I'm renovating a house and I'm looking to insulate it well. My question is about the roof.

It's today insulated with 80mm Extruded Polystyrene Foam (EPF) panels, Styrodur. I don't know the product reference for now. Under those panels are wood planks between rafters.

I would like to have advices from experts of this solution but I'm finding controversal speech on internet.

This roof is on top of living places and, most important, there will be a biomass heating system. I'm heading for a stove with granulated wood.

Some say it's really efficient and others indicates warm issues during summers. I'm living in the southern France, temperatures are around 0 °C (32 °F) during winter and 35°C (95 °F) in summer. Does it melt under high temperatures?

Is it suffisant to insulate the house?

  • 1
    EPF will melt under high temperatures, but it'll have to be significantly higher than 35°C to melt it. According to wikipedia its melting point is 240°C. Melting is not going to be your main concern unless, possibly, the chimney pipe for your stove runs through the insulation and does not have the proper gaps to it.
    – FreeMan
    Mar 1, 2021 at 12:01
  • Note that Polyisocyanurate is about 26% better that EPS on a per-inch basis, R-5.8/inch vs R-4.6/inch.
    – SteveSh
    Mar 1, 2021 at 12:45
  • Thanks for your comments. Indeed I found strange that it could melt at "low" temperatures. The chimney pipe won't run through this layer of insulation so I'm not scared about this. @SteveSh, the EPF are already in place, do you advise to add Polyisocyanurate or to replace them? Anyway, thanks for the note, very useful!
    – Paul Rey
    Mar 1, 2021 at 13:45
  • @Paul Rey - It sounded like you were thinking about re-doing the roof. The current 80 mm of EPS gives you an R value of ~14, which is low by today's standards for roof insulation. What does your jurisdiction recommend/require for roof insulation in new construction? I bet it's more like R-30 or R-38.
    – SteveSh
    Mar 1, 2021 at 13:52
  • @SteveSh - The current roof insulation allows me to add extra insulation layers. I have some spare space between rafters. There is no attic, I don't plan to build one and I would like to preserve in a specific room the vision of the roof frame. I was mainly asking to know if the current isolation would be suffisant. By your last comment I think it's not. EDIT : for what I know today, the required roof insulation is to be below R = 6 m².K/W
    – Paul Rey
    Mar 1, 2021 at 14:10

1 Answer 1


An easy method for simulation is offered on the site www.ubakus.com. It is worth to play around with width and materials before planning or construction work begins. Parameters like dew points, moisture and drying time can be seen in seconds, thus disclosing any critical layer combination that could produce mold and damage to the building.

And of course the U or R value are calculated, which might be important for subsidies. F.e. some EU countries demand an U value for public subsidies.

Best way is to open a roof example and change/add layers. Each layer's width can be decreased/increased and the layer can be switched on/off by a click to instantly see any changes.

The outside temperature and humidity levels (winter average) should be adapted.

  • I didn't know about this simulator. It looks really great, I'm playing aroung with it. You're right, France asks about U value to obtain subsidies. Maybe I should edit my OP to ask what U value I should head to. Do you have any idea? I believe that the lowest is the better. Am I wrong?
    – Paul Rey
    Mar 1, 2021 at 13:49
  • 1
    Yes. Lower U value is equivalent to higher R value. See my comment above.
    – SteveSh
    Mar 1, 2021 at 13:54

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