ᐅ Condensation Under OSB Panels in New Construction, October 2014
Created on: 10 Dec 2014 08:27
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ml_coolHello everyone,
I have been browsing through various forums for several days but only found fragmented information that doesn’t quite match my problem.
On October 1st of this year, I moved into a newly built one-story L-shaped bungalow. Basically, everything is fine so far.
During the construction phase (when the ceiling was still open), I placed 40m² (430 sq ft) of OSB boards on two piles on the ceiling joists to later install them on the future "cold floor" after moving in.
After moving in, I laid some OSB boards in the attic as a walkway to access the roof and to place some boxes on. I placed these directly on the joists; contact between the boards and the insulation cannot be ruled out, as the insulation is almost flush with the joists.
The insulated ceiling structure from the living area side is as follows: 1. gypsum board, 2. aluminum profiles, 3. vapor retarder (blue foil), 4.+5. joists with 200mm (8 inches) insulation wool in between. The roof is uninsulated, essentially a cold attic; insulation is only in the floor ceiling.
Now, after about 9 weeks, I accidentally noticed that some of the boards are significantly wet on the underside (facing the insulation). It clearly looks like condensation. One of the boards already shows slight signs of mold.
My immediate action: I stood all the boards vertically to prevent further moisture buildup and to let the already wet boards dry out.
I have also called the construction company and explained the situation. They advised me to avoid contact between the insulation and OSB boards by installing battens on the collar beams to create a minimum gap of around 5cm (2 inches) between the insulation and the OSB boards, to allow for ventilation. I have provisionally laid 6 boards this way to observe the situation.
What do you think of the construction company’s advice? Is it generally normal that this kind of condensation occurs as I described, or am I dealing with construction defects?
How would you address this issue to prevent moisture buildup and mold growth?
Thanks for some knowledgeable answers.
I have been browsing through various forums for several days but only found fragmented information that doesn’t quite match my problem.
On October 1st of this year, I moved into a newly built one-story L-shaped bungalow. Basically, everything is fine so far.
During the construction phase (when the ceiling was still open), I placed 40m² (430 sq ft) of OSB boards on two piles on the ceiling joists to later install them on the future "cold floor" after moving in.
After moving in, I laid some OSB boards in the attic as a walkway to access the roof and to place some boxes on. I placed these directly on the joists; contact between the boards and the insulation cannot be ruled out, as the insulation is almost flush with the joists.
The insulated ceiling structure from the living area side is as follows: 1. gypsum board, 2. aluminum profiles, 3. vapor retarder (blue foil), 4.+5. joists with 200mm (8 inches) insulation wool in between. The roof is uninsulated, essentially a cold attic; insulation is only in the floor ceiling.
Now, after about 9 weeks, I accidentally noticed that some of the boards are significantly wet on the underside (facing the insulation). It clearly looks like condensation. One of the boards already shows slight signs of mold.
My immediate action: I stood all the boards vertically to prevent further moisture buildup and to let the already wet boards dry out.
I have also called the construction company and explained the situation. They advised me to avoid contact between the insulation and OSB boards by installing battens on the collar beams to create a minimum gap of around 5cm (2 inches) between the insulation and the OSB boards, to allow for ventilation. I have provisionally laid 6 boards this way to observe the situation.
What do you think of the construction company’s advice? Is it generally normal that this kind of condensation occurs as I described, or am I dealing with construction defects?
How would you address this issue to prevent moisture buildup and mold growth?
Thanks for some knowledgeable answers.
That's correct; I was told the same and it makes sense. I also have a bungalow with a cold roof and insulation between the collar beams. On the room side, it is sealed downward with a vapor barrier. We screwed 30x50mm (1.2x2 inches) battens directly onto the beams to ensure ventilation behind the OSB boards.
OSB boards are simply not vapor-permeable. In hindsight, I might have preferred to install tongue-and-groove boards, but that seemed like too much work. The nice large panels were just more tempting.
OSB boards are simply not vapor-permeable. In hindsight, I might have preferred to install tongue-and-groove boards, but that seemed like too much work. The nice large panels were just more tempting.
As Kikolool already mentioned: OSB boards are not vapor-permeable. I would never use them as flooring in the attic space! Just for comparison: I have a 22 cm (9 inches) thick layer of mineral wool insulation between the collar beams, also with a vapor barrier on the living space side. For the attic flooring, I installed planed tongue-and-groove boards directly screwed onto the collar beams, with contact to the insulation. I have had no moisture issues with this setup.
I wonder whether your vapor barrier was installed and sealed properly to ensure it is airtight.
I wonder whether your vapor barrier was installed and sealed properly to ensure it is airtight.
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MaxPower13018 Feb 2015 06:43Good morning, if there is a vapor retarder on the underside and you have installed OSB on the top side, you are trapping the remaining moisture because both materials are diffusion-tight.
The correct approach would have been to use a climate membrane on the underside instead of a vapor retarder, or to install a breather membrane on the top side.
Otherwise, you will continue to have problems.
Best regards, Carsten
The correct approach would have been to use a climate membrane on the underside instead of a vapor retarder, or to install a breather membrane on the top side.
Otherwise, you will continue to have problems.
Best regards, Carsten
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Erbsenzähler707 Oct 2021 10:19MaxPower130 schrieb:
Good morning, if there is a vapor barrier on the underside and OSB is installed on the top side, you are trapping the residual moisture because both materials are vapor-tight.
The correct approach would have been to use a climate membrane on the underside instead of a vapor barrier, or to install battens on the top side.
Otherwise, you will continue to have problems.
Best regards, CarstenHello,it also depends on how the house is positioned and which way the wind blows. In that case, ventilation behind the OSB and insulation can work.
Alternatively, you can properly finish the attic and heat it adequately. Then there will be no issues with OSB boards.
Regards
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