Hello,
we have purchased an old house where the roof is currently insulated with aluminum-coated insulation wool placed between the rafters.
The roof tiles are laid on wooden battens. In the upper roof area, there is a ventilation tile installed in each rafter bay.
We would like to re-insulate the roof from the inside because the old insulation is partly falling out and torn.
However, the roof will only be re-covered in 5-8 years.
My question is whether it makes sense to install 120mm (5 inches) of insulation batts between the rafters (rafter height is 140mm (5.5 inches)) plus an additional 50mm (2 inches) of insulation below the rafters,
or if it would be better to double the rafters and install 180mm (7 inches) of insulation batts from the inside.
In both cases, of course, with an interior vapor control layer and an appropriate finish with drywall/plasterboard (Rigips/Fermacell).
The idea would then be to add further insulation from the outside later with a wood fiberboard when the roof is replaced.
Thank you very much in advance. Best regards from the North
we have purchased an old house where the roof is currently insulated with aluminum-coated insulation wool placed between the rafters.
The roof tiles are laid on wooden battens. In the upper roof area, there is a ventilation tile installed in each rafter bay.
We would like to re-insulate the roof from the inside because the old insulation is partly falling out and torn.
However, the roof will only be re-covered in 5-8 years.
My question is whether it makes sense to install 120mm (5 inches) of insulation batts between the rafters (rafter height is 140mm (5.5 inches)) plus an additional 50mm (2 inches) of insulation below the rafters,
or if it would be better to double the rafters and install 180mm (7 inches) of insulation batts from the inside.
In both cases, of course, with an interior vapor control layer and an appropriate finish with drywall/plasterboard (Rigips/Fermacell).
The idea would then be to add further insulation from the outside later with a wood fiberboard when the roof is replaced.
Thank you very much in advance. Best regards from the North
B
Bauwiesel9 Oct 2019 16:00Hello Borderpuschel,
Thank you for your reply. Would you also skip the insulation below the rafters? I was planning to use 120mm (5 inches) compressed mineral wool and 50mm (2 inches) insulation below the rafters. Both with a thermal conductivity of 0.035 W/(m·K).
Thank you for your reply. Would you also skip the insulation below the rafters? I was planning to use 120mm (5 inches) compressed mineral wool and 50mm (2 inches) insulation below the rafters. Both with a thermal conductivity of 0.035 W/(m·K).
Are you sure the roof will be replaced within the next few years? If it’s damaged, it should be fixed immediately, and if not, why would it be damaged in five years?
In general: after removing the old insulation, check all roofing felt strips for tightness and proper fit. If any corners are loose, water can seep in during heavy rain. This isn’t a problem in an open roof space without insulation, but it’s not ideal with insulation behind it. It’s best to watch for damp or moldy spots while removing the old insulation. You should repair those areas right away. Fortunately, roofing felt strips can be replaced individually.
Maintain ventilation under the roofing felt strips with a 2 cm (1 inch) gap, preferably 4 cm (1.5 inches). For limited build-up height, use insulation with a thermal conductivity of WLG032; WLG035 is also acceptable and won’t make much difference in the end.
Insulation between the rafters if there is enough space. Whether you use EPS composite boards or battens with boards on top doesn’t really matter in the long run. Mineral wool is easier to handle in corners and hard-to-reach areas. The thickness depends on the available space. If you end up with about 24 cm (9.5 inches) of insulation including the exterior insulation, adding more thickness won’t make much difference.
In general: after removing the old insulation, check all roofing felt strips for tightness and proper fit. If any corners are loose, water can seep in during heavy rain. This isn’t a problem in an open roof space without insulation, but it’s not ideal with insulation behind it. It’s best to watch for damp or moldy spots while removing the old insulation. You should repair those areas right away. Fortunately, roofing felt strips can be replaced individually.
Maintain ventilation under the roofing felt strips with a 2 cm (1 inch) gap, preferably 4 cm (1.5 inches). For limited build-up height, use insulation with a thermal conductivity of WLG032; WLG035 is also acceptable and won’t make much difference in the end.
Insulation between the rafters if there is enough space. Whether you use EPS composite boards or battens with boards on top doesn’t really matter in the long run. Mineral wool is easier to handle in corners and hard-to-reach areas. The thickness depends on the available space. If you end up with about 24 cm (9.5 inches) of insulation including the exterior insulation, adding more thickness won’t make much difference.
B
Bauwiesel27 Oct 2019 17:04Hello "dertill",
thank you for your explanation. So far, the roof is still in good condition. The reroofing is planned for later, as it is a semi-detached house and we want to maintain a uniform appearance. Would the effect be the same if I double the rafters and omit the insulation below the rafters? Of course, keeping the total insulation thickness the same. Since we are going to remove the entire interior roof construction anyway, doubling the rafters would also be easily possible. I would then use 180mm (7 inches) compression fleece with a thermal conductivity of 0.035 and later add 60mm (2.4 inches) Staico boards from the outside.
thank you for your explanation. So far, the roof is still in good condition. The reroofing is planned for later, as it is a semi-detached house and we want to maintain a uniform appearance. Would the effect be the same if I double the rafters and omit the insulation below the rafters? Of course, keeping the total insulation thickness the same. Since we are going to remove the entire interior roof construction anyway, doubling the rafters would also be easily possible. I would then use 180mm (7 inches) compression fleece with a thermal conductivity of 0.035 and later add 60mm (2.4 inches) Staico boards from the outside.
Today, the roofer advised against combining cavity insulation between the rafters with insulation above the rafters, as this can cause issues with the dew point between the layers. Is there any truth to this? Ubakus also flagged this, but as a layperson, I initially ignored it.
The recommendation was simply to use 200mm (8 inches) of cavity insulation between the rafters with a double layer of drywall.
The recommendation was simply to use 200mm (8 inches) of cavity insulation between the rafters with a double layer of drywall.
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