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I am renovating my house and am completely redoing my flat roof: ceilings, insulation and roof covering (I am planning to use EPDM rubber). In case it is relevant, I am living in the Netherlands.

My plan was to use rockwool on the inside as insulation, but now my builder is suggesting insulation on the outside. He wants to use 50mm (2 inches) PIR insulation.

Does anyone have ideas on the following:

  1. Is there a good argument to go with insulation on the outside of the roof instead of on the inside?
  2. Is PIR insulation better than rockwool?
  3. Or ... should I insulate both the inside and the outside?
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Dow (manufacturer of XPS panels) recommends their panels OVER the flat roof waterproofing. You do have to cover with an aggregate or a green roof. The practice is called "inverted roof"

PIR panels are similarly rated for inverted roof use.

PIR is "better" than rockwool in U-value vs thickness :

Mineral wool fibre loft insulation (sold as Rockwool), would also need to be 270mm deep to achieve a U-value of 0.16. Mineral wool fibre is claimed to be less irritating to the installer’s skin than fibreglass, although my own experience is actually the opposite. It is also more expensive than fibreglass, and would cost around £8.20 per sq m. ... The most thermally efficient insulation material is polyisocyanurate, or PIR, sold as lightweight rigid foam boards – Kingspan, Celotex, and some other brand names. The 0.16 U-value could be achieved using a PIR board 175mm (6¾in) thick. excerpted from Jeff Howells article in The Telegraph

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  • Thanks for the answer. My builder cited the same risk as mentioned in the article (If insulation is placed below the structural deck (cold roof construction) the structure remains cold and there is a considerable risk of condensation). – Mauritz Hansen Jun 21 '13 at 18:43
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Commercial flat roofer here with 18 years experience.

I cannot stress enough how useless and incorrect inverted roofs are. Having the XPS panels ABOVE the roofing membrane continues to be one of the dumbest things that I see on specs.

No matter what the insulation manufacturer tells you, those insulation panels will eventually absorb water. I have removed hundreds of these systems over the years and after 7-8 years of the insulation constantly sitting in water, they begin to absorb it. And sit in water is what they do, they cover the roof and never allow it to dry.

I have removed inverted roofs with 3" XPS and those 2'x4' panels can weigh up to 20 lbs once they suck up enough water.

Once this happens, all the R-value that they once had is completely negated.

Putting the insulation over the roofing membrane is like putting your car seat on the roof of your car to drive and wondering why your cold and wet.

In most residential situations it is usually better to install 7" ROXUL on the inside between the joists. The only time I see different is if the owner is looking for an exposed ceiling look. https://www.timbermart.ca/sites/default/files/files/brands/pdfs/us_accordian_brochure.pdf

If that is your case, I would suggest a Ballasted 60mil EPDM system over two staggered layers of 2.6" polyisocyanurate (Poly iso), this will give you an R-Value of 30.

If you are determined to use and inverted system, I would highly recommend installing a dimple or drainage board over the roofing membrane and ensuring you are using XPS panels that have pre-cut reglets in the bottom of them that will help with drainage and help keep the XPS out of the water. If you are looking for an r-30 you will need about 6" of XPS.

  • Yeah, even if you want the roof insulation outside the structural sheathing, you'll still want the roof surface system atop the roof insulation! (Who would build a house where the wall insulation was outside the siding?!) – ThreePhaseEel Dec 18 '18 at 0:06
  • Hello, and welcome to Home Improvement. Great answer; keep 'em coming! – Daniel Griscom Dec 18 '18 at 2:24
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I am not sure on your exact setup but I would almost always choose my insulation to be inside. If you have roofing issues you don't have to replace, you don't have to worry about the insulation getting damaged, and the biggest point is that I would always try to have the insulation as close to possible to the living space. I guess both would be better but I would put more money into extra rock wool on the interior than rigid sheets outside. Even with 10 inches of PIR on the outside you still have an inside space that will eventually be cold.

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