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polokrausi22 Oct 2014 17:35Hello everyone,
First of all: I am a complete construction novice and joined this forum out of deep frustration to get an objective opinion.
We are currently planning to convert the attic of our house built in 1982. The roof consists of wood, roofing felt, and tiles on top. The masonry is made of 36.5cm (14.4 inches) bricks with the then-standard 4cm (1.6 inches) external polystyrene insulation.
The eaves face towards the roof structure – so there is currently no air exchange.
The rafters are 18cm (7 inches) thick.
At the moment, the attic space is completely uninsulated.
As part of the renovation, the outer roof will be insulated to meet the 0.24 requirement of the energy saving regulations.
My building planner has planned for a ventilated insulation between the rafters with 16cm (6.3 inches) of insulation material with a thermal conductivity of 0.035 W/(m·K) and an additional 8cm (3.1 inches) of insulation with the same thermal conductivity as an overlay on the rafters, placed in front of the drywall partition. The whole system would include a vapor retarder. Vents would need to be installed in the walls for ventilation.
The drywall contractor I trust said that, personally, he would not install ventilation here and, to save space, would only insulate with 18cm (7 inches) of insulation material with a thermal conductivity of 0.035 W/(m·K), covered by a climate membrane. According to his calculations, this should achieve a U-value of 0.22, which should be enough. He also inspected the roof and measured that it is completely airtight. Therefore, he believes his solution is more sensible in this configuration. I also told him that I do not want to go overboard with insulation.
Further calculations did not reveal any issues regarding the dew point or similar concerns. He sees the main problem in a possible future retrofit of ventilation, as this would expose the existing roof to more moisture (from the humidity in outdoor air).
Calculated values with this insulation variant:
U = 0.224
Dew point offset 11.8 (not perfect but no problem)
What do you think? What should I do? I’m caught in the middle and cannot professionally assess who is right or wrong. Cost-wise, the architect’s solution is about twice as expensive. I’m looking for the solution that will definitely keep me out of trouble with rot, mold, or similar issues.
Thanks for your help,
A desperate home renovator – Matthias
First of all: I am a complete construction novice and joined this forum out of deep frustration to get an objective opinion.
We are currently planning to convert the attic of our house built in 1982. The roof consists of wood, roofing felt, and tiles on top. The masonry is made of 36.5cm (14.4 inches) bricks with the then-standard 4cm (1.6 inches) external polystyrene insulation.
The eaves face towards the roof structure – so there is currently no air exchange.
The rafters are 18cm (7 inches) thick.
At the moment, the attic space is completely uninsulated.
As part of the renovation, the outer roof will be insulated to meet the 0.24 requirement of the energy saving regulations.
My building planner has planned for a ventilated insulation between the rafters with 16cm (6.3 inches) of insulation material with a thermal conductivity of 0.035 W/(m·K) and an additional 8cm (3.1 inches) of insulation with the same thermal conductivity as an overlay on the rafters, placed in front of the drywall partition. The whole system would include a vapor retarder. Vents would need to be installed in the walls for ventilation.
The drywall contractor I trust said that, personally, he would not install ventilation here and, to save space, would only insulate with 18cm (7 inches) of insulation material with a thermal conductivity of 0.035 W/(m·K), covered by a climate membrane. According to his calculations, this should achieve a U-value of 0.22, which should be enough. He also inspected the roof and measured that it is completely airtight. Therefore, he believes his solution is more sensible in this configuration. I also told him that I do not want to go overboard with insulation.
Further calculations did not reveal any issues regarding the dew point or similar concerns. He sees the main problem in a possible future retrofit of ventilation, as this would expose the existing roof to more moisture (from the humidity in outdoor air).
Calculated values with this insulation variant:
U = 0.224
Dew point offset 11.8 (not perfect but no problem)
What do you think? What should I do? I’m caught in the middle and cannot professionally assess who is right or wrong. Cost-wise, the architect’s solution is about twice as expensive. I’m looking for the solution that will definitely keep me out of trouble with rot, mold, or similar issues.
Thanks for your help,
A desperate home renovator – Matthias
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