condensation in loft

14 Dec 2007
Reaction score
United Kingdom
hi just insulated my loft roof with 50mm kingspan with a 30 mm air gap between that and the roof felt.also come down with kingspan on borh sides at 90 deg to create both walls,the floor i have put 10 inch floor beams and fully insulated between these with rockwool.the space on the other sides towards the eaves is insulated between the joist but the roof is un insulated (just storage space) i have noticed alot of moisture condensation on the north facing roofing felt. but nothing on the south facing roof ?any ideas
Sponsored Links
There won't be any underside of the kingspan as it will be the warm side, so the temperature won't be cold enough to condense water vapour. The felt being exposed to the outside will be cold so that's why the water collects.

I think the fact that it's North facing is a moot point at this time of year.

If you didn't completely seal/tape all the kingspan joints then you might also have condensation on the felt behind where you have fitted it. This could cause problems.

Ultimately the area that's just felt will be a major weak spot for heat loss, so all that insulation you put in will be compromised until you insulate on all sides.

Look into condensation/ventilation problems on other posts in this section. The moisture that's forming is likely to be coming up through the floor/hatch/stairwell from the rest of the house.
i didnt tape the joints they where all a tight fit between the roof joists. but why would the condensation only be forming on this north facing roof ? if warm ait was coming through the loft hatch both roof sides would create condensation no?, please note the floor joists under neath the insulated ares in the loft i did not insulate because i thought the heat from the house will help warm the loft area. i might pull down the t and g and create a vapour barrier between the t and g and kingspan ?
water will condense on a surface that is colder than the warm moist air.

your options are:

1) ventilation to put the warm moist air outside. (Lofts are usually quite well ventilated so perhaps the eaves have been blocked with insulation.)

2) insulate all exposed surfaces so that they are not cold.

If you are getting condensation on the felt, it sounds to me like you have excessively moist air in the loft. Possibly you have water tanks which are warm or not tightly covered or both

Possibly you have warm moist air rising up through gaps in the ceiling below, or a badly fitted hatch, or round downlighters or pipework.

if your bathroom does not have an extractor fan which is constantly used during and after every bath or shower, or if you have damp washing draped about the house, you are causing excessive moisture.

There is a faint chance that some buffoon might have ventilated the bathroom into the loft.

Sponsored Links
thanks for that. the bathroom is under the north facing roof and i have no extractor in the bathroom ? just checked the temperature in the storage space (north facing) and its colder by 3.0deg than the south facing one, this tells me its to cold on this side thus causing the condensation on this side only
you need an extractor in the bathroom. All that steam is going to go somewhere.
ok i will install a extractor in the bathroom. do you think this will stop all the condensation on this side of the roof (the opposite roof no condensation whatsoever) i have lived hear 19 years and had no extractor in the bathroom in that period. the fact that i have now insulated the centre loft space has highlighted the problem ?
bathrooms generate a lot of steam. if you can take it outside the house that will prevent it condensing on the roof felt.

I don't know if you have any other causes, such as blocked loft ventilation, or uncovered water tanks. It might be the "room" in the loft has obstructed airflow.
the eave vents are not blocked i checked. since i have insulated with the kingspan 50 mm air gap its prob restricted the airflow in the roof space.with the south facing roof being condensation free that tells me the air gap should be sufficient because that section of kingspan etc is the mirror of the problem side (north)
I have the same kind of problem in my house. When it gets cold like it is at the moment I get major condensation on the roof felt in my loft. It starts on the north side and as it time goes on it progresses to the south side of the house. This is a problem I have had since we moved in and this summer I put in an extractor fan in the bathroom that was vented straight outside and also new insulation up there thinking this may solve my problem. However with the cold snap at the moment it has come back, so my work hasn't stopped it. There seems to be plenty of air movement up there at the moment with air circulating in from under the eves and out of a vent in the ridge of the roof.
I have had a couple of roofers out to have a look at it and no body has said that this is the solution. However one did think that the roofing felt was an old type that doesn't breath and so several vented tiles in the roof may sort out the problem. This may be what you need?
If anybody has any other ways I may be able to solve my problem they would be greatly appreciated.
had a lok this morning the condensation has gone, it was a really windy night last night that tells me it got more ventilation than normal ?
hi me again no condensation in the loft near the eaves (this part not insulated roof kingspan) but some condensation in between the kingspan and roof felt. got approx 40 mm air gap should i increase this gap for better air flow ?
Hi, i have read the previoue loft condersation advice given, my loft on the left of the roof there is felt which has wet batched on the lower section of the roof and on the right side of the roof there is a black plastic type of material under the roof tile which is full of condersation, you can see the water droplet on all of the surface area, now and again you can hear water droplets, i have a bathroom under the right side of the loft whitch does have an extracor fan, and we leave the window open about 2inched. the loft is insulated and i was putter extra lolft insulation at each end of the loft because it has an old pink type of inulation, in the middle of the loft i have a good quatity 200 thick insulation 3 yrs old now. and today i have put the same insulation on the right side of the loft, havent had time to do the left side yet or the loft hatch.
the joist on the right side of the loft which touches the roof is wet in looking dont know if this is just due to condersation, and of course there is a lot of snow on the roof could this the problem. there in no ventilation in the roof, but i can feel cold brease in the loft coming from both sides.

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links