Dampness in loft, would a PIV system solve it

We have a victorian house that has some dampness in the loft, the ends of the nails poking through the sarking boards have water beading on them, and the north facing sarking boards are damp to the touch. Some background info : the house has been reroofed with a quality membrane and reslated, We’ve installed double glazing, (the double glazing will be trapping more moisture than the old single glazing) we dont dry clothing on radiators and have a dehumidifier that runs in the main bathroom. To be truthful, the windows are not opened enough in the winter and the loft hatch needs to be sealed properly. We don’t have any vents in the loft space, there are no sofits, so if we want to ventilate I’m guessing it’s a case of replacing some slates with vents like these

https://www.roofingsuperstore.co.uk/product/nature-vent-500mm-x-250mm-slate-vent-black.html?gclid=EAIaIQobChMIhc7XqbOb9QIVZWHmCh1lEARzEAQYASABEgJjRvD_BwE

? I’m wondering if a PIV system would be a good idea , or maybe just an appropriate number of vents fitted.

Thoughts?

First you need to get a cross flow of air in the loft space – install a couple of those vent tiles onto each elevation as low down as possible and then some lap vents in the membrane (I got a pack of 50 lap vents which were very easy to fit). After I did this my condensation went to zero. If you have a properly ventilated loft, and can minimise the air leakage back into the loft from below a piv system will then reduce moisture levels in the living spaces and improve air quality. We have a purair one and now that I have properly ventilated the roof it’s made a big difference – just feels a bit cold when stood underneath it!

Thanks Fizik. Great info. I’m not sure lap vents in the membrane will be an option as there will be sarking boards in the way. I’ll press ahead with low level slate vents and stick a fan in the loft to promote air movement in the first instance.

Whilst I’ll have a roofer do the work, I always think it wise to have a rudimentary knowledge on what needs doing, I hadn’t considered adding vents on each elevation, I’m guessing the roofer won’t want to do this as it will probably require scaffold to be erected at each side of the house and they won’t want the hassle of having to come back and forth each time the scaffold is moved. Maybe they’ll be happy working from ladders… Do you know of any simple calculators to define the number of vents required by loft space or square footage of the house?

Vents at eaves and near ridge level are best, let nature take its course with wind and cross flow etc…

If you just stick Vents at low level it won’t work quite as well.

It’s a slate roof and by the design has minimal eaves ventilation (which cannot be increased). Likewise we can’t disturb the ridges, so realistically slate vents are the only option I think
.

We had our loft insulated via whatever govt scheme was running at the time, after which we had terrible dampness and condensation (cold water and heating header tanks were up there) – there was condensation dripping from the rafters and roof lining. The insulation guy came back and put in some of those lap vents and the problem went away immediately, even in winter.

Understand the lap vents won’t work if you have sarking boards, but won’t that be the same for a vent tile? Or would you cut a hole in the sarking board where the tile goes?

@tonyd, yes, we would cut holes in the sarking boards to fit the slate vents.

I think your roofer will be able to advise how many vents and as above also fitting them closer to the ridge line will improve further particularly if lap vents aren’t an option. I’ve put vents on the front and back elevation of a modest size 3 bed bungalow, none at the ridge line but I haven’t got any noticeable condensation now and the pic system has stopped blowing moist/mouldy air into the house!

I’ve just fitted a PIV in a 300+ year old cottage. When I first began exploring the idea I went up into the loft to find that the insulation was up to a metre deep in places.

When I dug down it was wet at about 30cm up from the ceiling. Reading up on it it was interstitial damp: warm air contains more water and as it rises from the ceiling, if there’s too much insulation it can’t evaporate and condenses to water as it cools through the insulation.

Even the eaves had been stuffed full of insulation.

I’ve now reduced this to a more normal level and unblocked the eaves completely so there’s a good cross flow of air. I’ve fitted the Nuaire drinaster eco (non heated) and have seen a massive difference in window condensation (it was like a waterfall on every window every morning). According to my Amazon cheapy moisture measuring thingy it’s gone down from 79% to mid fifties. It was down to 49% this afternoon when I got home from work and the house had been empty of people for a few hours.

So, I don’t know if the PIV would work in drying your loft in isolation, but certainly in combination with vents to create airflow, I can’t see why not.

The biggest difference I’ve noticed is my breathing is much better; I’m asthmatic and for tears I’ve coughed fairly constantly when I go to bed. From about the 3rd day of having the PIV fitted I’ve had virtually no coughing whatsoever. I wish I’d heard of these things years ago.

@AC505 I should have been clearer, sorry. I just meant having slate vents near the top and as near the bottom of the roof slopes as possible. You are just creating airflow and as long as you can have the benefit of stack effect (warm air rises and cool falls) you should ventilate the roof void well enough. Hope it all works out.

You’d be amazed at the difference a couple vents will make.

My God, he can yap!

Hi got a nuaire fitted a few years back in my 1968 house with surprisingly no air vents. Teenagers in the house mean there’s lots of damp towels on the go at any one time, but the PIV did actually make a positive difference to the moisture buildup on the windows on cold mornings.

(Part of the reason for getting it was my asthma too.)

The only real downer was when my neighbour had fires and the smell of smoke quickly came inside. Oh and when we’ve had Covid in the house, I’ve turned it to the max…

Like above, vented loft space is needed. A friend put one in their cottage that was forever damp, due to the side of the house being part of a rock face, and it also made a difference.

The only real downer was when my neighbour had fires and the smell of smoke quickly came inside.

no firebreaks in the loft space? Or a shared chimney stack that is leaking?