We have a pitched roof (1971 house, The Netherlands) which consists of (outside to inside):

  1. Tiles
  2. 2cm PUR insulation
  3. 2cm wood

We asked a few companies for quotations to improve the insulation (currently Rc=0.8).

Company 1 is suggesting to add rockwool to the inside, while company 2 says that any insulation on the inside will lead to condensation issues.

Are there any options for adding insulation on the inside of the roof in this case?

Edit: picture of the roof material: Picture of the roof material

  • Hello, and welcome to Home Improvement. Good question; let's see if one of our pros can help. And, you should probably take our tour so you'll know how best to participate here. Jul 20, 2019 at 15:09
  • Can you add a picture of the current insulation please? Jul 20, 2019 at 15:09
  • I have added a picture to the post of a slice of the roof (on top are the roof tiles).
    – Ruben
    Jul 21, 2019 at 8:46

3 Answers 3


We had insulated the heated attic area with rock wool. After the winter, mold had formed on the wood. The reason was ice crystals between the wood and the rock wool. When thawing, the moisture has accumulated more and more on this layer. However, I don't have 2 cm PUR insulation on the outside. This certainly reduces the problem of condensation on the wood layer. It may not be easy to assess whether this will help 100%.

In our case we had newly installed all insulation with an intermediate ventilation space (2 cm) between insulation and wood. There were no problems until now for over 10 years.


If you have your insulation on the roof sheathing and the roofing covers it, presumably there is adequate ventilation for the attic so condensation hasn't been an issue already since the foam is non permeable.

Another assumption I will make is that you have an attic space that would be a conditioned space since the roof insulation is above that.

With those parameters in mind you should be able to add as much insulation you need or like as long as it is permeable like the rock wool suggested in your post, fiberglass, or other loose material that may be available for you.

  • Thank you for your answer Jack. The attic is heated in winter (but not cooled in summer other than opening windows). So if I understand correctly, we could add for example rock wool, can we then cover with plasterboard + paint? Or would that not be permeable enough?
    – Ruben
    Jul 21, 2019 at 8:49
  • Evidently I don't understand quite yet the construction of the roof/ rooms. I figured there already was sheetrock finish on the ceilings of the rooms to support the rockwool if added, but it sounds as if you are looking at the backside of the roof sheathing, so any added insulation needs to be supported... Yes the added layers are permeable enough to all moisture to pass through.
    – Jack
    Jul 23, 2019 at 14:19

As an update on our final solution: we've added 16cm of rockwool sheets with climate foil to regulate moisture buildup in the wood/insulation. The climate foil should allow moisture to evaporate from the insulation layer, but not get into it from inside the house.

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