ᐅ Renovating Roof Insulation from 1977 – Are Wood Fiber Insulation, Vapor Retarders, and Construction Methods Effective?
Created on: 15 Dec 2025 00:31
G
Golzner
Hello dear experts,
Our roof dates back to 1977. The attic above the top floor is unheated. The sloped ceilings and the top floor ceiling are currently insulated with 120 mm (5 inches) yellow glass wool (probably from the late 1970s) – insulation is patchy and damaged. The entire upper floor / top floor is to be gutted and renovated.
Details about the roof structure:
- Rafters 135 mm x 100 mm (5.3 inches x 4 inches) – spacing 75 cm (30 inches).
- Underlay membrane is present but appears to be vapor-tight.
I would like to remove all glass wool insulation and install new insulation.
My current idea – new assembly:
Sloped ceiling build-up from inside to outside:
- 12.5 mm (0.5 inch) drywall
- 60 mm (2.4 inches) Rockwool Formrock 035 as insulation below rafters
- Vapor retarder – preferably Pro Clima Intello PLUS (variable moisture permeability)
- 140 mm (5.5 inches) Naturheld-Flex-50 036 insulation between rafters (compressed by 5–10 mm (0.2–0.4 inch))
- vapor-tight old underlay membrane
Room height is 240 cm (7 ft 10.5 inches). The top floor ceiling will later have a U-value of 0.14 (300 mm (12 inches) Naturheld-Flex-50 036 – planned).
Top floor ceiling build-up:
Top floor ceiling from bottom to top:
12.5 mm (0.5 inch) drywall / 24 mm (0.9 inch) air gap (suspended ceiling) / vapor retarder (Isocell ÖKO Natur) / insulation between joists: 140 mm (5.5 inches) Naturheld-Flex-50 036 / insulation above joists: 160 mm (6.3 inches) Naturheld-Flex-50 036 laid crosswise over the joists / 20 mm (0.8 inch) rough spruce boards as a walkable surface.
There is a bathroom in the attic (currently still in the shell stage) within a masonry dormer without sloped ceiling (possibly important regarding the vapor retarder (Isocell ÖKO Natur) in the ceiling).
Above is an unheated attic space.
I expect these measures to reduce heating costs and in summer to keep the top floor cooler, as wood fiber boards are said to retain heat better. Also, the new assembly will achieve the 240 cm (7 ft 10.5 inches) room height (previously there was an additional 30 mm (1.2 inch) of polystyrene between battens below the roof beams).
Would this build-up be feasible?
P.S. I plan to insulate the top floor ceiling this way because for the other part of our house (no upper floor, only ground floor with a cold roof above), I insulated the ceiling there with the same build-up on the advice of my roofer. I am familiar with the materials and was satisfied with the workmanship and results.
Many thanks
Our roof dates back to 1977. The attic above the top floor is unheated. The sloped ceilings and the top floor ceiling are currently insulated with 120 mm (5 inches) yellow glass wool (probably from the late 1970s) – insulation is patchy and damaged. The entire upper floor / top floor is to be gutted and renovated.
Details about the roof structure:
- Rafters 135 mm x 100 mm (5.3 inches x 4 inches) – spacing 75 cm (30 inches).
- Underlay membrane is present but appears to be vapor-tight.
I would like to remove all glass wool insulation and install new insulation.
My current idea – new assembly:
Sloped ceiling build-up from inside to outside:
- 12.5 mm (0.5 inch) drywall
- 60 mm (2.4 inches) Rockwool Formrock 035 as insulation below rafters
- Vapor retarder – preferably Pro Clima Intello PLUS (variable moisture permeability)
- 140 mm (5.5 inches) Naturheld-Flex-50 036 insulation between rafters (compressed by 5–10 mm (0.2–0.4 inch))
- vapor-tight old underlay membrane
Room height is 240 cm (7 ft 10.5 inches). The top floor ceiling will later have a U-value of 0.14 (300 mm (12 inches) Naturheld-Flex-50 036 – planned).
Top floor ceiling build-up:
Top floor ceiling from bottom to top:
12.5 mm (0.5 inch) drywall / 24 mm (0.9 inch) air gap (suspended ceiling) / vapor retarder (Isocell ÖKO Natur) / insulation between joists: 140 mm (5.5 inches) Naturheld-Flex-50 036 / insulation above joists: 160 mm (6.3 inches) Naturheld-Flex-50 036 laid crosswise over the joists / 20 mm (0.8 inch) rough spruce boards as a walkable surface.
There is a bathroom in the attic (currently still in the shell stage) within a masonry dormer without sloped ceiling (possibly important regarding the vapor retarder (Isocell ÖKO Natur) in the ceiling).
Above is an unheated attic space.
I expect these measures to reduce heating costs and in summer to keep the top floor cooler, as wood fiber boards are said to retain heat better. Also, the new assembly will achieve the 240 cm (7 ft 10.5 inches) room height (previously there was an additional 30 mm (1.2 inch) of polystyrene between battens below the roof beams).
Would this build-up be feasible?
P.S. I plan to insulate the top floor ceiling this way because for the other part of our house (no upper floor, only ground floor with a cold roof above), I insulated the ceiling there with the same build-up on the advice of my roofer. I am familiar with the materials and was satisfied with the workmanship and results.
Many thanks
Very good, thank you.
So, we would have a theoretically possible system.
Are there still some things that could be improved here?
Two questions regarding this:
1. I have read about variable vapor barriers that allow for drying back toward the interior (e.g., Pro Clima Intello Plus). Would that improve the system? I would then stick with the manufacturer’s system (connection tapes, etc.). I watched a video to assess whether I could handle it technically – it should be doable.
2. Does the loss of insulation value also apply to flexible wood fiber mats, or could one possibly skip the second layer as long as the 40mm (1.6 inches) thickness is maintained ("spacing battens" placed vertically in the rafter corners as a tactile stop + the mats sloping more inward by sagging rather than bulging outward)?
This topic has been occupying me nonstop for a week now (the shell is finished upstairs, and we currently need the rooms, but nothing can be done until a practical solution is found).
Thank you very much for your help!
So, we would have a theoretically possible system.
Are there still some things that could be improved here?
Two questions regarding this:
1. I have read about variable vapor barriers that allow for drying back toward the interior (e.g., Pro Clima Intello Plus). Would that improve the system? I would then stick with the manufacturer’s system (connection tapes, etc.). I watched a video to assess whether I could handle it technically – it should be doable.
2. Does the loss of insulation value also apply to flexible wood fiber mats, or could one possibly skip the second layer as long as the 40mm (1.6 inches) thickness is maintained ("spacing battens" placed vertically in the rafter corners as a tactile stop + the mats sloping more inward by sagging rather than bulging outward)?
This topic has been occupying me nonstop for a week now (the shell is finished upstairs, and we currently need the rooms, but nothing can be done until a practical solution is found).
Thank you very much for your help!