condensation/droplets in rafters

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poppy1

Registered User
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Hi
We have a new build and last nite i was in the attic and noticed that there was droplets on the felt, only on one side of the house!! It was raining at the time but this was not there as a result of rain, can anyone explain what could be the cause of this?

Thanks
 
If this is condensation it means that you havent the attic insulated well enough or securely enough.

Heat is escaping up into the attic cavity heating the air which contain lots of moisture (usually 60-80%). This warm air is rapidly cooled when it meats the cool felt.. thus the warm water vapour is turning in to moisture droplets on the felt. Your felt isnt allowing the water vapour through.

are you completely sure it wasnt from the rain... was it uniform throughout the attic, or at least uniform on one side???
 
I'm not sure I aggree with syd's synopsis. My take on it is this: warm air can hold a lot of water vapour; this moisture laden warm air travels from the heated habitable space (below the roof space) and rises into the attic through gaps and cracks in the structure (especially if no vapour check is incorporated in the ceiling lining). The moisture laden air enters the roof space, temperature drops and moisture condenses on the coolest part of the roof- the sarking felt. I think you need to investigate the ventilation of the roofspace, is this adequate? Also are water tanks fitted with covers? Are extract fans ducted to the external air?
 
Whilst i agree fully that the ventilation should be checked, would you not agree that what is happening now is flagging that there is a problem with the insulation levels?? Increased ventilation may solve the condensation forming on the felt, but isnt solving the primary issue of the heat escaping into the attic cavity.

oh, and the extractor fan point you made is excellent.... that could be the source of your water vapour.
 
Had the exact same thing on my build. The felt on the inside was soaking with moisture and it wasnt the rain.
The rafters were highly insulated and i think as the house was drying out after plastering, there was excessive condensation which couldnt get out even though I used breathable felt.
I installed 4 ridge vents and the problem disappeared.
 
thanks for your replies!! it is uniform on all rafters on one side only, yes all water tanks are covered, must check out the extractors though.
 
sydthebeat said:
Whilst i agree fully that the ventilation should be checked, would you not agree that what is happening now is flagging that there is a problem with the insulation levels?? Increased ventilation may solve the condensation forming on the felt, but isnt solving the primary issue of the heat escaping into the attic cavity.

oh, and the extractor fan point you made is excellent.... that could be the source of your water vapour.
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Sorry, can't agree; no level of insulation will prevent moisture vapour passing through a construction; only a vapour check will prevent the migration of this vapour. The solution is two pronged: firstly, limit generation of water vapour in the habitable space (don't dry clothes indoors, do open windows/ use mechanical extraction when showering, bathing, cooking etc), ensure pathways for air leakage are sealed (around pipes, loft door etc). Secondly, ensure that the roof space is adequately ventilated. Regardless of the level of insulation at ceiling level the (uninsulated) attic space will always be colder than the (insulated) habitable space and condensation will occur in this space unless ventilation is adequate. Ventilation is the key.
 
Ive changed my view to agree with yours carpenter, although i would clarify some issues.

i would just point out that kitchens / bathrooms are not an exclusive source of water vapour. Relative humidity in ireland is very high all year round.. going from 60% in summer to 80% on average in winter... thus all air contains this humidity. The trapped air in the cavity has this amount of water vapour in it... and i appreciate that ventilation will / should exhaust this air to the external.
This air is being heated up by :
1. (primarily) heat rising from underneath.... inefficient insulation
2. stagnant air currents.. inadequate ventilation
3. solar gains on the dark coloured roof finish

Thus if the space is ventilated adequately then the attic air will be as cold as the external and the construction thus condensation will not occur.
 
sydthebeat said:
1. (primarily) heat rising from underneath.... inefficient insulation
2. stagnant air currents.. inadequate ventilation
3. solar gains on the dark coloured roof finish
quote]
Ok, but I think the issue of insulation is a red herring, whether the ceiling is insulated or not there is a condensation risk in the roof space if there is not adequate ventilation. Simple example: sit in your car on a cold day with windows closed- the windows will quickly fog up as water vapour condenses on the cold glass; if window was left open water would not condense as ventilation ensures that saturation point of air is not reached. The warmer the air the more water vapour it can hold, when temperature drops the air can hold less vapour and condensation is the result. The old maxim: "keep vapour barrier on warm side of insulation" applies.
Click to expand...
 
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