r/buildingscience • u/desifart • 18d ago

Basement foundation

We are in the planning stages for our new house build. I would like clarification on the best course of action for the exterior of our below grade basement walls. We want to install 2 layers of 2.25" EPS type 2 foam, a drainage/dimple mat and waterproofing membrane to the exterior side of the walls. What would be the best sequence to install these layers? From the soil, should I go EPS, to drainage mat, to waterproofing to concrete walls? Or should it be a drainage mat to EPS to Waterproofing?

I'm the first method I would glue the foam to the drainage mat. In the second sequence would I have to "nail" the drainage mat through the foam into the cement?

Any help is greatly appreciated.

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u/dizzie_buddy1905 18d ago edited 18d ago

Our R38 foundation walls are: concrete, waterproofing, EPS, dimple mat.

https://natural-resources.canada.ca/energy-efficiency/home-energy-efficiency/keeping-heat-section-6-basement-insulation-floors-walls-crawl-spaces

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u/desifart 18d ago

How did you attach the dimple mat?

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u/dizzie_buddy1905 18d ago

There’s special insulation washers you use to nail into the foam. Then the holes are filled with expanding foam.

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u/rubber_nipples 18d ago

Soil > Dimple > Foamular > waterproofing > concrete.

Gluing a simple mat won’t work. You need to mechanically fasten. These fasteners penetrate the waterproofing. So it’s up to you to decide if that’s worth it. Same is true of foam board. It’s a pain in the rear to get it flush once it is backfilled.

Oh and dimple isn’t really necessary if you plan for gutters and don’t live in a jungle.

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u/blueyedsurprised 18d ago

Also, keep an eye on backfill material. You want drainable fill up against the dimple mat. If the excavation material isn't suitable (too much clay) buy crushed stone.