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Hi there,
We moved into a Victorian house in the summer, however a few weeks ago when up in the loft I noticed some mold growing on some of the rafters against the roof, and the rafters themselves felt a bit damp to the touch.
The roof it fairly new (approx 10 years old), felted and not leaking. The loft space is very well insulated - for example during some recent cold weather the temperature in the house was around 20oC, in the loft it was ~7oC. The loft in general seems to stay at around 2 degrees higher than the outside temperature. The loft appears fairly well ventilated (when windy can feel air movement), and we have extractor fans for the 2 upstairs bathrooms. The rest of the house has a RH of around 55% (range of 50-60%).
I will admit, we were a bit foolish when we first moved in until recently - we were drying some washing indoors, and we were a bit lax in using the extractor fans in the bathrooms during showers, etc. This has been stopped now.
Now my questions are the following:
1) In a well ventilated loft, should the indoor humidity approximately match the outside humidity? Unless there is heavy rain, the loft humidity appears to be around 5-10% higher than outdoors. As mentioned above, temperature in the loft is typically around 2oC higher than the outside.
2) Does relative humidity really matter, or is dew point more relevant? I keep reading that the maximum RH should be no more than 60% else mold can grow. However, if the temperature is 12oC and the RH is 71% then the dew point is 7oC, so as long as the loft is higher than 7 (in that set of conditions), no condensation will occur?
3) Essentially i'm trying to work out if I have a bigger problem. I have put a desiccant dehumidifier in the loft, and that takes out plenty of water, and the rafters are now dry to the touch and no new mold has grown. Is this just a one off situation that has been rectified by our changed habits (washing indoors, extractor fans), or am I really aiming to get to a loft humidity of 60%?
4) Similarly, we have one room in the house (a 1960s extension) that has 3 external walls, and is always a bit colder than the rest of the house. It seems to sit almost constantly at 60% RH and around 18-19oC. I can use a dehumidifier and bring the RH down to 40-50%, but over the next 12 hours, it gets back to 60%. Is this a problem, if the dew point is calculated as 12oC?
Thanks for the advice!
We moved into a Victorian house in the summer, however a few weeks ago when up in the loft I noticed some mold growing on some of the rafters against the roof, and the rafters themselves felt a bit damp to the touch.
The roof it fairly new (approx 10 years old), felted and not leaking. The loft space is very well insulated - for example during some recent cold weather the temperature in the house was around 20oC, in the loft it was ~7oC. The loft in general seems to stay at around 2 degrees higher than the outside temperature. The loft appears fairly well ventilated (when windy can feel air movement), and we have extractor fans for the 2 upstairs bathrooms. The rest of the house has a RH of around 55% (range of 50-60%).
I will admit, we were a bit foolish when we first moved in until recently - we were drying some washing indoors, and we were a bit lax in using the extractor fans in the bathrooms during showers, etc. This has been stopped now.
Now my questions are the following:
1) In a well ventilated loft, should the indoor humidity approximately match the outside humidity? Unless there is heavy rain, the loft humidity appears to be around 5-10% higher than outdoors. As mentioned above, temperature in the loft is typically around 2oC higher than the outside.
2) Does relative humidity really matter, or is dew point more relevant? I keep reading that the maximum RH should be no more than 60% else mold can grow. However, if the temperature is 12oC and the RH is 71% then the dew point is 7oC, so as long as the loft is higher than 7 (in that set of conditions), no condensation will occur?
3) Essentially i'm trying to work out if I have a bigger problem. I have put a desiccant dehumidifier in the loft, and that takes out plenty of water, and the rafters are now dry to the touch and no new mold has grown. Is this just a one off situation that has been rectified by our changed habits (washing indoors, extractor fans), or am I really aiming to get to a loft humidity of 60%?
4) Similarly, we have one room in the house (a 1960s extension) that has 3 external walls, and is always a bit colder than the rest of the house. It seems to sit almost constantly at 60% RH and around 18-19oC. I can use a dehumidifier and bring the RH down to 40-50%, but over the next 12 hours, it gets back to 60%. Is this a problem, if the dew point is calculated as 12oC?
Thanks for the advice!
