Condensation in the loft

NJR999

In the last month I have had condensation starting to occur in my loft space. It has happened twice, on both days the weather was very cold but the sky was cloudless and the sunshine warm.

My property was built in the 1980’s and had 50mm of fibreglass insulation between the joists. The loft space has no eaves ventilation but each of the 3 gable ends has 2 large air blocks at rafter level. I have just added 200mm of Knauf space blanket insulation (wrapped in silver foil) perpendicular to the rafters leaving ample space around the air blocks.

I have no cold water tanks in the loft. The bathroom and shower room both have extractor fans that are piped via the loft to the outside but there is no sign of leakage. No leaks in the roof itself are apparent. I have dried the loft out once with a dehumidifier and it stayed dry for a couple of weeks until the day with bright sunshine. I am tempted to remove all the insulation as there has never been condensation in the loft before I installed the space blanket.

Could it be the case that the loft gets very cold as it is well insulated and when the sunshine heats the external roof the temperature difference creates the condensation even though the ventilation was sufficient before topping up the insulation? Should I add more ventilation at rafter level or ridge?

Any advice gratefully received.

jennifernil

We have eaves ventilation all round (slots with stainless steel mesh covering) and have no problems with condensation, so I would say you need more ventilation.

We have 200mm of Rockwool insulation in the ceilings, it is very cold in our loft.

WestonDave

I wonder if condensation on a cold night is almost inevitable - you get condensation on your car windscreen - we call it frost but in effect its condensation that has frozen. If you have successfully cut all the heat loss from your house then the same air which is forming moisture and freezing on your car windscreen is circulating in your loft. When it hits a nice cold surface you will get the same as on the windscreen.

What may have happened is that in the past the escaping heat from the house was enough to keep the condensation up there as dew but now its colder its freezing and the daylight warmth is only enough to melt it, not evaporate it off again.

By all means look at more ventilation but I wouldn't take the space blanket out because trying to dry your draughty loft by letting heat escape out of the house is a very expensive option, and I'd also be careful with using a dehumidifier in a vented site because you end up drying air which escapes outside and is replaced by wet outside air - before you know it you are paying to try to dry half the northern hemisphere! Keep an eye open for pools forming where melted frost has run down into a trap but of its self moist outside air won't do much damage - consider that most garages are draughty and unheated etc yet their roof timbers don't rot unless there is a leak.