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Condensation in loft space
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akis:
I have a serious issue on my new built house where there is a lot of condensation in the loft. This happens in the (warm) mornings after very cold nights (down to 0C or thereabouts).

The loft is habitable and the insulation is in between and under the rafters.

The house is not finished yet, is not heated, and no one lives in it.

This condensation is created on the underside of the membrane under the roof tiles. The membrane is at outside temperature and in the mornings warm and humid air condenses as it contacts it. This happens on both sides of the membrane, but it is the inside surface we care about. The underside of the membrane gets soaked, and it starts to run down the rafters until it finds a point where it drips onto the floor (and that is how I see it).

This is morning dew but it happens inside the house.

To combat this you are supposed to ventilate the underside of the membrane with eaves vents and ridge vents and a gap between the membrane and the insulation. As the house is under construction, there were areas of no insulation at all in between the rafters, and that was the maximum ventilation one could hope for. Yet, it condensed. I also saw a little condensation on the inside of brand new roof windows.

I am not sure how to proceed. If anyone has experience with rooms in the loft and condensation please help!
IanB:
What do you mean by "the loft is habitable"? Is it actually fitted out as an extra room in the house, of is it just partially finished as a storage space?

If it is an actual room for living in, like a bedroom, then it should presumably be very thoroughly insulated with no outside vents or membrane visible on the inside? So I'm a bit puzzled by your description. It seems like you should probably talk to the builder about it, as they should be most qualified to discuss the issue?

I do recall when I moved into a new home in the UK, the builder gave all owners a leaflet about condensation, saying that it was a common concern in new houses. I think the issue is partly that there is a lot of residual moisture left inside a home after it is built, which takes time to disperse, and that houses are thoroughly insulated and sealed to prevent drafts and heat loss, and this can trap moisture inside. I think the advice was to open windows for ventilation, and to wait for the house to dry out and reduce the problem.
tggzzz:
A couple of observations, with no definite conclusion. I am not a civil engineer, but I did replace my roof :)

Where do you think the water is coming from?, e.g. is some part of the construction still "drying out"?
Are other properties suffering the same issues?

The membrane between the rafters and the tiles should be breathable, i.e. porous to moisture but not to rain. Examples: Tyvek, Daltex.

Your statement "...there were areas of no insulation at all in between the rafters, and that was the maximum ventilation..." is too loose to be correct.

There is a difference between a "warm roof/loft" and a "cold roof/loft". Different designs have the temperature gradient in different places (i.e. thermal insulation is in different places), and humidity barriers are placed so as to ensure that the dew point occurs in a safe place. You need to understand where the humidity and thermal barriers should be.

Do not assume that the building has been built correctly and to design. There are many many examples of on-site people cutting corners and/or just doing what they've done before. The former allowed an entire cul-de-sac in Sadly Broke to be built with only 25% of the specified cement in the mortar => mortar could be abraded by fingers, and (IIRC) Lovell had to buy all the properties back. The latter was the source of the "high alumina cement" building failures. You may have heard the phrase "Jerry built", referring to poor design/construction. That originated in the 20s/30s and referred to the various problems found in homes rapidly constructed after WW1.

I suggest that your first step should be to complain the builders and the NHBC - with other people if they are affected too. If there is a problem, do not expect the builders to respond easily. If that fails, then you will need to find a structural engineer/surveyor and solicitor that are prepared to take on the case. That may not be easy, since the building industry is a small community (think Masonic Halls).

If you can back out of a purchase, you might like to consider that. I would not rely on a builder's statement that "it will be alright when finished", unless they state why that will become the case and you believe the cause and effect.
Gyro:
I suggest checking out the Skill Builder channel on YouTube... https://www.youtube.com/user/skillbuilderchannel/videos

I agree with tggzzz on the 'official' route to follow, but it would be helpful to have a bit more background on construction practices.
tggzzz:

--- Quote from: Gyro on November 27, 2023, 10:27:21 am ---I suggest checking out the Skill Builder channel on YouTube... https://www.youtube.com/user/skillbuilderchannel/videos

I agree with tggzzz on the 'official' route to follow, but it would be helpful to have a bit more background on construction practices.

--- End quote ---

Yes, you need to know how to demolish bullshit explanations and fob-offs.

"The David and Charles Manual of Roofing" isn't a bad starting point, but manufacturer's application notes will be more up to date.

"Defects in Buildings" by HMSO/PSA is more general, and will make you wonder why there are any buildings left standing :)
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