can help act as a moisture barrier. For eg, I'm looking to use Halo Subterra for my subfloor, and Halo Interra for my interior foundation insulation (plus the 2x4 studs with Roxul). This EPS brand has an R5 per inch and is quite affordable.
As per my earlier response to you, I guarantee that exterior dimpled membranes are EXACTLY the same thing as interior dimpled membranes. It's all about marketing. The latter, however, will cost you 4x as much. Though, they do come in larger rolls so if you have a small space you might have leftovers.
As for the landscape fabric underneath, I know I've seen several people recommend this but personally I'd avoid this, as it could become saturated and not ever dry, and it may provide a medium for mold to grow on. If you have the dimpled membrane, then ~1" of foam, then your T&G 3/4" to 5/8" plywood or OSB, then you have a thin layer of soft subfloor material and then your engineered flooring... I I'm reasonably confident that you won't have the clicking sound. That's normally when people go dimpled membrane, then final flooring overtop.
I'm also planning to have a floating floor, so I'm ideally looking to avoid using fasteners like Tapcons. If my base is relatively flat and everything is properly sealed, the weight of the wood and final flooring plus any internal walls built on top should, imo, be sufficient. I could always apply a little adhesive to the foam and slab, and foam to plywood.
Those are my thoughts and recommendations.
Cheers
Todd
Terminology: The 1" foam is a thermal BREAK, not a thermal bridge.
Floating floors over slabs has some risk of developing wave or "potato chip curl" with normal seasonal indoor humidity changes. Don't be surprised if you have to throw down a few TapCons at some later date.
Both EPS and concrete are highly tolerant of water, so it doesn't matter if they stay damp. It's the susceptible materials above that you're trying to protect.
The temperature of the piece of slab under 3" of foam is likely to a few degrees cooler than under 6 mils of polyethylene, which may be what made the visible difference.
Sheet polyethylene is cheap stuff, and thus cheap insurance, even with a sump. Dimpled membranes are good for managing visible bulk water seepage, but if that's never been an issue it's not clear it's worth the additional expense.
In locations with high uranium / radium geology and the correspondingly high radon levels, a reasonably air-tight membrane (either cheap poly or dimpled goods) can make a measurable difference in the indoor air's radiation levels, even without radon mitigation systems (such as sub-slab depressurization.)
Certainly outside water management is of upmost importance. I doubt I'll get to my finish floor immediately, so spring time work will help (grading, gutters, etc).
The DMX product is $70 for 100sqft and comes with the appropriate tape. Given that the platon rolls (I can see) provide way, way more than I need (for more money, not per square foot but in totality) DMX might make sense. I'll cross that bridge when I come to it.
The XPS I had was scrap and was just a test. We have Insulfoam R-Tech (I think it's called) at all the big boxes and lumber yards (I think it's made in WA state). Foal backed and 'sealed' with a thin coating of something on the other. It's cheap enough to buy. If that was used, would it be foil side down?
Radon is a concern. I did testing and my number (forget what it was specifically) was under the threshold by a bit and considered safe. When I get everything and tidied up, I'll test again. Good to know that even with a porous concrete floor my levels were low and adding poly, dimpled membrane, etc will only help.
Sidenote: I see names of posters sometimes, sometimes just a user_ID or na/na?
Sean,
The irregularities in the screen names below comments (sometimes N/A N/A, sometimes "user-72446") is a web site glitch that our technical team has been working to fix for over three weeks. We apologize for the confusion -- hoping for a fix soon.
-- Martin Holladay
The box-store EPS is usually "Type -I" ~1lb per cubic foot nominal density, R3.9-ish/inch with a 10-12psi compression rating. That's going to be OK, but Type-II goods (1.5lbs nominal density) are more rugged, with better compression ratings- typically 15psi or higher. Type-I can also take on more moisture than higher density EPS, which is an issue if it's truly wet soil under the slab. If foil-faced one side only, yes the foil side is down, since that's where the moisture drive is coming from.
With either you'll want to keep the seams of the subfloor a foot or more away from the edges of the foam to distribute the higher load at the subfloor edges between sheets of foam. If the subfloor sheets are directly aligned with the sheets of foam, compression of the foam at the seams can happen over time.
Thx Dana, yes, I meant to say thermal "break" not bridge :)
With respect to the potato chip curling, I can't see that ever becoming an issue unless moisture is able to penetrate the moisture barrier and being able to get into your subflooring materials.
I'm not concerned about standing water under the subfloor. My main concern has been about excessive vapor transmission and moisture wicking of the slab.
I just thought an air gap that allowed for some vapor transmission and drying might be better than trapping moisture under poly or the foam itself. An external dimpled membrane would cost me ~$CAN300 to cover 1400 sqtf. In my tests I've done, I've noticed that I only get some wetness when the slab is not allowed to breath. So I'm still trying to decide to just lay poly then foam, or to use the dim membrane and then foam.
I haven't yet tested for Radon, and on the map it shows much of our area is med-high but looking at the gov't stats for my area, and 93.4% were below 200 Bq/m3. I still plan to buy a kit and do as much as I can to mitigate the risks, irrespective of me having high levels or not.
I also stupid forgot that I still have to rough in a bathroom, so will have to cut into my slab anyway. Where I plan to put the bathroom is reasonably close to the main sewage line under the slab, which itself is, coincidentally, in the part of my basement with the efflorescence. So I may just install a sump pump yet, I just don't know if I'll go to the trouble of doing french drains. Running a radon vent up through my attic could be difficult unless I could possibly tie it into an existing vent line in the basement.
Regards
Todd
Sean, you're not only doing 100 sqft?? External foundation rolls come in a variety of sizes and are literally 1/4 of the price of DMX.
Look for a type 2 or 3 EPS in your area that is made of Neopor, which has ~R5 per inch. The panels you could consider are often used for under-slab application, so are of higher density, tho some internal application ones are also sufficient... somewhere around 15 psi. (http://neopor.basf.us/files/pdf/Neopor_Plus_GPS_2016_v3.pdf)
What city and state do you live in and I'll have a couple of Canadian foam manufacturers forward me some local dealer info for you.
Then as Dana said, stagger your plywood seams with each other, but also with respect to those of the foam panels, which themselves should also be staggered (standard brick laying pattern). Martin said in a diff post that 2 layers of plywood on top is best, staggered, and with each layer running in opposite directions of each other. I'm not going to go to that extent due to cost.
Cheers
Todd
Thanks Martin. I figured it was a technical glitch of some kind.
No Todd, I was just noticing how it's sold.
When the time comes for the finished floor I'll have to check my ceiling heights and see what I feel comfortable going with. Looking at under 7' height. And while it's mostly bonus space (tv, kids area) and a spare room, if the height will be crazy low and uncomfortable feeling I may just have to go with dimple mat and lvt or poly and lvt and keep an eye on it.
With respect to potato chip curling, the moisture exchange creating the issue is seasonal shifts of moisture from the subfloor to & from the room air, not the slab.
In a 7A climate the dew point of the summertime ventilation air is SUBSTANTIALLY higher than the wintertime air, even if you keep it above 30% RH in winter and under 50% RH in summer.
Only the top of the subfloor is exposed to the room air, so when moisture is leaving the wood, the top side get's drier sooner than the side next to the foam or moisture barrier, causing it to shrink, causing the curl. When it's taking on moisture from room air it's a bit slower, since the vapor permeance of dry plywood is quite a bit lower than when it's at a higher moisture content. You're most likely to see potato chipping arise in the fall season as the air becomes rapidly drier.
One method of preventing this in a floating floor is double-layering the plywood with staggered seams, gluing the layers together with construction adhesive. That makes the size of the smallest "potato chip" the entire floor rather than a single 4x8 sheet. There is still some potential for getting wave, but there won't be any vertical separation at the edges of the sheets.
BTW: 200 Bq/m3 is above the US EPA recommended action level of 4pCi/l (= ~150 Bq/m3), not that it's worth chasing too hard if you're between 150-200 Bq/m3. (IIRC the European regulations limit it to 250 Bq/m3.)
One the first things a radon abatement contractor would do is put an air-tight lid on the sump, which of course makes it useless for draining and liquid water from on top of the slab, though still useful for keeping the water table from overtopping the slab, assuming you have a pump. If you're handy this is DIY-able, fabricating a heavy plexiglass or polycarbonate lid (with the appropriate holes for sump pump plumbing & electrical), sealing the lid to the slab with a polyurethane caulk formulated for sealing cracks in concrete, and sealing the penetrating plumbing & electrical to the lid with the same stuff (yes, it's messy & ugly.) The clear plastic allows you to visually inspect the quality of the air seal, and add more caulk (if necessary) before the caulk sets. If it ever has to come off to service the pump it's likely to be a destructive process for the lid.
While it's all still open it's worth cleaning out then polyurethane caulk sealing the perimeter crack between the slab and foundation wall, and any other cracks in the slab &/or foundation wall.
Thanks, Dana.
The radon stat only said under 200... what the exact average levels were, I'm not sure. But the point of the stat was that the vast majority of people in my area likely don't have to be concerned.
As stated, I'm really not worried about liquid water on my slab. If I didn't get any water in after half the house was surrounded by water a foot deep, then I'm pretty confident. If I do put a sump ion, it's more about controlling groundwater levels. Thx for the info on the sump pump install... I haven't yet done any research but like your ideas.
Regarding the potato chip concern, on top of my subfloor I still plan to put something like engineered flooring for the finished layer. And under that I may put a very thin cushioning subflooring product to go under laminates to help with noise. Some of these are also rated as vapor barriers. So once done, I'm quite confident that no moisture can penetrate from either the slab or the interior space and that any changes in RH levels (likely/ hopefully) won't be an issue. I will build the subfloor such that any possible moisture can safely evaporate into the wall assembly and dry into the interior.
Cheers
Todd
> After a day, I have no moisture under the squares of poly
I suggest that the moisture seen today says very little about worse case moisture levels that you need to design for.
Jon, I appreciate the input and get your point. It's been several days now and only under a few of the squares of poly is there some very minor condensation. I'm not at all worried about water penetrating the slab/foundation and flooding the basement, even after excessive rain or melting. I've made substantial changes to the exterior grade, so all is being addressed.
My only lingering concern that needs to be addressed is the control of the subterranean water table level, as (1) I don't currently have a sump pump, (2) I still have to do a few things outside to better redirect the surface run-off from one of my neighbors. While this can't flow to my house, per se, it can seep down to the water table, which can make it's way to my foundation. I'm currently evaluating some options and will do something about this in the spring/ summer when I finish my landscaping.
Todd