Hello forum,
I have been told that our exhaust fan and duct for the exhaust (we have mechanical exhaust) in the cold attic should be better insulated. The reason given is that otherwise, you might have issues with fungus/mold.
I'm wondering why?
The air extracted from the house using this fan is typically warmer than the air in the attic, right? During winter, the exhaust air is dramatically warmer than the air in the attic. In that case, it's not a problem – there won't be condensation on a surface that's warmer than other surfaces in the same space, right?
Or is the problem that there could be condensation on the exhaust duct in the summer? This seems unlikely to me – the exhaust air would practically never have a temperature below the dew point. Although we have air conditioning in the house, it still shouldn't be an issue. The exhaust air will realistically always be >22 degrees warm, and it's very rare for the dew point in Sweden to be greater than 22 degrees.
I do understand that it's extremely important that the exhaust air itself is not released into the attic. This air is warm and humid, and if it were released into a cold attic, there could definitely be condensation. But assuming that the exhaust duct does not leak air, it shouldn't need to be insulated at all, right?
Am I missing something?
I have been told that our exhaust fan and duct for the exhaust (we have mechanical exhaust) in the cold attic should be better insulated. The reason given is that otherwise, you might have issues with fungus/mold.
I'm wondering why?
The air extracted from the house using this fan is typically warmer than the air in the attic, right? During winter, the exhaust air is dramatically warmer than the air in the attic. In that case, it's not a problem – there won't be condensation on a surface that's warmer than other surfaces in the same space, right?
Or is the problem that there could be condensation on the exhaust duct in the summer? This seems unlikely to me – the exhaust air would practically never have a temperature below the dew point. Although we have air conditioning in the house, it still shouldn't be an issue. The exhaust air will realistically always be >22 degrees warm, and it's very rare for the dew point in Sweden to be greater than 22 degrees.
I do understand that it's extremely important that the exhaust air itself is not released into the attic. This air is warm and humid, and if it were released into a cold attic, there could definitely be condensation. But assuming that the exhaust duct does not leak air, it shouldn't need to be insulated at all, right?
Am I missing something?
The problem isn't moisture in the attic but in the duct itself, right? When the warm, humid indoor air meets cold outdoor air, it loses the ability to carry as much moisture, which instead risks precipitating as condensation inside the pipes and flowing back into the system.
An uninsulated duct in a cold attic acts as a heat sink, lowering the temperature and thus decreasing the dew point.
Edit: For various reasons, I haven't insulated the plastic duct to my exhaust fan from the bathroom. It runs about 2m in the cold attic, and that's enough for it to condense and flow back water if you shower when it's -15C outside. Higher fan speed reduces the problem but doesn't eliminate it completely, and we're not talking about a small exhaust fan but a robust 100mm duct fan.
An uninsulated duct in a cold attic acts as a heat sink, lowering the temperature and thus decreasing the dew point.
Edit: For various reasons, I haven't insulated the plastic duct to my exhaust fan from the bathroom. It runs about 2m in the cold attic, and that's enough for it to condense and flow back water if you shower when it's -15C outside. Higher fan speed reduces the problem but doesn't eliminate it completely, and we're not talking about a small exhaust fan but a robust 100mm duct fan.
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The problem is exactly what MMO says. You get condensation inside the fan duct, and you have no idea where that water is going. If you're lucky, it flows back into the bathroom where it annoys you and you solve the problem, but if you're unlucky, it leaks from a joint in the fan ducts that you can't see. In such a case, a little moisture could slowly seep out every day, causing the wooden structure in the attic to mold. If you have also insulated the attic with sawdust (like almost all older houses have), the mold problem becomes even worse.
Many thanks for all the answers! Of course, it's as you write!
I was so focused on there being some sort of condensation risk on the outside of the pipe.
So the idea is that the insulation keeps the pipe at such a high temperature that no condensation precipitates? That sounds very reasonable, I'll make sure to address this.
Thanks again!
I was so focused on there being some sort of condensation risk on the outside of the pipe.
So the idea is that the insulation keeps the pipe at such a high temperature that no condensation precipitates? That sounds very reasonable, I'll make sure to address this.
Thanks again!
Or it gets filled with water and then it becomes a pump = the fan is overloaded and stops. In the best case.