Hello everyone,
We built a house in 2015 and shortly afterward covered the attic floor with OSB boards (including cross battens). Looking back, I’ve read that OSB might not be the best choice and can potentially lead to mold growth, but I’m not keen on tearing everything out again. It’s an unheated attic.
Now, one could cut open a few spots to check for mold, or—here’s the question—should I use a moisture meter? I have an electronic one with a metal probe on the front, which measures moisture content. I’ve already taken several readings in the attic, and all the values were good—so dry.
Would it be possible to use the meter to detect damp areas without causing damage, allowing me to identify spots with a higher risk of mold? OR is that not advisable, and should I actually remove the boards and check directly?
Thanks for your opinions.
We built a house in 2015 and shortly afterward covered the attic floor with OSB boards (including cross battens). Looking back, I’ve read that OSB might not be the best choice and can potentially lead to mold growth, but I’m not keen on tearing everything out again. It’s an unheated attic.
Now, one could cut open a few spots to check for mold, or—here’s the question—should I use a moisture meter? I have an electronic one with a metal probe on the front, which measures moisture content. I’ve already taken several readings in the attic, and all the values were good—so dry.
Would it be possible to use the meter to detect damp areas without causing damage, allowing me to identify spots with a higher risk of mold? OR is that not advisable, and should I actually remove the boards and check directly?
Thanks for your opinions.
Regarding the question from the original poster.
In most cases, using OSB as flooring in an attic with a cold roof is avoided. Here, tongue-and-groove boards installed on cross battens with a 1 cm (0.4 inch) gap are the preferred solution. The specifications or recommendations come from the home builder. From what I see here, this concerns a timber frame construction.
Reason: Naturally, both heat and some moisture from below enter the attic, regardless of whether a vapor barrier is installed. Especially in the area of the upper floor ceiling / attic floor, the dew point is often reached (warm, dry air from below, cold and usually moist air in the attic).
OSB cannot store moisture, unlike tongue-and-groove boards. The moisture that condenses then occurs in the insulation of the upper floor ceiling towards the attic. This often leads to mold formation, accompanied by the gradual deterioration of the insulation material due to increasing moisture over time. Tongue-and-groove boards, on the other hand, can absorb and release moisture, so this kind of damage does not happen.
You can borrow a moisture meter (from an architect, tool rental, or online). However, it may not be very useful. What exactly would you measure? The moisture content of the OSB panel? The moisture already present in the insulation? Well, you already have such a device. In this case, I consider it impractical.
As unpleasant as it may be: I would remove the OSB panels there and replace them with tongue-and-groove boards (cost for DIY work approximately 250-400 euros + OSB disposal). Please discuss this again with your home builder to see what they recommend (I assume the same).
There is a high probability that if you leave the OSB on the attic floor under the cold roof, you will experience significant building damage over the years. The subsequent repairs will be far more expensive and complex than simply replacing a few OSB panels (as painful as that may sound).
Perhaps Mr. Pickartz may kindly add a more professional perspective on this issue.
Best regards,
Thorsten
In most cases, using OSB as flooring in an attic with a cold roof is avoided. Here, tongue-and-groove boards installed on cross battens with a 1 cm (0.4 inch) gap are the preferred solution. The specifications or recommendations come from the home builder. From what I see here, this concerns a timber frame construction.
Reason: Naturally, both heat and some moisture from below enter the attic, regardless of whether a vapor barrier is installed. Especially in the area of the upper floor ceiling / attic floor, the dew point is often reached (warm, dry air from below, cold and usually moist air in the attic).
OSB cannot store moisture, unlike tongue-and-groove boards. The moisture that condenses then occurs in the insulation of the upper floor ceiling towards the attic. This often leads to mold formation, accompanied by the gradual deterioration of the insulation material due to increasing moisture over time. Tongue-and-groove boards, on the other hand, can absorb and release moisture, so this kind of damage does not happen.
You can borrow a moisture meter (from an architect, tool rental, or online). However, it may not be very useful. What exactly would you measure? The moisture content of the OSB panel? The moisture already present in the insulation? Well, you already have such a device. In this case, I consider it impractical.
As unpleasant as it may be: I would remove the OSB panels there and replace them with tongue-and-groove boards (cost for DIY work approximately 250-400 euros + OSB disposal). Please discuss this again with your home builder to see what they recommend (I assume the same).
There is a high probability that if you leave the OSB on the attic floor under the cold roof, you will experience significant building damage over the years. The subsequent repairs will be far more expensive and complex than simply replacing a few OSB panels (as painful as that may sound).
Perhaps Mr. Pickartz may kindly add a more professional perspective on this issue.
Best regards,
Thorsten
To return to the original question:
Basically, using OSB sheathing as the upper layer on the insulated collar beam layer leading to the unheated attic is structurally not optimal and potentially harmful. The moisture ingress mechanism has been described by T21150. OSB boards are, compared to tongue-and-groove spruce solid wood, relatively more vapor-retarding: the vapor diffusion factor (μ-value) for OSB, depending on the manufacturer, ranges from about 50/30 to 550/300, whereas the μ-value for wood is around 20/50, with the lower value relating to moist material and the higher one to dry material.
The so-called sd-value (diffusion-equivalent air layer thickness) is calculated by multiplying the μ-value (water vapor diffusion resistance factor) by the material thickness in meters. For example, for a dry 21mm (0.83 inch) OSB panel, this would be 50 x 0.021 = sd-value 1.05 meters (3.4 feet). Materials with an sd-value from 0.5 meters (1.6 feet) upwards are considered vapor-retarding.
Depending on the OSB manufacturer, the panel is therefore significantly more vapor-retarding than tongue-and-groove wood. Indeed, the OSB panel absorbs less water than solid wood, meaning the sd-value decreases only slightly when wet (water absorption coefficient (kg/m² × h × 0.5): OSB 0.05 / spruce 2.0–3.0). Additionally, the panel is dense, installed with tongue and groove, and has a significantly lower seam proportion than tongue-and-groove boards, through which moisture could escape.
In short: this construction is not recommended and can lead to long-term moisture accumulation in the insulation. If the insulation is mineral wool or glass wool, it is vapor-permeable but not capillary active, so it does not support drying out.
The proper assembly would be a ventilated OSB layer, comparable to a ventilated facade, but then with a windproof yet vapor-permeable membrane on the collar beams. As an alternative, a strongly vapor-retarding or humidity-variable vapor barrier on the underside of the beams may be used, but this must be installed extremely carefully and without errors. I try to avoid all these membranes as much as possible because they are not fault-tolerant.
Basically, using OSB sheathing as the upper layer on the insulated collar beam layer leading to the unheated attic is structurally not optimal and potentially harmful. The moisture ingress mechanism has been described by T21150. OSB boards are, compared to tongue-and-groove spruce solid wood, relatively more vapor-retarding: the vapor diffusion factor (μ-value) for OSB, depending on the manufacturer, ranges from about 50/30 to 550/300, whereas the μ-value for wood is around 20/50, with the lower value relating to moist material and the higher one to dry material.
The so-called sd-value (diffusion-equivalent air layer thickness) is calculated by multiplying the μ-value (water vapor diffusion resistance factor) by the material thickness in meters. For example, for a dry 21mm (0.83 inch) OSB panel, this would be 50 x 0.021 = sd-value 1.05 meters (3.4 feet). Materials with an sd-value from 0.5 meters (1.6 feet) upwards are considered vapor-retarding.
Depending on the OSB manufacturer, the panel is therefore significantly more vapor-retarding than tongue-and-groove wood. Indeed, the OSB panel absorbs less water than solid wood, meaning the sd-value decreases only slightly when wet (water absorption coefficient (kg/m² × h × 0.5): OSB 0.05 / spruce 2.0–3.0). Additionally, the panel is dense, installed with tongue and groove, and has a significantly lower seam proportion than tongue-and-groove boards, through which moisture could escape.
In short: this construction is not recommended and can lead to long-term moisture accumulation in the insulation. If the insulation is mineral wool or glass wool, it is vapor-permeable but not capillary active, so it does not support drying out.
The proper assembly would be a ventilated OSB layer, comparable to a ventilated facade, but then with a windproof yet vapor-permeable membrane on the collar beams. As an alternative, a strongly vapor-retarding or humidity-variable vapor barrier on the underside of the beams may be used, but this must be installed extremely carefully and without errors. I try to avoid all these membranes as much as possible because they are not fault-tolerant.
Good! – It’s unfortunate that the building physics proved to be correct. You can of course use the OSB boards for building shelves, but not as the bottom layer (base panel) directly on the tongue-and-groove boards. The OSB boards need to be ventilated on all sides. Good luck.
B
Bieber081512 Jan 2016 21:32Majo83 schrieb:
Can I at least use the OSB boards as shelves in the unheated attic/loft? Since the cause of the mold was identified so easily, the next step is to remove the existing mold, possibly from the OSB boards as well. After that, you can use them (see #16).
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