I often get asked about how to deal with a concrete roof—or a green roof for that matter. Whether it’s over a flat-roofed extension, a commercial tenancy, or someone dreaming of a rooftop veggie patch, the same question comes up:
How do I insulate this properly without causing long-term issues?
So I’ve done what I always do: asked my American building science friends. In particular I like to refer to Building Science Corporation for their online advice. It’s a treasure-trove of information well worth exploring.
Here’s the best advice I have been given of concrete roofs, distilled down into something practical.
1. Always Insulate on Top of the Concrete
This is the biggest takeaway. If you remember one thing from this blog, make it this:
Don’t insulate underneath a concrete roof slab unless you absolutely have to.
Why? Because concrete acts like a giant cold plate in winter. If the insulation is below the slab, the concrete stays cold. Then, warm, moist air inside the building hits that cold underside—and bang: condensation, mould, problems.
Instead, keep the concrete warm by putting the insulation above the slab.
2. Use the “Upside-Down” Roof System
The building science crowd calls it a “protected membrane roof” or “inverted roof.” It basically means you’re flipping the normal roof layers so the insulation is on top of the waterproofing.
Here’s what that looks like from top to bottom:
- Pavers or gravel (something to weigh it all down and protect from UV)
- Rigid insulation (specifically, XPS—extruded polystyrene)
- Filter fabric or drainage mat
- Waterproof membrane (fully adhered to the concrete)
- Concrete slab
It might feel weird putting insulation above the membrane, but it actually works really well. The waterproofing layer is protected from the sun and temperature extremes, and the slab stays warm and dry.
3. XPS Is the Hero Here
XPS (extruded polystyrene) is the go-to for this kind of roof. It doesn’t absorb water, it’s strong enough to walk on, and it holds its R-value over time. Don’t substitute it with EPS or polyiso unless you know exactly what you’re doing—they behave very differently when wet or compressed.
4. Slopes Still Matter
Even flat roofs need to shed water. You can either taper the insulation itself or make sure the concrete slab has a fall built in. Ponding water kills roofs.
5. What About Green Roofs?
The same principles apply. Insulate above the slab, waterproof it properly, and then build your green roof layers (root barrier, soil, plants) on top. Just be extra careful with drainage. A soggy rooftop garden sounds fun until it leaks into someone’s bedroom.
6. Can’t Insulate from Above? Okay, But…
Sometimes, access is limited and you have to insulate from the inside. Just know you’re entering tricky territory.
You’ll need:
- A proper vapour control layer
- Careful detailing around edges and penetrations
- No sub-optimal “batts between battens” approaches
And even then, there’s a higher risk of condensation. Do it only if you’ve got no other choice, and get the detailing right.
In Summary:
If you’ve got a concrete or green roof to insulate, here’s the cheat sheet:
- Insulate on top of the concrete slab
- Use XPS insulation
- Protect the waterproofing membrane by placing it under the insulation
- Avoid insulating from the inside unless there’s no other way
- Slope the roof so it actually drains
It’s one of those cases where the old-school method (insulation inside) can create more problems than it solves. Above-slab insulation may cost a bit more upfront, but it’ll save you a fortune in repairs, call-backs, and future mould dramas.
For more, detailed information about this topic and much more, check out Building Science Corporation
Image above from Building Science Corporation.