Insulating interior basement walls after interior drainage membrane applied
My plan is to apply a dimpled drainage membrane (Platon, Delta, SuperSeal) to the interior foundation walls from grade level to newly layed weeping tile beneath the floor of the basement (exterior solutions are not an option due to limited access). The exterior of the foundation walls is uninsulated. Poured concrete basement walls are 7’8” high from floor level. Grade is at 6’ mark. I am planning to insulate and frame the basement walls. How should I frame and insulate the walls to prevent condensation on the surface of the plastic drainage membrane? The drainage membrane is an air gap, but will likely be the temperature of the soil and I am concerned about the warm basement air going through the insulation and condensing on the membrane.
Is there any danger of condensation developing on the interior side of the membrane? Would a layer of XPS (extruded polystyrene) with sealed and taped joints prevent condensation? I would also like to add additional batt insulation – Can I place it touching the XPS? To reduce costs, can I build the face of the wall 5 ½” from the drainage board and insert 2×6 batt insulation in my 2×4 wall or do I run the risk of condensation developing on the batt insulation? Is there a need for a vapor barrier of poly sheeting and if so, how and where to apply it?
Replies
Basement, water vapor, condensation, insulation
The drainage membrane will allow the water from outside to run down the wall into the drain. The room side of the membrane will be warmer than the wall side so you will get condensation on the membrane, this will also drain down = water running down both sides, not a problem goes into same drain.
The air in the basement will be warmer and wetter than the dew point/temperature of the basement walls, therefore without proper water vapor sealing, this wet air will condense on the plastic membrane, or slip round it onto the wall, but as I wrote earlier, not a problem.
Your proposal, polystyrene sheet, with can foam sealing will keep the warm wet air away from the frame and the drainage membrane. Keeping in mind that it is very difficult to design and make a water vapor proof wall, if the water vapor does get through it will head for the nearest cold surface and will condense either on the drainage membrane or the concrete wall, in either case it will do no harm.
Do not use batts, they are useless, wet air can easily pass through them, the batts become wet and cold and useless.
Damp Basement - Insulating
I recently watched an episode of Holmes Inspections with Mike Holmes. I have a basement with high humidity (home is over 60 years old in Michigan) and we have a dehumidfier constantly running. The damp air transfers to the upstairs causing a damp smell. On the Holmes show they used a dimpled membrane on the walls with treated lumber framing, spray foam insulation, then mold resistent drywall to fur out the walls. I liked the looks of this application but was wondering if this would help take care of my problem (my edge drains around the house seem to be working) ( do have a crawl space with an earth floor and I put vapor barrier down on when I moved in 14 years ago which seemed to work then)?
What A Flood Taught Me ...
FHB just had an article on building basement walls. While the article departs from earlier 'conventional advice,' I still have some differences of opinion.
I keep telling folks: Your first step is to FIX THE FLOOR. Place your eye at floor level and you'll see ... all manner of dips and ridges. The floor needs to be able to drain. Basement floors are carelessly installed, not nearly as flat as your driveway, and the sump opening is often at the highest point of the floor! There are various self-levelling products (like Gypcrete) that can be applied in a thin (1/4") layer to address this. You don't want water pooling and stagnating on the floor.
As for the walls ... First, leave a space between the foundation walls and your insulation. The dimpled sheets you mention will do this. This will let any moisture (from condensation or seeping) drain.
Next, find a way (shims, etc.) to keep the wall frame OFF the floor. Again, this is to provide drainage under the wall. Use either light steel or pressure-treated wood. You'll want at least a (nominal) 3" wall, to allow space for wiring and plumbing.
There's no substitute for foam panels as insulation. Either attach to your dimpled membrane, or to the blind side of the wall framing - taking care to allow for drainage under it. Foamboard won't absorb water. Leave your wall cavities open for the wiring, etc.
Whatever you cover the walls with, leave an inch or so gap at the bottom. This will prevent the material from wicking up any moisture on the floor. Conceal this gap with PVC molding. Again, leave a gap, or make weep channels, to let water drain from within the wall cavities. This is NOT a place for caulk!
Which brings us to ... floor covering. These days I'm favoring a layer of coreboard (that 'plastic cardboard' used for signs) to allow drainage, covered by those 24" square, rubber-backed carpet tiles. If the tiles get wet, it's a simple matter to pull them up and let them sun-dry on your driveway.
My concerns may seem exaggerated .... but, having seen how less than an inch of water caused $15,000 damage that's to be expected. That little bit of water, with less than 12 hours of exposure, wicked 4" up the masonite panelling, 6" up the drywall, and 12" up the fiberglass insulation. Mold odors began to be noticed less than 6 hours into the exposure.
Which, of course, is a reminder to us that you also need to provide separate HVAC for tha basement- with a bias towards greatly increasing the ventilation!
Good advice here
Closed cell spray foam would be a good option, but rigid XPS is fine too. Tape the seams, use canned foam on any gaps. Be sure to install an air barrier on the bottom of the joists that spans between the mudsill and the interior face of your wall framing. This can be plywood. Also, do not use a vapor retarder on the inside of this wall.
On my two most recent jobs with basements (both 2 years ago, same original builder, uncoated block foundations in clay soil) I used closed cell foam each time and it was cheaper installed then rigid foam for what I believe is a better result - amongst the advantages, it is continuous and it locks in the studs to increase the wall regidity.
I considered having a membrance like Delta to the outside, but in consultation with my insulation sub I concluded it was not necessary as long as I used 2"
My sub says you needs at least 2" to be a vapor barrier, which is what I used, but while I'm sure that is technically right I have trouble believing 1" would not have been adequate, but not being certain I took no chances.
I am at both houses regularly and I would charactorize the air quality as excellent, and the HVAC load as minimal (one of them is walk out on two sides with only a few registers returns for 1800 sf, yet we never ended up installing the suplemental baseboard heat, though it is wired for, because it remains comfortable in winter in Zone 4).
something to consider
My understanding is that closed cell foam should not be sprayed on the interior of foundation or crawl space walls; particularly those foundations that are moisture or vapor permeable. This is because the closed cell foam does not provide a capillary break between the foam and a potentially wet foundation wall interface. If the foam is sprayed up onto framing (sill plates and band) then capillary action can transfer moisture verticially to these areas that "care" about moisture penetration. Of course it depends on how high your foudnation continues above for this to be a major concern. If there is a large area of exterior foundation exposure that allows this migrating moisture to readily dry to the outside, then your probably safe. If not, then you may be promoting rot in an area that is covered by insulation. You probably won't reap the results of this bad choice until it's too late.
My foam insulator will only use open cell foam sprayed directly onto moisture permeable foundation walls for the reasons outlined above.
If one were to install a capillary break dimpled memberane on interior wall as O.P. indicates, then this would not be an issue with closed cell foam. Then again, you might as well use open cell foam at that point which is cheaper to install-- and skip the dimpled membrane all together. This logic tells me that open cell is the way to go for the O.P. if they want to foam.
BTW, we usually frame interior basement walls in light guage galv. steel studs if we choose to install an interior surface vapor retarder and batt insulate with fiberglass. This framing cares very little about moisture. Condensation would only be an issue if dew point is achieved within the wall cavity insulation; vapor drive from the outside is really not in the equation with a vapor retarder. In my opinion any condensation would have to be caused by warm, moist air leaking past fiberglass insulation from the inside. Doubtful. Again, IMO, this would mainly be caused either by a really crappy batt insulation job or a forced air system driving warm moist air through drywall and into wall cavities. Crappy insulation job, perhaps. But you have control over that. The latter pressurized scenario, not so likely. The home would have to be pretty airtight otherwise in order to force air toward this (least likely) path of least resistance during the heating season. I don't see that usually happening.