Hello forum
Here are the key points:
Multi-family house. Built in 1961. Converted attic. Thermal insulation. Above-rafter insulation.
The roof is dry and the roof tiles are old but still watertight.
Now I have received quotes from different roofers mentioning vapor barrier and vapor retarder.
It is not completely clear to me why these are necessary.
I have read that a vapor retarder or vapor barrier is supposed to minimize or prevent moisture entering the thermal insulation.
My question is:
Moisture can also come from the outside air into the thermal insulation.
There are days in winter or summer when the relative humidity is very high.
This humidity also gets into the thermal insulation.
Or am I mistaken?
An alternative—which would also be cheaper than above-rafter insulation—would be to insert insulation mats into the space between the roof and attic.
See sketch.
Thank you very much for your feedback.
Jochen
Here are the key points:
Multi-family house. Built in 1961. Converted attic. Thermal insulation. Above-rafter insulation.
The roof is dry and the roof tiles are old but still watertight.
Now I have received quotes from different roofers mentioning vapor barrier and vapor retarder.
It is not completely clear to me why these are necessary.
I have read that a vapor retarder or vapor barrier is supposed to minimize or prevent moisture entering the thermal insulation.
My question is:
Moisture can also come from the outside air into the thermal insulation.
There are days in winter or summer when the relative humidity is very high.
This humidity also gets into the thermal insulation.
Or am I mistaken?
An alternative—which would also be cheaper than above-rafter insulation—would be to insert insulation mats into the space between the roof and attic.
See sketch.
Thank you very much for your feedback.
Jochen
K
Knallkörper23 Sep 2016 17:30Hello Jochen,
The installed thermal insulation must be able to dry if it is exposed to moisture “from below.” Warm, moist air rises from the living space into the insulation. Somewhere between the cold and warm side of the insulation, condensation occurs. If the insulation is not adequately ventilated, it will remain damp for a long time or permanently, which will inevitably cause building damage. “Adequate ventilation” must be carefully planned and correctly implemented. From my experience, not all architects know exactly how to do this. As a rough guideline: with a 45° pitched roof, there should be a continuous 4cm (1.5 inch) wide gap above the insulation layer, running from the eaves to the ridge, with ventilation at the ridge. This creates sufficient ventilation through the “chimney effect.” This would not be sufficient for a flat roof.
Alternative: A vapor barrier “below” the top floor ceiling. Unlike a vapor retarder, a vapor barrier does not allow any moisture to pass through.
The installed thermal insulation must be able to dry if it is exposed to moisture “from below.” Warm, moist air rises from the living space into the insulation. Somewhere between the cold and warm side of the insulation, condensation occurs. If the insulation is not adequately ventilated, it will remain damp for a long time or permanently, which will inevitably cause building damage. “Adequate ventilation” must be carefully planned and correctly implemented. From my experience, not all architects know exactly how to do this. As a rough guideline: with a 45° pitched roof, there should be a continuous 4cm (1.5 inch) wide gap above the insulation layer, running from the eaves to the ridge, with ventilation at the ridge. This creates sufficient ventilation through the “chimney effect.” This would not be sufficient for a flat roof.
Alternative: A vapor barrier “below” the top floor ceiling. Unlike a vapor retarder, a vapor barrier does not allow any moisture to pass through.
K
Knallkörper23 Sep 2016 19:17spassig schrieb:
Do we agree that roof insulation needs to be ventilated?
JochenWell. If it is possible to install a vapor barrier under the top-floor ceiling with a 100% airtight seal at all connections, ventilation would not be necessary. Since this is hardly achievable in practice, I would say: yes, it needs to be ventilated.
Why don’t you use insulation between the rafters? Did I miss something?
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