Condensation in roof space, not sure what’s wrong

[FFCForever]

Hi,
I went up into the roof space very recently during the extremely cold weather we have had, it’s a fair size, you can stand up on it and walk around, however I’ve noticed quite a bit of condensation on the underside of the roof.

I’ve never noticed this before and if I’m honest it freaked me out a bit.

FYI - I have also recently started to put extra insulation up in the roof space, I’ve maybe done only 1/3 off the space up there however but have stopped laying any more for now. I had 100mm insulation in between the batons, so I got the 200mm Rockwool top up stuff to lay over the top. In addition I have also had new double glazing installed upstairs this year.

I thought I may have disrupted the airflow by laying too close to the gap between the rafters and the soffit, so i have went back around and pulled all insulation away from that gap to ensure any air can circulate, I have also bought a dehumidifier for the house which indicates anywhere from 50-60% humidity but generally it sits around 55%.

However I noticed that there are no vents in the soffit around the house (other than for extractor fans), I also noticed that there are no vents on the soffits of any house in the estate however, so I’m curious as to whether I should install some and how the air is supposed to flow around my loft to improve the airflow or if that’s a mistake, my house was built around 2001.

Also, other than what I have already done, does an anyone have any suggestions?

 

[mrrusty]

So your house is at 55% humidity at let's say 20 degrees. (ignoring the fact that RH is really a ratio of vapour pressure and not density - the figures are very similar at lower temps) your air is carrying 17.3 x 0.55 g/m3 water vapour = 9.5g. You will see that a saturated vapour density of 9.4 equates to 10 degrees, so this is the dew point below which condensation will occur.

I bet the underside of your roof is a bit colder than that yesterday with all that extra insulation stopping the heat from the house getting to it!!

Assuming you don't have a breathable membrane under the slates/tiles that allow the water vapour out but stops condensation getting back in (that's how a breathable membrane works - it doesn't prevent condensation under the tiles/slates, but it does mitigate against it raining back down), you can try and keep that loft hatch shut and as far as possible prevent warm house air getting in the loft. A dehumidifier will help reduce the relative humidity, but when the roof is close to freezing or below it, you will get condensation.

As soon as the ventilation air is below 100% relative humidity it will start to evaporate the condensation, which is why ventilation is important - the air in contact with the moisture needs to be able to replenish and move away once it has "collected" the surface water. If it can't move, evaporation stops because the local air becomes saturated.

 

[FFCForever]

Many thanks for the, slightly technical, response but I think I get the jist of it.

It does bring me back to my question though, with no vents in the soffit, should I be installing vents to allow air to circulate more freely up th there as currently there are none that I can see?

 

[mrrusty]

Now you have more insulation and the loft space will hence be colder, improving ventilation will certainly help to keep nuisance condensation down.

Keeping the loft hatch shut helps and if you have un-sealed downlights that provide an open link between house and loft, it will be worth thinking about sealing those. It's not the fact the loft is cold that is the problem, it is the possibility of warm humid air from the rest of the house getting in there.

 

[harrythefurrysquid]

should I be installing vents

Yes, I think that would be a good idea if you're getting condensation.
You could also add Manthorpe felt lap vents. Is your felt breathable, or is it the bituminous type?

Also check whether there are any obvious ways that moist air could be entering:
Is your loft hatch airtight? If not, consider fitting P-profile draught excluders.
Do you have any penetrations for pipes?
Do you have any penetrations for downlighters?
Does your bathroom extractor fan exhaust enter the loft?

 

[FFCForever]

Ok, first off many thanks for all your input here, very valuable.

Here is a picture of the membrane that sits in my roof space behind the rafters. I'm not sure what type this is, other than it's not felt, would lap vents be a good way forward here initially rather than venting the soffit?

Regarding other points raised, this is an in progress thing but we had a downlight that was loose right above a shower that I have corrected with sealant and I suspect this was doing is no favours, but I think I'm going to buy some proper ones that will form a better seal around the ceiling. The loft hatch is new but could do with some adjustment/insulation to isolate the roof space better. Noted and I'll get these done.

The extractor fans DO enter the loft and extract via vents in the soffit (these vents are purely for fans though and do not vent or allow air into the roof space.

I can't "see" how well they are connected to the vents however, but I ran the showers and looked for steam in the loft space and could not see any.

I'm going to hire a scaffold tower in the summer and check all this (as well as repaint the soffit etc anyway).

 

[harrythefurrysquid]

It doesn't _look_ breathable to me.
Lap vents are quick and easy to fit yourself, so definitely worth trying.

I'm in a similar position. I fitted extra insulation, and then during the cold snap found quite serious condensation - albeit judging by water stains I think this had been an ongoing thing. Fitting lap vents seems to have mitigated the problem a fair bit, but it's still almost 100% humidity up there. Like you, I think I have some moisture ingress from the bathroom ceiling.

In my house I have a dormer section front and rear so there's nowhere to put soffit vents - but if the lap vents don't fix it for you that seems like a sensible and hopefully inexpensive next step. After that would be tile vents, followed by ridge vents, I believe.

 

[FFCForever]

Are there any recommendations for lap vents? Number per sq meter or similar that you know off? And by the way - thanks for the recommendation, i didnt even know this was a thing.

Finally, i had stopped insulating because of this issue, but it sounds like if anything continuing to insulate would be recommended also as it stops warm air coming through better. I assume therefore i should be ok to continue?

 

[Justin Passing]

You could try a humidistat fan up there maybe. I have one in the kitchen which is dead reliable. It comes on whenever it rains.

 

[harrythefurrysquid]

Are there any recommendations for lap vents?

The Manthorpe vents are as good as any, I believe:

https://www.amazon.co.uk/Vents-Prevents-Condensation-Attic-Ventilation/dp/B00M97O5YY

I suggest putting at least one between each pair of rafters.

I have installed one row during the cold snap, and I think I'll go on and do a second row since it appears to have improved matters even if the problem's not completely solved. Soffit vents are probably a better technical solution as I believe there is some increased risk of tiles being lifted during gales with lap vents.

You may as well carry on insulating IMO, but keep in mind that wool insulation won't prevent moist air entering the loft. It can make the problem worse, but it does so by making the roofing membrane colder (since it's not being heated from underneath), which will cause more condensation. Similarly, it's a good idea to run a dehumidifier in the house, but it's not going to drop the moisture content down far enough to make any real difference to condensation in the loft, per the table above.

Basically a twin solution of:
- try to reduce moist air entering the loft
- make sure there is enough air flow to dry it out

The lap vents will help with the second bit.

I'm personally tempted by Justin's humidistat fan idea, since the previous owner of my house already ran power up there for lighting, but I'll still need vents so that the fan is pulling air from the outside, not the inside.

 

[FFCForever]

Yeah, it does sound a good idea, like you say though i think they key here is getting fresh air into the loft space. I'm going to tackle that first before i try things like this out.