ᐅ Improving Insulation Between Rafters in Older Homes

Hello everyone,

I hope someone can give me a quick piece of advice...

We bought a house from the 1960s. The roof was renovated in 2001 (roof tiles, underlayment, glass wool insulation). Unfortunately, the rafters are only 110mm (4.3 inches) deep. Below the rafters, there were “sauerkraut boards” attached, which we have removed. What is now the best approach?

1. Leave the old glass wool insulation in place (it still looks good), double the thickness of the rafters, and clamp another layer of glass wool underneath.
2. Double the rafters and completely replace with new, thicker glass wool.
3. Attach new beams perpendicular to the existing rafters and clamp new glass wool between them (offsetting to avoid thermal bridging).

For all options, I want to avoid losing too much interior space. What should the minimum thickness of the new rafters be?

We are also considering an additional 30mm (1.2 inches) insulation below the rafters (battens 50x30mm (2x1.2 inches)).

Thanks in advance!

If the old glass wool insulation still looks good, I wouldn’t see a reason to replace it right now.
Installing the insulation perpendicular to the rafters makes the most sense, as this eliminates thermal bridges.
If you’re already taking on the work, I’d recommend insulating properly—an absolute minimum of 50mm (2 inches), but closer to 100mm (4 inches) of insulation is much better. The additional cost for better insulation is minimal, allowing you to end up with a total of 180 to 200mm (7 to 8 inches) of insulation. I think this is worth it in your case.
Is the loss of space only on the sloped ceiling?
Do those few extra centimeters (inches) really make such a big difference that you want to skip better insulation?

Hello Garfunkel,

Thanks for your reply!

You’re right. The additional costs compared to the rest are relatively low, and everything is transparent at the moment anyway. I’m also concerned about the structural integrity. Using 110mm (4.3 inches) rafters and then adding another set of rafters of the same thickness underneath? A roofer will come for a different matter. I can ask him, but that will take some time.

Rafters 110mm (4.3 inches)
Crosswise: battens 40x60mm (1.6x2.4 inches) (still available), possibly screwed in on edge (= +60mm / 2.4 inches)
Vapor barrier membrane
Framework for drywall (50x30mm / 2x1.2 inches) with insulation beneath the rafters (= +30mm / 1.2 inches)
Drywall

That would add up to 190mm (7.5 inches). Or is this generally not how it’s done?

Thanks in advance!

Sounds pretty good. Let's wait and see what the roofer says, then get started with the cordless drill 🙂