Hello dear forum,
unfortunately, I have to start with a problem and ask for help here.
I have an issue with mold forming on the roof sheathing in my attic during winter.
Here is the roof structure: The house has a roof with a 42-degree pitch. The roof has sheathing, covered by a Tyvek membrane. Between the rafters, there is full insulation with glass wool. So there is no ventilation behind it, unfortunately. The ceiling is also fully insulated and sealed with a vapor barrier membrane. In the ceiling, there is an attic access hatch, which has already been sealed with Tesamoll tape, and additionally, a 6cm (2.4 inches) polystyrene board was glued onto the wooden panel.
In the roof apex area above, one part is insulated and covered with a membrane and additionally boarded on the inside; another part is not. The attic only has a small roof gap for ventilation, but otherwise no ventilation. The gap is mostly closed, so there is little airflow.
Now to the problem.
In winter, when it is cold outside, water freezes on the uninsulated roof sheathing and later thaws, or water condenses on the sheathing. There are many droplets, and the sheathing becomes quite wet. Humidity in the attic in winter is around 80–85%. In summer, the area dries out again. The water amount is so high that it runs down along the sheathing behind the insulation. Black mold develops on the sheathing.
I think there is simply too much moisture in the attic, rising through the ceiling or the roof gap. I also suspect that the vapor barrier membrane on the upper floor is not properly airtight. Replacing the membrane is difficult because I cannot remove the wooden ceiling due to built-in cabinets.
How should I proceed now?
Should I insulate the remaining part of the attic as well? But it is difficult to seal the membrane completely above because there are many counter battens on the roof frame, many cables running, and many corners. So it would be very difficult to make it airtight. Wouldn’t the water then also condense behind the insulation if the system is not fully airtight?
Thank you very much for your help!
Best regards
Marcus
unfortunately, I have to start with a problem and ask for help here.
I have an issue with mold forming on the roof sheathing in my attic during winter.
Here is the roof structure: The house has a roof with a 42-degree pitch. The roof has sheathing, covered by a Tyvek membrane. Between the rafters, there is full insulation with glass wool. So there is no ventilation behind it, unfortunately. The ceiling is also fully insulated and sealed with a vapor barrier membrane. In the ceiling, there is an attic access hatch, which has already been sealed with Tesamoll tape, and additionally, a 6cm (2.4 inches) polystyrene board was glued onto the wooden panel.
In the roof apex area above, one part is insulated and covered with a membrane and additionally boarded on the inside; another part is not. The attic only has a small roof gap for ventilation, but otherwise no ventilation. The gap is mostly closed, so there is little airflow.
Now to the problem.
In winter, when it is cold outside, water freezes on the uninsulated roof sheathing and later thaws, or water condenses on the sheathing. There are many droplets, and the sheathing becomes quite wet. Humidity in the attic in winter is around 80–85%. In summer, the area dries out again. The water amount is so high that it runs down along the sheathing behind the insulation. Black mold develops on the sheathing.
I think there is simply too much moisture in the attic, rising through the ceiling or the roof gap. I also suspect that the vapor barrier membrane on the upper floor is not properly airtight. Replacing the membrane is difficult because I cannot remove the wooden ceiling due to built-in cabinets.
How should I proceed now?
Should I insulate the remaining part of the attic as well? But it is difficult to seal the membrane completely above because there are many counter battens on the roof frame, many cables running, and many corners. So it would be very difficult to make it airtight. Wouldn’t the water then also condense behind the insulation if the system is not fully airtight?
Thank you very much for your help!
Best regards
Marcus
Not completely clear to me, but okay.
The uninsulated attic must be protected from heat and moisture coming from the occupied upper floor below. Both are absolutely harmful, as you have now noticed. If metal surfaces in the attic develop condensation, something is wrong, and heat or moisture (warm air) is reaching the attic. This MUST be prevented, no matter what. Of course, any existing mold must be removed first.
Improving ventilation alone won’t help much here, because without ventilation, there should be no moisture in the roof structure. The back or cross ventilation is more of a secondary factor and won’t solve your problem. Good ventilation might reduce the effects of mold but won’t address its cause.
If you’re unsure what to do, you should consult a professional (carpenter). For new builds (entire house or roof structure), report the defect to the contractor. As an owner, there’s little you can do wrong, except for leaving the attic hatch open all the time. Usually, there is a notice on the hatch reminding you to keep it closed at all times.
The uninsulated attic must be protected from heat and moisture coming from the occupied upper floor below. Both are absolutely harmful, as you have now noticed. If metal surfaces in the attic develop condensation, something is wrong, and heat or moisture (warm air) is reaching the attic. This MUST be prevented, no matter what. Of course, any existing mold must be removed first.
Improving ventilation alone won’t help much here, because without ventilation, there should be no moisture in the roof structure. The back or cross ventilation is more of a secondary factor and won’t solve your problem. Good ventilation might reduce the effects of mold but won’t address its cause.
If you’re unsure what to do, you should consult a professional (carpenter). For new builds (entire house or roof structure), report the defect to the contractor. As an owner, there’s little you can do wrong, except for leaving the attic hatch open all the time. Usually, there is a notice on the hatch reminding you to keep it closed at all times.
S
Sebastian7922 Aug 2016 21:45Heat always rises into the attic – insulation only delays this.
The important thing is to prevent any moisture from rising.
And ventilation in the uninsulated attic is also mandatory – either mechanical or through a window. And this must be continuous...
The important thing is to prevent any moisture from rising.
And ventilation in the uninsulated attic is also mandatory – either mechanical or through a window. And this must be continuous...
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