ᐅ How to Properly Insulate a Roof with Sheathing?

Created on: 19 Dec 2024 14:23
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Falcrum
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Falcrum
19 Dec 2024 14:23
Hello fellow home builders and experts,

I am currently working on converting the attic of my old house. There is one room that is still completely unfinished because it used to house the main electrical connection. This will now become a new children's bedroom, and since the situation is what it is, I will structure this post accordingly so that all information comes across clearly and you can skip parts if needed.

Roof condition/construction:

It is a gable roof with 12cm (5 inches) thick rafters, fully clad on the outside with boards, on which roofing felt shingles have been nailed. The roof is currently not sealed towards the masonry, so there is a constant draft. The condition is still very good and it is weather-tight for now; there are no plans to renovate it at the moment. The gable wall is 24mm (1 inch) thick with a small double-glazed window.

What are my plans?:

First, I want to build up the floor. I will fill the cavities with Fibotherm as ceiling insulation for the room below and then close everything with 22mm (0.9 inch) OSB boards (screwed and glued). I want to keep the room as large as possible while still having decent insulation because of the sloping roof. Currently, the room has a floor area of 30m² (320 square feet), which I want to preserve, so only drywall construction will be used for the walls and sloping ceilings. I also want to build another layer in front of the gable wall to add insulation there.

My questions are:

- Firstly, should I install a foil or vapor barrier between the roof and the insulation, and what would be best to use here? Should another vapor barrier be installed between the drywall and the insulation?

- Is 12cm (5 inches) of insulation sufficient, or will it have hardly any effect and I should rather double it? However, doubling would make the room feel very cramped; perhaps there is a good material that doesn’t cost a fortune yet insulates well despite being thin?

- Do I need to seal the roof towards the masonry first, or is it essential that there is always some airflow there? (In my understanding, ventilation would make sense.)

- Do you have any other comments or important points I should consider? Recommendations, etc.?

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Thanks in advance to everyone who contributes and wants to help.

Best regards
11ant19 Dec 2024 18:23
The purely textual description of the property is insufficiently suitable to discuss your planned measures.
https://www.instagram.com/11antgmxde/
https://www.linkedin.com/company/bauen-jetzt/
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Falcrum
19 Dec 2024 19:08
11ant schrieb:

The purely textual description of the property is insufficient for properly discussing your planned measures.

So what exactly is unclear now?
Nida35a19 Dec 2024 20:30
You ask the basics and answer yourself with construction drawings.
Warm and insulated contradict continuous ventilation.
Inform yourself about converting a cold roof (open, uninsulated, ventilated) to a warm roof (closed, insulated, vapor barrier).
11ant19 Dec 2024 22:27
Falcrum schrieb:

So what exactly is unclear now?
Everything. You’re talking about your house without realizing that we don’t know it and can’t see it. A picture is worth a thousand words.
https://www.instagram.com/11antgmxde/
https://www.linkedin.com/company/bauen-jetzt/
F
Falcrum
20 Dec 2024 09:21
Well, of course I have my own ideas, and maybe I’ve already answered my own question.

Unfortunately, I don’t have a better picture at the moment, but I think the structure is quite clear here.
Attic with wooden beams, crumbling brick wall, cables, wood scraps, and dust.