Choice between Expanded or Extruded Polystyrene Insulation Boards

Question

Am living in Lahore, Pakistan, and these days in process of my house construction. Note, here locally we have concrete flat roofs. Summer time maximum temperature upto 50C, and in winters minimum temperature -1C. Looking for a Insulation board for my concrete roof top.

Traditional way, is that over the concrete slab , a layer of molten bitumen is laid with a Plastic sheet over it to water proof the roof, then a mud layer of about 6" inches is applied, over the plastic sheet, and over it clay brick are grouted with cement. The mud in this scenario is supposed to act as thermal insulator.

I am looking for a more efficient way than described above. Local vendors are offering EPS ( Expanded Polystyrene Sheet ), and XPS ( Extruded Polystyrene sheet ) boards. In both cases vendors recommend a 2" ( 5 cm ) thickness of these boards.

Installation method as described by vendors is quite similar too : (1) Concrete roof slab (2) molten bitumen layer with a plastic sheet over it (3) Lay the EPS or XPS boards (4) another layer of plastic sheet (6) 6" inches of mud layer (7) clay bricks grouted with cement over it.

My query is that out of these 2 options : EPS and XPS, whichone is better, as price difference between the two is about 50%.

Thanks & Regards. Omar

Answer 1

This article describes the difference:

• EPS is formed as beads (as used in cushions) then heated and pressed together
• XPS is formed as a sheet or whatever final shape is desired.

So EPS will break up more easily and be less waterproof, which makes it likely to degrade faster and be less effective as insulation. XPS will be stronger and better insulating. This US article gives similar values in their table, but is less clear about the differences.

That article suggests 50mm of each will give R1.2 for EPS, and R1.78 for XPS - 48% more insulation for the same amount of XPS. If it costs 50% more that's because you get nearly 50% more value from it. But if you can just use a 50% thicker layer of EPS, that might not matter. But I suspect that both come in 50mm thick sheets, so the choice is really 50mm of XPS or 100mm of EPS (two layers), if you can fit two layers on. The installers are likely to damage from of the fragile EPS panels than the XPS one, so it may cost more to have installed.

My inclination would be to prefer XPS. But I'd also prefer more insulation rather than less, so 100mm of EPS while it costs 50% more, might be better because it's also 50% more insulating. On the other hand, if you instead shade some of the roof using solar photovoltaic panels the insulation doesn't matter as much. And I'd prefer to do that, if you can afford it.