ᐅ New Insulation for Attic (Between and Under Rafters)

Hello,

I would like to have my attic apartment newly insulated. I already removed some of the cladding and found 10cm (4 inches) of aluminum foil-faced PU foam boards.

Now I have the option to add 15cm (6 inches) of insulation between the rafters and then another 5cm (2 inches) of insulation beneath the rafters during the battens installation for the drywall cladding.

Can you give a general recommendation whether this makes sense, and if so, which insulation you would suggest? More around 0.32 or would 0.35 be sufficient?
In total, about 115sqm (1238 sq ft) of roof would be insulated – it’s mainly a question of costs. Does this even make sense or is it not financially worthwhile?

I’m grateful for any tips.

Here are some additional details about the building structure:

- Foundation: concrete slab with waterproofing
- Basement walls exterior: B25 concrete, interior: 12 cm (5 inches) perforated bricks
- Exterior walls: Unipor blocks, 49 cm (19 inches), except the west side where there is a garage connected with 24 cm (9.5 inches) blocks, here 36 cm (14 inches) blocks were used
- Load-bearing interior walls: 12 cm (5 inches) perforated bricks
- Non-load-bearing interior walls: 12 cm (5 inches) perforated bricks
- Ceilings: B25 concrete
- Floor: screed and insulation
- Roof structure: purlin roof frame
- Roof covering: tile roof, 34-degree pitch, with 10 cm (4 inches) aluminum foil-faced PU foam below it

Best regards
André

Hello André,

just a quick preliminary question: Do you own the entire house, or are you the owner of the apartment according to the condominium ownership law (WEG)?

If it is the latter: Have you obtained approval for the structural modification to the building from the owners’ association?

Regards,

Dirk

Hi,
sorry, I am the homeowner 🙂
Good luck

Any tips?
After doing some reading, I would now choose 14cm (5.5 inches) of 0.32 density, with a vapor-retardant but breathable membrane underneath (possibly an Isover Vario climate membrane), then the battens for the drywall (without insulation), followed by the drywall panels.
I was advised not to insulate the battens anymore in order to leave an air gap for "breathing."
What do you think about this?

You should install a maximum of 20% of the insulation value of the insulation behind the vapor barrier (usually the insulation between the rafters) on the room side in front of the vapor barrier to prevent condensation. For 14cm (5.5 inches) 032 insulation between the rafters, this corresponds to about a maximum of 3cm (1.2 inches) 032 insulation below the rafters. I actually find insulation below the rafters only really useful where it is important to use every centimeter of insulation. If that is not the case for you, I would rather double the rafters, add more insulation between the rafters accordingly, and do without insulation below the rafters. Doubling the rafters means less work for the same insulation effect than adding an additional insulation layer without doubling. In addition, this gives you a free installation space.