Basement floor insulating drain question

[Esser]

Hello everyone, fish time poster. I tried to search for this answer but I came up blank.

Quick note about my project. I am finishing a basement in my house in Edmonton, AB where the temperatures can go as low as -40. The house was built in 1974 and has zero basement insulation. While I am at it, I am cutting in some windows and converting the entire house to in-floor heat.

I am insulating the floor of the basement with 1.5" EPS with 5/8" particle board on top. The walls have 2" XPS durofoam on them plus a 2x4 framed wall over the XPS on 24" centres willed with Roxul comfortbatt.

Now, for my questions. I have two drains in what is becoming a bedroom that slope sharply into the drain. over a 3' area surrounding the drains it slopes 4" down towards the drain like a big dish. What is the best way to subfloor over this? Part of my thinks it would be good to have those drains and the concrete as in under the floor so is there was ever a flood could help drain under the sub floor (my property is all sand and I have never had a water issue in the basement.) I guess in this situation I would just cut 2x4s to match the dish so I can build my subfloor across? My second thought was to use self levelling concrete and bring the drain up to the height of the surrounding basement slab and build the floor on top as I have been doing. And lastly, is there any benefit to having the drain come up to the top of my finished floor?

 

[Esser]

Additional info. The drains are in what is currently the laundry room and what looks like an area they might have roughed in for a bathroom. They aren't the main sump but they do drain to the sump.

 

[Sponsor]

 

[Dana]

Even though it's labeled R10 a 2" sheet of XPS is only warranteed to R9, which isn't quite enough for dew point control for R14-R15 rock wool on the above grade section of wall. Averaging over 5000 HDD (base 18C) Edmonton is in IECC climate zone 7.

The IRC prescriptive for dew point control on 2x4 framed walls with only a class-III vapor retarder (standard interior latex paint) in zone 7 is R10 on the exterior side, with a presumptive cavity fill of R13, which would be 43% of the total R as foam. With R9 on the exterior of R15 you're at about 38%.

Making the gyprock as air tight as possible and painting it with "vapor barrier latex" primer would be more than enough to mitigate this fairly modest risk. Alternatively using 2-mil nylon (Certainteed MemBrain) detailed as an air barrier in the same methods used for polyethylene vapor barriers behind the gyprock will work.

Using pressure treated 2x4s to compensate for the dips in the floor around sumps and drains would be a sound approach

 

[Esser]

Thanks for the reply Dana. The Durofoam is foil backed on the one side which from what I am told makes an adequate vapour barrier which the product advertises at 0.5 perms. Although I am new to the vapour barrier thing so I could be mistaken.

 

[Dana]

Thanks for the reply Dana. The Durofoam is foil backed on the one side which from what I am told makes an adequate vapour barrier which the product advertises at 0.5 perms. Although I am new to the vapour barrier thing so I could be mistaken.

It's fine as a vapor barrier against ground moisture drives wetting the stud wall cavities, but has inadequate R-value to fully keep condensation or frost from accumulating on the rock-wool side of the foam during the winter, potentially hitting mold-potential humidity levels inside the wall cavity by early spring. The more fiber-R there is, the colder the interior facing side of the XPS becomes, the more condensing hours there are over a winter. The more foam-R, the warmer that face becomes, and the fewer condensing hours there are. The IRC prescriptive is for the foam to be a minimum of 43% of the total-R, so you're not too far under that, but it's still under that, which means you need something more vapor tight than standard interior latex paint on the interior.

With a Class II vapor retarder such as vapor barrier latex (about 0.5 perms) or a "smart" vapor retarder such as 2-mil nylon at the wallboard layer that risk goes away. The 2-mil nylon becomes more vapor open than standard interior latex paint when the humidity levels inside the wall cavity are high enough to support mold, but it's about twice as expensive at 4-mil polyethylene. Vapor barrier latex is never more than 1 perm no matter how damp it gets, but it will prevent wintertime moisture accumulation and still has some drying capacity. Installing polyethylene on the interior side creates a moisture trap, and is not recommended (though you'll usually get away with it if your soil is well drained.)

 

[Esser]

I understood. Thanks for the explanation

 

[Esser]

OK after comtemplating this a bit, my options are more foam board (Can I put two layers of 1.5" XPS or does that create issues too?), the 2mil nylon, the vapour paint, or the unrecommended polyethylene. The only reason I'm asking is I'm not planning on fulling finishing out the basement immediate so I wasn't planning on painting etc.

 

[Dana]

More foam (or all-foam) would be the better solution.

But it's greener to use something other than XPS which is the least-green insulation in common use. All XPS in North America is blow with HFCs, predominantly HFC134a, (automotive AC refrigrant) with a 100 global warming potential (GWP) of about 1400x CO2. EPS and polyiso are blown with hydrocarbons, usually a variant of pentane, with a 100 year GWP of about 7x CO2.

The reason XPS is only warranteed to 90% of labeled R is that the HFC that give it the performance edge diffuse out over time. Most of the hydrocarbon used for blowing other rigid foam leaves the product at the factory where it recovered to meet local air pollution requirements, often burned for process heat. The performance of EPS is very stable over time, and cheaper per R than XPS.

To hit IRC code-minimum for basement insulation (not thermally bridged by studs) takes R15 continuous insulation, which take about 2.5-3" of polyiso, or 4" of EPS, or 3.5" of XPS (at the 90% of labeling warranteed R-value.)

 

[duomit]

Self-leveling option sounds easier and better to me.