Positive Input Ventilation

Together with trickle vents on windows in a 1950s bungalow in the UK

Would really appreciate people’s points of view on this

Adequate ventilation is essential. Whole house heat recovery ventilation is the way to go. Direct vents, whether in- wall or “trickle vents” common in double glazing units in my experience are very unsatisfactory, at best being an unacceptable path for heat loss.

Fair enough, but when adding such a system, it is advisable to fully insulate the house on the outside as well as the roof space as well and depending on the age of the glazing, replace all that. That would be a massive investment especially these days after all the price increases due to “cough cough” and inflation

However yes I agree with you, but no ventilation at all is a recipe for disaster during the cold months that we experience now

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I don’t have any direct experience with PIV but if you have a problem with excess moisture in your home then it’s a solution that maybe of help. I guess you have read up on it?

Vent-Axia provide a small scale solution that might be worth installing in a problem area to trial


Yes I did look into this a bit. Downsides are it heats up the place in summer due to hot loft space and cools during winter. The pulsed heating component can push up energy consumption in winter of course.
Overall though tenants that don’t ventilate property properly by opening windows, it can be a useful solution to avoid severe issues with moisture and mould

Well that is advisable anyway! And I don’t think has any bearing on whether MVHR (mechanical ventilation with heat recovery) is worthwhile, only that when considering overall thermal efficiency insulation is more cost effective. But MVHR itself is not about thermal efficiency, just the HR part minimises the negative effect of the ventilation on heating cost in winter. If a house has problems with damp, often the case when an older house that originally was naturally vented via leaky window and door frames, open fireplaces etc has those closed up to prevent draughts and heat loss, then something is needed while in my experience trickle vents such as those invariably fitted to double glazed window frames reduce the effectiveness of the double glazing both on heat loss and noise insulation.

The system extracts moisture and odour laden air from bathrooms, toilets and kitchens, feeding fresh air from outside the house to living rooms, and has a heat exchanger to use the exiting air to warm the incoming air, no energy used on heating.

Back in the early 1990s I installed one in a 1920s semi, which was not difficult. We appreciated the benefit without insulation of external walls, though at the same time we did put in loft insulation. (It is crazy not to insulate the loft anyway, and I don’t understand why anyone wouldn’t, at least in Britain and anywhere with similar or colder winters.)

I put a more sophisticated one in our present house 11 years or so ago, which was more difficult due to being a chalet bungalow and needing to get ducting from one side to the other, and recently put one in my son’s Victorian terraced house, which also took quite some effort to find routing. In both these cases we insulated both the roofs and external walls, but the ventilation system is to prevent damp problems and provide fresh air. We also put triple glazing in ours, son’s already has double - old without trickle vents. Son’s house is not yet finished, but we’ve been very happy with ours, no draughts yet no house smells, no need to open windows either at night or to ventilate in the morning, kitchen smells only in the kitchen while cooking, similarly toilet smells, and no requirement for trickle vents in window frames


How did you insulate the external walls, presumably some form of cladding?

We have a Victorian end-terrace and since replacing carpet with engineered oak downstairs and a few other adaptations we seem to have significant draughts in recent years.

The front wall might be tricky to insulate without impacting on the visuals/character simply as it would likely stand out compared to adjacent properties, the rear I’d imagine would be less likely to matter. The main issue is the non-cavity side-wall of the property which looks nice in brick as does the adjacent end-terraced house, that clearly is the different weak point vs a mid-terraced property - 3 exposed walls vs 2.

We couldn’t do anything externally as its in a conservation zone. Mid terrace. Solid brick, exposed at front, rendered at rear. First problem was water pouring through the front wall (SW facing in wet windy location) - needed repointing (a big job - my wife and I are now experts!). We removed panelling and stripped of plaster inside, repointed inside as necessary, then “tanked” with an epoxy paint. Rear render not so bad, just a few render bits to repair, just removed loose plaster. Built internal stud wall, 75mm thick with ‘Kinspan’ type insulation, with air gap between it and brick wall, floorboards and ceiling cut back to link from void below ground flood all the way up to above insulation in the sloping roof, allowing airflow in case of future damp.

Wall below windows have additional 50mm insulation (ideally we wanted whole wall 100mm for intended U value, but at window level the reveals would be too deep, so this compensates and so gets best close), the step out to be covered with extended windowsill material (probably lipped PVC). Tricky bit was how to do window reveals without timber touching front wall: Fixed timber pieces to the uPVC frame edges, with 50mm insulation between that and the stud wall (tricky to describe - happy to try further some time if you are serious about doing similar). Plasterboard wall facing except the reveals which use ‘backer board’ as more insulating over the timber fixed to PVC edge and over the 50mm insulation. One or two bits needed some pondering, but all coming together nicely. The rooms lose about 170mm in length, but the alternative of a cold house costing a fortune to try and heat was not tenable.

I hope this gives a reasonable impression…

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Thanks very much for the detail, I follow most of it, but some bits will require re-reading with a clearer head!

Happy to clarify if still unclear once you’ve shad a chance study further.

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Looking at the link @JOF provided, a type of product I hadn’t come across, whilst in a typical house with air leaks it would provide a flow of air, the effectiveness of that in terms of removing moisture from the air would depend greatly on the air leak paths via rooms with the moisture problem vs other paths. E.as a simple example, if the outlet is on the landing, and the hall has very poor fitting door and loose letterbox flap, with the windows in those rooms with tight fitting windows and no other major exit paths, then most or all of the incoming air will simply flow out via landing, stairs and hall, making no difference to the damp rooms.

It might be better than nothing, but isn’t a proper solution to the problem. There is a reason why building regulations set minimum levels of extraction from bathrooms, toilets and kitchens, and requirements for air exchange capability for other rooms (= air inlet drawn by the extractors, providing a low level flow). MVHR is the modern and better way to do it.

I note the reference to tenants in your post, and implication of no or insufficient insulation. If this is a property you are renting out and profiting from, shouldn’t it be insulated anyway, at the very least the loft to the current recommended level (= 270mm if using fibreglass, the simplest and cheapest material to install)? I don’t know where you are, but in UK there have been all sorts of Government schemes in recent years assisting with cost, and whilst I haven’t looked at the current one others have been available to landlords.

The system in the link has been around for quite some time, since the 1970s when it was conceived I believe. It produces a positive pressure within the property which helps to exhange trapped moisture laden air via vents in the property.
The bungalow has some of the issues that you mention and not really amenable to having thick slabs of polystyrene plus rendering. It would look totally out of character with the neighbourhood if this was done. People already mentioned the difference of the new roof w.r.t. the old
We did insulate though from the inside which is not the best compared to exterior walls. Also a generous layer of insulation with over boarding the ceilings, which I believe is the main source of heat loss compared to exterior walls. Next is the glazing but that is of decent quality where some glass has been changed due to signs of misting in between the glazing. Anyway I think we can agree that closing up a property removing ventilation leads to a disaster which sadly is the case with so many properties in the UK. One could argue that many places should have a severe overhaul which many will not receive due to the current financial climate even though the governments provide means, which are not available to everyone unfortunately

Yes, external wall insulation, which can be great for some properties, can be a challenge for others, or impossible as in my son’s house. When we did up our own, a chalet bungalow, the choice of doing internally was mainly for cost reasons and we could afford the space. In a bungalow the loft if uninsulated is indeed is the primary heat loss path, but you say that is done. I expressed my concerns about the positive input ventilation - it would need an assessment of likely airflow in the property to determine if it would be at all beneficial and if so the best siting. However installation of MHVR in a bungalow is a very straightforward process assuming there is loft access, the only disruption to living spaces being the cutting of a hole in the ceiling (with a hole saw) and fitting an air vent, usually in every room, which takes just a few minutes, and with care makes little and easily contained mess. If eaves have wide enough soffits the external air intake and vent can be fitted into them, which is the easiest if it can be done, otherwise a hole is needed for each in the roof with associated flashing and weatherproof cowl, which would be the most significant work required.

I suppose the ideal installation is something aided by geothermal. Also a solar pack plus battery as well. Not to mention a solution for heating water independent of the gas central heating. Aero thermal on its own and the heat pump will consume a lot of electricity especially during the winter months that we have at the moment and not ideal for such climates. Another thing about UK property is the lack of cavity walls in the earlier constructions. Also in some constructions in Cardiff where the foundations and floor boards above the ground :crazy_face:. The delights that our daughter had to suffer when she studied in Cardiff with slugs making there way through the floor boards into her room. With the delightful discussions with the handyman telling us that slugs are intelligent creatures​:rofl:
I have the impression a PIV system in a bungalow that is permanently closed up is not a final solution but at least avoids the discussions of the importance of ventilation. The neighbours installed one in the hallway which is central to the building with window trickle vents and they are happy with the result and significantly reduced levels of condensation on double glazing and more importantly fresher air and no harmful mould growths.
With the new roof, new gas central heating plus radiators plus bathroom and kitchen improvements and complete replastering and ceiling over boarding works with redecoration, this will be a start and we see how things evolve

That sentence seems a little concerning, however if there is a damp problem doing something is better than nothing, and that is important. I can well understand tenants not wanting to open the window and let their expensive warm air out - I would only open windows myself briefly after having a shower to let the obvious steam out, but would very much prefer not to (and since installing MVHR in my home there has never been a need) - and would certainly not open any windows for longer periods in winter. As I suggested, if using the PIV approach it would be a good idea to do some assessment of airflow to verify most effective location - which may mean discussing with the tenants. Good luck with it.

Quite right and I’m talking about after getting a surveyor in and discussing the results with them as well as the results from inspections organised by an estate agent, which are not as detailed as I like, but at least show how the living space is being used. We are doing something to find solutions and I am not just talking about winter here either. Also there are results from the survey which are not very pleasing but I will not go into further details.
Regarding the bathroom and kitchen spaces there are already extractors to the outside in there installed during renovation which is important to keep the humidity down there.
Like I said, our neighbours have gone with a centrally installed PIV which leads into all rooms and they are very happy with it, where it has removed all signs of condensation on the windows. This makes the most sense to me as far as installation is concerned

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