Complete basement retrofit for 1974 home in Climate Zone 7a
Hi, I purchased a 1974 bungalow in Alberta, Canada, so climate zone 7a. Some specific info on the home:
- Basement unfinished… foundation would be like this http://bit.ly/2kadR6I, and may or may not have a bitumen layer around foundation. There is definitely no exterior insulation, poly under the slab, foundation drainage, or sump pump. The foundation rises approx 2’ above ground-level.
- The main floor is 2×4 wood stud construction with 2 layers of 3/8” gyprock, poly vapor barrier, yellow fiberglass insulation, and the external sheathing was built using some type of fiberboard sheathing like black board, gray board, or buffalo board; followed by a layer of tar paper, metal mesh, and finally stucco.
- No water leakage in basement and no cracks in the floor. The water table is high here and neighbors on one side are higher so I have to manage their surface water to some extent. I’ve raised outside grading so all should be good.
- On that side of the basement, the slab shows recurring efflorescence and some spalling in areas. Given the basement is unfinished, moisture can evaporate so all is dry. I have some concern that if not done correctly, the obviously damp soil may become a problem. Rest of basement is fine.
My internal basement wall construction plan is to follow this guideline. The only notable difference is I have no under-slab insulation. The 2” and properly sealing should give me ~R10, which should provide sufficient thermal break to prevent condensation. I know XPS is less enviro-friendly but from all accounts it appears to be the best option. If here is an EPS someone can recommend, that I’ll consider.
I’ll then build a 2×4 wood stud wall at 24” OC with a pressure treated base plate on top of a gasket seal. I’d use 2×6 but this would take up far too much livable space from the basement. I plan to fill the walls with Roxul. Now I’m aware that ComfortBatt R14 is the product designed for this, but I can purchase some CavityRock at about 1/3 retail so I’m strongly considering using this instead. It’s not as dense as ComfortBoard and I’m aware that it will require a little more cutting and time to install, but I’ll achieve the same R-value and have all of the same benefits as ComfortBatt at a 1/3 of the cost (ignoring my time). Your thoughts on this?? The CavityRock is 4″ thick so I can place the 2×4 studs slightly away from the wall.
Alberta Code 2014 requires a vapor barrier (CAN-CGSB 51.33M). And I believe this would make all drying go to the outside. Taking into consideration the work done between Roxul, Doug Tarry, Gord Cooke, Dr. John Straube and John Godden I will somewhat emulate their “Optimum Basement Wall” design with respect to the vapor barrier, being lower half 6 mil poly and upper half a more permeable Smart Membrane. I’ll also seal all joists with 1” XPS and cover with either poly or a smart membrane and then cover with ~5.5” of Roxul R22 for the thermal barrier and added insulation in those areas.
I would also like to have an insulated subfloor. Dow and Corning both show installs of placing XPS directly on top of the slab. That may work in newer constructions, but on a slab like mine (as described earlier) the concrete will always be wicking moisture so I’m wondering if leaving an air gap between the slab and the insulation is best? Thoughts?
The solution I came up with is to lay a dimpled waterproof foundation membrane on slab, tape all seams, cover with 1/2″ XPS, then lay either plywood or OSB tongue and groove panels. I can do this after building the walls, or I could build the walls on top of this. This will provide a sealed vapor barrier, radon barrier, etc.
Manufacturers of the dimpled membranes advise to leave a 1/4″ gap from wall (the XPS on wall, in my case) to leave for some evaporation. Any of that can go into the wall cavity and evaporate out the top through the smart membrane. This is really no different that building a wall on the concrete slab and vapor can rise up through concrete into wall.
Given my situation, what are your thoughts on my construction concepts?
Thanks in advance!
Todd
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Replies
The missing links are: My internal basement wall construction plan is to follow this guideline: http://bit.ly/2kadR6I
... work done between Roxul, Doug Tarry, Gord Cooke, Dr. John Straube and John Godden I will somewhat emulate their "Optimum Basement Wall" design with respect to the vapor barrier http://bit.ly/2j5W575
Todd,
I'm not in favor of installing any vapor barrier on the interior side of a 2x4 wall in a basement. If you are forced to put a membrane there by a code official, I would install a so-called smart vapor retarder like MemBrain.
What to do with the damp slab is a judgment call. The conservative approach is to install an interior French drain along the side of the basement where you think the moisture might be entering. The French drain should lead to a sump pump with an airtight lid. And the sump pump should discharge outdoors, either to daylight or a dry well, or (if permitted by your local authorities) into the town sewer lines.
If you've got the moisture under control, you can install rigid foam above the concrete slab -- with or without a layer of polyethylene under the rigid foam -- with or without the dimple mat. Ideally, you'll squeeze out room for more than 1/2 inch of rigid foam.
For the walls and floor, you don't have to use XPS. EPS will work fine.
-- Martin Holladay
Thanks, Martin.
I don't think I have a water issue, per se, only some visible signs of moisture in that area due to the recurring efflorescence. I have been thinking about installing a sump pump and still may, but the basement has never had any water in it, even when that half of the foundation was literally surrounded by a foot of water a couple of times after heavy rains soon after I first bought the house. The grading on that side was sloped towards the house and I have since added a lot of clay and properly graded. We're in the dead middle of winter to I'm thinking that I'll wait to do the floor until the summer when the ground has thawed and I can see if the moisture issue has subsided. So I could just do the walls now and wait on the flooring.
Yes, I have no problem using MemBrain... I have been looking at it already and as long as it conforms to CAN-CGSB 51.33M it's all good. Regarding the XPS vs EPS, I've done a lot of research on this and was originally planning to use EPS. From info from various credible sources incl Building Science Corp, most were recommending XPS over EPS due to the permeability factors and the slightly lower R-value between the two with respect to creating the thermal barrier to prevent any interior condensation. Most info was saying that in climate zone 7 at least R10 is required to achieve this. What are your thoughts on this? From your original response you don't see it being an issue.
Regards
Todd
And to just add to the EPS vs XPS, it wasn't just the difference in R-values but, as stated, the diff in permeabilities. The higher permeability foams were better suited to warmer climate zones or places with higher humidity, but the XPS in colder and less humid. So when asking on your thoughts in my previous question, this is what I was more referring to.
Btw, you didn't say anything about me using the CavityRock in place of the ComfortBatt so I'm assuming that you have no issues there. I did speak with Roxul and, while they advise against it, it was mainly do to the extra cutting and less "bounce" of the product to fit between the studs. But it's easy enough to cut and not too dense where I can't get a nice friction fit. They had no other technical reasons and said all their products are made of same material and off the same stated R-value per inch of product.
Thx
Todd
On the damp slab adding the dimpleboard without a place for the moisture to go is contradictory. If you're concerned at all about moisture in the basement I recommend breaking up the slab around the perimeter and creating a mechanism for the moisture (including condensation) to drain below the slab and into a drainfield with a sump system. I'm not a fan of the simpl drain track systems - there are too few scenarios where they actually work.
https://www.mtidry.com/wet-basements-101 has some DIY resources you could reference - you'd simply add the insulation as you mentioned, tape the seams and use a good sealant at the edge of your new poured slab and at the top of your rigid insulation. If you're going through this much of a remodel it would be worth getting a radon test and determining whether you need a mitigation system.
A sort of combination of the two approaches attached would work.
Thanks, Tyler, I appreciate your input.
Fyi, I was only thinking of putting the dimpleboard on the slab so as to create an air gap and a water + vapor proof barrier between the insulated subfloor. It is either this or just put the foam panels directly on top of the slab... only foam is not 100% vapor impermeable. I could put poly and then foam, but this still won't stop moisture/vapor from wicking up through slab. My debate between the 2 options was centered on should there be an air gap or not?
As for any moisture (not water)/ vapor wicking through the slab, with the dimpleboard on the slab my idea was the vapor would dissipate around the perimeter edges, into the enclosed wall cavity. The required vapor barrier can be something like MemBrain, as Martin suggested, so this construction will allow any moisture to safely dry to the interior.
So to be clear, I don't have any water penetration or condensation on the walls, etc and I don't consider my situation to be a wet basement. I've just had some recurring efflorescence and a little spalling in a few places so it's clear that there has been a lot of water in the soil. The grading around that side of the house was abysmal but I fixed that this autumn so water can no longer drain towards the house.
I don't at all disagree with considering putting in a sump pump, but I honestly don't believe that a french drain is necessary at this point.... though I won't rule it out. I'm thinking that I can hold off on the floor for now and in the spring when the snow melts and the ground thaws that I'll have a better indication if I still have a sub-slab moisture issue.
In the interim, however, I would like to insulate the walls because we've been getting a lot of -20*C and colder weather, which can leave the main floor a tad chilly!
If you disagree with anything I've said, pls feel free to comment back. I'm all ears and my objective is to do everything correctly.
Regards
Todd