Ridge vents and airflow

We have a cold loft space which had a damp problem . During the summer we cleared the space and re- insulated. Additionally, knowing ventilation wasn’t good we added felt lap vents. Everything has been fine and dry up there through the summer, but this week I have been up twice and condensation is pouring down the roof liner.
Can anyone advise why this might be. I have been researching the problem and have read about ridge vents to enable better airflow.
Has anyone else had this type of condensation problem in a cold loft?

Just wondered what “felt lap vents” were, and came across this: http://www.glidevale.com/uploads/9b36108ead3faa2df4933e194aefdef9.pdf

I haven’t read it all, but from a quick look it seems as if your vents are likely to be defunct.

See also http://www.greenbuildingforum.co.uk/ for general UK green building discussions on a huge variety of topics (free to join).


Hi Paul,
thankyou so much for this. It definitely does seem that the felt lap vents are defunct as we have discovered this winter!! :sweat_smile: The link you sent explains well why they are so ineffective. I’m doing a lot of learning on here.
We had our retrofit handover this morning and discussed the roof. A ridge ventilation system seems to be what we need, with a wind wash layer for on top of the sheep wool insulation.
Additionally we need a good reliable wiry roofer who is happy and competent to go on the roof and do the job in a safety harness with his cat ladders.
Are you by any chance related to the Hadfields from Hadfields Farm in Littlemoss?

(Already PMd by email re the Hadfields, as being somewhat off topic.)

As regards a wiry roofer, I needed to spend some time on our main roof ridge here some years ago, to remove some tiles and saw through the ridge beam, to install a Glidevale (previously Passivent) intelligent passive ventilation extract. Fortunately we have a flat roof extension at the back here, so I could put a board on the flat roof and a ladder on that, and so didn’t have too far to fall.
For a simple, low-maintenance ventilation system in a still-leaky house such as ours I can highly recommend Glidevale’s iPSV system as far less disruption, space-consumption and trouble (and hence much less embodied energy) than MVHR. For ‘good-enough’ or interim-stage retrofit I believe it is often well worthwhile, although I haven’t seen a full comparative life cycle analysis of the two. On the leaflet linked above, Glidevale quote a study by the Institute of Energy and Sustainable Development at De Montfort University here in Leicester into the performance of Passivent iPSV which they say demonstrate that the system “provides sufficient ventilation even under challenging or non-optimum conditions”. They say that these studies “provided the independent proof of the performance of the (i)PSV system which led to its acceptance in the Approved Document to Part F of the Building Regulations (and where it’s type is covered as ‘System 2’ of section 5)”.
But those who sell high-cost MVHR systems tend to argue against this much simpler, low-cost system for obvious reasons, and it hasn’t gained wide acceptance except in affordable housing, where its zero-maintenance aspect is the unique selling point for social landlords. I haven’t yet seen a comparative life cycle analysis in terms of carbon emissions for retrofitted MVHR versus retrofitted intelligent passive stack ventilation, which would be interesting, but prompted by thinking again about all this I have now asked Glidevale (/Passivent) if there has been any such study done. The newer version of their brochure doesn’t mention these studies in so much detail I see.


As far as I recall, passive stack has been removed from the latest version of building regulations. I think that must give an indication of its efficiency given a few years of hindsight.

Yes it seems it has been removed, largely I believe because it hasn’t been much taken up. But that may be because it is not widely promoted, which is possibly because the suppliers can’t earn much from it. But for a stepwise retrofit, at least as an interim measure, it has been good for us and could be good for many customers. I haven’t heard from Glidevale about any Whole Life Carbon analysis however, probably for the same reasons - it has never earned them very much money, being so inexpensive, so they can’t justify devoting much time to it.