ᐅ Renovating Roof Insulation from 1977 – Are Wood Fiber Insulation, Vapor Retarders, and Construction Methods Effective?
Created on: 15 Dec 2025 00:31
G
GolznerHello 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
Basically okay – but I would take a closer look at two points. The old vapor-retardant breathable underlay is the limiting factor; moisture-variable function only works properly if the airtightness on the interior side is truly 100%, otherwise moisture accumulates exactly where you won’t be able to get rid of it. Intello PLUS is a good option for this, but only with absolutely clean sealing, especially at purlins, dormer walls, and the bathroom. The combination of counter-batten insulation plus 140mm (5.5 inches) of insulation between rafters is energy-wise acceptable; summer heat protection is improved but don’t expect miracles with a total thickness of 200mm (8 inches). Wood fiber insulation helps but cannot replace thermal mass. I find the top floor ceiling with two insulation layers and a vapor retarder more critical; here you must clearly decide whether the attic is part of the thermal envelope or not, otherwise you create unnecessarily complex moisture pathways. Why do you separate the roof slope and ceiling differently when the area above is unheated? This is exactly where the typical calculation errors happen later on, including a small thinking mistake...
The first upper floor (with knee walls and sloping ceilings) – the attic floor – is part of the thermal envelope. The top floor ceiling is already insulated. The insulation in the sloping roof sections ends at the top floor ceiling. In the unheated attic above, there is no insulation between the rafters. You can see the underlay membrane directly here.
Important detail:
Our house has a part without an upper floor. Here, as described, I have already insulated the floor ceiling (under a cold roof) with 300 mm (160 mm + 140 mm) wood fiber mats. This cold roof intersects at a right angle (90°) with the roof that is actually the main subject here. At this junction, after the renovation work, there was no insulation left (an entire slope in one room measuring 3.5 m (11.5 ft) wide by 2.5 m (8.2 ft) high). This is the main reason why the roof topic was brought up.
Would option B be feasible:
- 120 mm (4.7 inches) glass wool remains in all rooms where it is still in the sloping ceilings and is supplemented with 60 mm (2.4 inches) of rock wool insulation below the rafters
- The ceiling will be redone with 300 mm (11.8 inches) wood fiber mats (a combination of insulation between the joists and insulation on top of the joists, laid directly)
- I will also add the missing insulation in the adjoining room using wood fiber (I won’t be able to access the slope later if the roof is professionally redone, as it overlaps with the other roof here).
Sorry for all the detailed input, this issue is really on my mind at the moment.
Thank you :-)
Important detail:
Our house has a part without an upper floor. Here, as described, I have already insulated the floor ceiling (under a cold roof) with 300 mm (160 mm + 140 mm) wood fiber mats. This cold roof intersects at a right angle (90°) with the roof that is actually the main subject here. At this junction, after the renovation work, there was no insulation left (an entire slope in one room measuring 3.5 m (11.5 ft) wide by 2.5 m (8.2 ft) high). This is the main reason why the roof topic was brought up.
Would option B be feasible:
- 120 mm (4.7 inches) glass wool remains in all rooms where it is still in the sloping ceilings and is supplemented with 60 mm (2.4 inches) of rock wool insulation below the rafters
- The ceiling will be redone with 300 mm (11.8 inches) wood fiber mats (a combination of insulation between the joists and insulation on top of the joists, laid directly)
- I will also add the missing insulation in the adjoining room using wood fiber (I won’t be able to access the slope later if the roof is professionally redone, as it overlaps with the other roof here).
Sorry for all the detailed input, this issue is really on my mind at the moment.
Thank you :-)
Option B is pragmatic – but: the combination of old fiberglass insulation plus 60mm (2.4 inches) of insulation below the rafters only provides limited energy efficiency. The thermal conductivity of old fiberglass varies, and you have different materials in one layer. This can cause small thermal bridges, especially at junctions. With the top floor ceiling insulated with 300mm (12 inches) of wood fiber, this is less of an issue, but you need to implement airtightness carefully; otherwise, moisture can be drawn through gaps into the fiberglass below, particularly at the transition to the pitched roof.
Adding wood fiber insulation later to the adjacent room where the slope is missing is possible. However, make sure the joints between the old and new insulation are really tight; otherwise, a small cavity could form that might condense moisture in winter.
Adding wood fiber insulation later to the adjacent room where the slope is missing is possible. However, make sure the joints between the old and new insulation are really tight; otherwise, a small cavity could form that might condense moisture in winter.
Hello, thanks in advance for the response.
According to the information I found, glass wool loses a significant amount of its insulation performance when exposed to air flow. Is that correct?
Therefore, I have developed a new approach:
From outside to inside:
The real problem is only the old vapor-tight underlay membrane, which I cannot properly replace from the inside, of course.
I calculated total material costs of around 4500€ (including disposal of old materials).
The roofer’s offer (of course much better with many advantages) is 58000€ gross, without subsidies.
That’s why I’m driving this effort myself.
Could you please evaluate this construction? Is a U-value of 0.21–0.22 achievable for the pitched roof?
P.S. The second underlay membrane is intended to protect the wood fiber insulation from wind in order to avoid impairment of its insulation value.
Many thanks!
According to the information I found, glass wool loses a significant amount of its insulation performance when exposed to air flow. Is that correct?
Therefore, I have developed a new approach:
From outside to inside:
- Remove everything from between the rafters
- Attach 4cm x 2.4cm (1.6 inches x 0.9 inches) battens on both sides of the rafters, aligned flush with the roof battens at the back (my roof has no counter-battens)
- Fix (and glue toward the rafters) a second, diffusion-open breathable membrane with an SD value of 0.02 to the battens
- This creates a ventilated, continuously open rafter field with 4cm (1.6 inches) air gap (air can enter at the eaves and exit into the attic at the top)
- On top of that, 100mm (4 inches) flexible wood fiber mat 036, flush with the front of the rafters (to increase from 130 to 140mm thickness — wood will be added there)
- Then the vapor retarder from the inside (vapor retarder membrane with an SD value of 6.45)
- 60mm (2.4 inches) Rockwool Formrock 0.5
- Interior cladding with plasterboard
The real problem is only the old vapor-tight underlay membrane, which I cannot properly replace from the inside, of course.
I calculated total material costs of around 4500€ (including disposal of old materials).
The roofer’s offer (of course much better with many advantages) is 58000€ gross, without subsidies.
That’s why I’m driving this effort myself.
Could you please evaluate this construction? Is a U-value of 0.21–0.22 achievable for the pitched roof?
P.S. The second underlay membrane is intended to protect the wood fiber insulation from wind in order to avoid impairment of its insulation value.
Many thanks!
Yes, glass wool noticeably loses its effectiveness when exposed to air flow, typically around 15–25%, which is not just theoretical. Your use of a second underlay as a wind barrier is therefore logical, but you are creating a rather sensitive system that only works properly if the 4cm (1.6 inches) ventilation gap remains continuously clear. The U-value of 0.21–0.22 for the roof slope is roughly correct; with the old vapor-tight membrane, achieving more is realistically hardly possible. The vapor retarder with an sd-value of 6.45 remains critical, especially at the eaves, the transition to the cold roof, and in the bathroom — here, the execution matters more than the calculation. The difference of 4,500€ to 58,000€ explains the appeal, but you are trading money for low error tolerance, which is easily overlooked.
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