Loft Condensation - already well vented

ThumbRemote

I've got a problem with loft condensation in my loft. 

The property is a late 80s detached house. A few years back I replaced much of the original loft insulation with Recticel 50mm thick insulation board, double layers. This means the house is now very well insulated but little heat makes it to the loft.

I am satisfied that little or no moisture is making it into the loft from the house. I ran an internet-connected humidity sensor in the loft for a number of weeks. There were no peaks in the humidity, just gradual rises and falls throughout the day caused by temperature changes.

The loft is vented by strip vents under the eaves to the front and rear. The insulation does not block this, especially now it's insulation board with a solid edge. In addition there are felt lap vents all across the loft. Then, because of condensation last winter, I had 8 vent tiles fitted. There was still condensation after all this...

The condensation was particularly bad when the temperature dropped below zero. To try and avoid moisture getting up there I didn't open the hatch too often, but on one occasion I noticed frost on the underside of the felt. The condensation is particularly bad on the south-western side of the loft.

So...

I suspect the only way I can stop the condensation forming is to remove some insulation to ensure the temperature remains warmer. Given heating costs I really don't want to do this. Are there any other suggestions - anything at all - that may help?

Or, failing this, is there any way I can mitigate the effects of the condensation? I'm most bothered about the water falling onto the chipboard as it won't take long for mould to form on this. I tried putting puppy training pads down to cover the worst affected area but it's not a great solution. But would a tarpaulin sheet work, or cause other problems? And what about painting the chipboard to prevent water soaking in - would that help?

Thanks for any suggestions you can come up with

tacpot12

It's not clear whether the moisture is rising from the house or is coming in through the vents in the roof. You say that you are confident that it isn't coming from the house, but do you have an airtight barrier membrane fitted across all the ceilings and tied into the walls? Moisture coming from the house is the most likely cause. During very cold weather, the outside air is usually very dry, so it seems unlikely that the moisture is coming from outside. 

Where did you have the vent tiles fitted? If they are not in the ridge, then there will dead areas where there is insufficient air movement. The pattern of staining on the floor suggests the problem exists across the majority of the roof width, so incorrectly located vent tiles seem an unlikely cause.

Do you have a water tank in the loft? If so, does it have a tight fitting lid? If your heating system had a feed and expansion tank in the loft, this could be a source of moisture in winter (especially on cold days when the heating will be working hard).  

You might try these as a solution: 10 x Felt Lap Vents Prevents Loft roof Condensation. Attic Space Ventilation : Amazon.co.uk: DIY & Tools

Anything that would pressurise the loft would help. Installing a fan that sucks fresh air from outside and injects it into the centre of the loft would force out moisture laden air through the eaves and other vents. It would be unusual to have to fit a PIV into the loft, but there is no reason why it wouldn't work, and it would cost a lot less to run than removing insulation. The biggest issue you might have is where to site the inlet duct. Clearly you can't draw air from the house into the loft - you must only draw fresh air from outside into it.   
 

daveyjp

If you have condensation in the loft the second part of the title can't be true.

You need movement and flow of air as well as ventilation which may be very localised.

ThumbRemote

daveyjp said:

If you have condensation in the loft the second part of the title can't be true.

You need movement and flow of air as well as ventilation which may be very localised.

Any suggestions how I could achieve this?

My lawn is very well ventilated, yet it still gets frost forming on it. Could something similar be happening here? Frost forms, then when it melts it's too much water for any amount of ventilation.

ThisIsWeird

TR, that is really bizarre. 

I presume you haven't been monitoring it on a near daily basis, as you don't want to allow 'house' air up there, but if you were to do this, I'd hope you'd find that the cond happens only on a small number of days, and - apart from the annoying stains on the chipboard - is nothing to be concerned about.

I suspect it's just 'weather conditions' as you have described - the loft is clearly cold enough at times for frost to build up from moisture in the ventilating air, as you have witnessed this happening. The main reason for ventilating the loft space is to prevent the timbers from being damp over prolonged periods which could lead to rot, but your timbers don't appear to have any untoward or concerning stains on them?

I suspect, therefore, that tho' it ain't ideal - a bit more ventilation would be good - it isn't actually a 'problem' (other than cosmetic).

May I suggest that a solution is definitely not to remove any of your insulation to try and make your loft warmer! 

Where is the moisture coming from? No idea. But I suspect it's mostly from the ambient outside air, and a small (and perfectly normal) amount trickling up from the house. I think even the draughtiest of lofts will have some winter days where cond forms on the underfelt. 

So, I'm speculating that what you have is on the edge of being 'normal', at least not particularly 'unusual', and will most likely cause no long term harm. It will almost certainly be limited to occurring for short periods of the year, and the loft will otherwise be perfectly dry. But, it does also look to be far less than ideal, so it's worth looking at further measures.

NB the above is pure suspicion and speculation...

What to do? As others have said, perhaps more targeted vent tiles? Who fitted the first lot, and where? When you are up in the loft on a windy day, does it feel draughty?

The soffit strip vents are def clear? There isn't an extractor fan outlet from a steamy shower room directly under one? Or a leaky kitchen extractor vent duct running through the loft?!

If the timbers are otherwise dry and free from obvious damp/mould stains, I don't think I'd worry too much. I'd monitor it, and try and work out under which weather conditions it mostly occurs. I wouldn't worry about letting puffs of house air up there when you investigate - these should be relatively trivial compared to outdoor ambient conditions.

As for protecting the chipboard. Yes, you could paint it, but you could also staple a panel of breathable Tyvek to the underside of the rafters to just cover the drippy area. Stop it well short of the eaves, and place something there to catch any drips. Also stop it short well before the ridge - wherever the drips start. Check it each year to ensure the timbers aren't affected.

You could also try increasing the soffit ventilation with extra round vent grills - these are easy to fit, requiring only a hole saw. Pepper the darned soffits with them.

Monitor, and try not to worry.

Chickereeeee

Have you any high-level ventilation, such as ridge vents? If not, that would be my first thought.  Any warm, moist air would go high up, especially  in a cold loft. Maybe existing ventilation is not causing sufficient circulation high up.

born_again

The condensation was particularly bad when the temperature dropped below zero. 

Frost forming on the inside of the tiles & felt. Which then melts once temp rises. 

Due to lack of heat filtering up to keep this from happening. 

ThumbRemote

Thanks for the comments.

Your thoughts all seem to mirror mine - the frost is probably from the ambient air. The loft is definitely well ventilated - I can feel the air moving when there's a breeze outside. Overall there are ridge vents the whole width front and rear, 4 circular soffit vents down each side, numerous felt lap vents, a high level vent on the 'point' of the roof, and 8 vent tile - two low down on each side.

Last winter we had a couple of prolonged cold spells (one before and one after having the vent tiles fitted) - these coincided with noticeably bad condensation. The photo is from the first spell before Christmas. Short of removing the roof entirely I'm not sure there's much extra venting I can do! 

While the condensation seems to be for relatively short periods, it can be hanging around for a week or so at a time - the length of a prolonged cold spell. It does make me worry about long term damage. There are water marks on the beams under the tiles, but I think these are just marks where it's been wet then dried out rather than anything more serious. 

Tyvek sounds an interesting idea to catch the moisture, I'll have a good look at that.

An alternative thought I had last night - might putting some insulation roll under the roofing felt help? (ie a bit like spray foam insulation, but easily removable. Held on with some thin pieces of supporting batten across the tile beams) I'm wondering if this would reduce the rapid temperature drop of the roof tiles so help prevent the condensation forming, and also give a less solid surface for the condensation to form on and stop it running down. 

Or to improve air flow, a normal household fan that comes on for a few hours if the temperature drops below 2 degrees. It might be just enough to force air to circulate. 

ThisIsWeird

ThumbRemote said:

Thanks for the comments.

An alternative thought I had last night - might putting some insulation roll under the roofing felt help? (ie a bit like spray foam insulation, but easily removable. Held on with some thin pieces of supporting batten across the tile beams) I'm wondering if this would reduce the rapid temperature drop of the roof tiles so help prevent the condensation forming, and also give a less solid surface for the condensation to form on and stop it running down. 

Or to improve air flow, a normal household fan that comes on for a few hours if the temperature drops below 2 degrees. It might be just enough to force air to circulate. 

I wouldn't add any insulation to under the existing felt. All I can see that doing is trapping the formed condensation, and leaving it damp around the rafters for a lot longer. Bear in mind that insulation doesn't make things warmer - only heat can do that. And that loft will - should - always be at around outdoor temps.

I also wouldn't bother with powered ventilation such as fans. Until there is good reason to suspect summat is actually amiss up there - actual signs of mould forming on the rafters, for example - I personally wouldn't worry.

The only thing I'd consider doing is adding more vents, either in the soffits, near the ridge, or both. From what you say, tho', this is already well catered for. And the proof is in the obviously detectable air flow up there. By all means try and increase the passive ventilation, but otherwise don't worry about it - unless there is a genuine cause to.

I think Tyvek will work to catch the drips since it's 'waterproof' whilst still being breathable. In any case, I'd suggest it's only fitted as a single horizontal strip where the drips form, leaving a goodly gap both above towards the ridge, and below towards the eaves - leave a good couple of feet at least exposed.

I don't think there's anything really amiss there - it'll be the same in countless other lofts during winter.

Chickereeeee

Incidently, what roof covering do you have? I assume it has a non-breathable underlay, by the look and age, but slates/clay tiles/concrete?

I have heard the slate (natural or man-made) is a bit more susceptible to this issue.