How thin a roof is possible to insulate?

[Earplugged]

Hello,
I have plans for an extension where, for aesthetic reasons, a flat roof (with the smallest possible slope) fits best. The concern is that the interior ceiling height risks becoming very low if you need 40-50cm of insulation and a few degrees of roof slope...
Is there any technique to achieve a well-insulated roof that is significantly thinner than normal? Are there any self-supporting superpanels that can be placed as a lid?

How thick does a flat roof with normal construction/insulation become if measured from the roofing felt on the outside to the underside of the plaster?

The room is estimated to be about 4x8m.

 

[huggan]

From the outside in:
Two layers of cardboard 10mm
Tongue and groove boards 22mm
Air gap 25mm
Insulation 300-500mm
Furring strip 22mm
Gypsum board 13mm
Total 392 mm to 592 mm

There are more efficient insulations that allow you to have 35% thinner layers than with traditional fiberglass.

 

[Earplugged]

Do you mean 35% less than 300mm, or is 300mm calculated with the more efficient variant?
Do you have an example of what the more efficient insulation is called?

I saw a variant of passive house where they came with a finished panel and placed it like a lid on the house, does anyone know how this works? (I think it was in last season of Husdrömmar)

 

[falkn]

I find it hard to imagine that you will get less than the thickness of the insulation if you also need load-bearing. The whole point of insulation is to have as much air as possible and air doesn't carry a load. You can use special insulations that are gas-filled, etc., but you still need a load-bearing structure.

 

[huggan]

Do you mean 35% less than 300mm, or is 300mm calculated with the more efficient variant?
Do you have any example of what the more efficient insulation is called?

I have seen some variant of passive house where they came with a finished slab and placed it like a lid on the house, does anyone know how this works? (I think it was in the last season of Husdrömmar)

35% less than 300 mm, i.e., about 200 mm. PIR insulation is an example.

 

[huggan]

I find it hard to imagine that you'll get less than the thickness of insulation if you also need structural support. The whole point of insulation is to have as much air as possible and air doesn't bear loads. You can use special insulations that are gas-filled, etc., but you'll still need a supporting framework.

4 meters should be possible with studs of 45*220 cc 600 mm.

 

[huggan]

TS where do you live? So we know the snow zone. Assuming that the rafters should be 4 meters then?

 

[Earplugged]

I find it hard to imagine you getting down to less than the thickness of the insulation if you're also going to have support. The whole point of insulation is to have as much air as possible, and air does not bear weight. You might use special insulation like those filled with gas, etc., but you still need a supporting structure.

Yes, the support must absolutely be there; with prefabricated elements, they are self-supporting, but I'm not sure about the thickness relative to the insulation value. With "normal" techniques, I think you could perhaps live with visible beams if the insulated space between the beams is higher up and creates space.

 

[Earplugged]

TS where do you live? So we know the snow zone. I assume the rafters should be 4 meters then?

I live in Vaggeryd, just south of Jönköping.
Yes, the rafters will be about 4m (with as little pitch as possible)

 

[huggan]

Yes, the load-bearing capacity must absolutely be there, with prefabricated elements they are self-supporting, but I don't know how the thickness compares to the insulation value. With "conventional" techniques, I think you might be able to live with exposed ceiling beams if the insulated space between the beams comes higher up and creates room.

Read here for prefabricated elements:
http://takochtillbehor-se.danaweb1.com/CustomerData/Files/Folders/2-pdf/55_se-dbs-9-11-web.pdf

 

[huggan]

I live in Vaggeryd, just south of Jönköping.
Yes, the roof trusses will be about 4m (with as little slope as possible)

Snow zone 2.5 or 3.0 then.

 

[attefall]

@Earplugged Which solution did you choose? I'm curious as I have the same problem.