Retrofit Ventilation

Hi all,

We have a 20 year old 3.6m x 3.6m dining room extension with a pitched roof ceiling that was as leaky as old boots. Upon inspection all it had up there was around 100mm of rockwool type insulation and thermal camera images showed how bad it was.

So we set about improving that. We ripped down the old ceiling back to joists and rebuilt. We’ve now ended up with:

200-300mm rock wool
70mm PIR between joists (taped underneath)
25mm PIR under the joists (taped)
12mm Plasterboard

Using Home Assistant and Xiaomi sensors I can see an instant positive impact on temperature drop off. Hopefully this will really help in winter.

But a by product of the work is an increase in humidity in both the dining room and adjacent open plan kitchen next door.

We have no room for any large space consuming MVHR and associated pipework as we are in a 1930’s semi.
We have a cooker hood which we use when cooking.
But was wondering if something like a constant trickle feed mechanical vent would help in the kitchen?

There is a small area of wall in the kitchen that is outward facing that we might be able to fit such a device.
But it would need to be quiet. And probably responsive to higher humidity readings, ie when washing/drying clothes etc.

Any recommendations would be very much appreciated.

Cheers, Mick


Hi Mick, welcome to the Carbon Co-op community forum!

I know issues with humidity after improving airtightness is an issue a lot of people have faced, so hopefully there will be some solid input from other members. Unfortunately, I don’t have any specific suggestions from my end, but am interested to see what come up since I have some similar concerns about work I have planned for my house.

It is great that you have been able to measure the improvements you have made with the Xiaomi sensors, are these the Aqara temp/pressure/humidity ones?

Thanks Matt.

Yes, just the Aqara square shaped ones. I’ve got them scattered around the house going back to Home Assistant (via a Conbee II).

I then squirt the data from HA to influx and graph with Grafana.

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There are. Number of single room non-ducted heat recovery units. I haven’t looked into them as I have a ducted system. Alternatively you could look into an on-demand system with built in triggers for humidity and air quality as provided by Aereco.

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There are many ventilation options to chose from. I presume you have a cooker hood? Is this ducted to outside? If not would it be worth doing / ie is it convenient to do that? For more continuous background ventilation you could try a single room heat recovery unit such as the Vent-Axia Tempra. Obviously with any extract vents like these the make-up air will have to come from somewhere in the house and as such it is probably worth looking round the house to review the whole ventilation strategy in the medium term.

I went for Kair single room heat recovery ventilator with humidistat KHRV150/12RH. It works well in bathroom and shower room. Fairly quiet, you can adjust humidity levels.
I would echo what zapaman says about overall ventilation strategy


Hi Mick ,
your lean-to kitchen moisture problem seems very similar to ours. We have a lean to with internal thermal bridging on the ceiling. We also know we need to remove the ceiling and re insulate. Additionally, the roof tiles on the kitchen are plastic which look like slate - this may be contributing to our leaky roof.

Also, in our retrofit survey it was mentioned about moisture build up from cooking, kettle and washing/drying clothes by the radiator , which coincidentally is positioned by the freezing cold wall near the back door - just like in your kitchen!! We got the impression the cold wall , warm and wet air situation is exacerbating the condensation buildup in that area. We were told it would help a lot if we used a dryer with a moisture outlet, as well as some additional venting.

@zapaman , @Zarch @Phoebe_Spence , @Tim_Gilbert @mattfranklin
Thanks everyone, this thread made for great reading and really full of great tips and possibilities. really appreciated here at my end.

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Driers with external venting are expensive and breach the thermal envelope. I spread out any drying in the bathroom and put a portable dehumidifier in the room. It is amazing how much water it gets out of the atmosphere and the clothes. The dehumidifier, of course, is more versatile than a dedicated clothes drier.


I’ll remember that - thanks Tim. Need all the advice I can get here :grin: I’m enjoying this forum and everyone’s shared experience.

Thanks for the replies.

We currently have a cooker hood above the gas cooker that we use when cooking. But this is noisy and only runnable during cooking. Yes, long terms plan to move to electric cooker! :slight_smile:

Our tumble drier is a pretty old condenser model which spews out warm air into the room. We hope to replace this with a A+++ rated heat pump model that a) should use half as much electricity and b) keep all the moisture inside the unit rather than out into the room. But £600-£800 ??

As for an overall house strategy, I think this is tough. I don’t think our 1930’s semi lends itself to anything other than single room opportunities.

On the very top floor we have the loft conversion. The windows have trickle vents and the Velux has a opener. The trickle vents are open 24/7 and the Velux vent gets opened most days. Maybe long term we consider a single MVHR?

On the middle floor we have a humidity sensor extractor in the bathroom that vents outside. Not subtle, but it does a bit of a job when moisture is high, then turns itself off. When we finally get around to redoing the bathroom we’d look at replacing that with perhaps something quieter/cleverer and maybe runs 24/7?

That just leaves ground floor.

So it does sound like I need a 24/7 or perhaps mini MVHR unit that can sit on a single wall and vent out?

Single room MVHR systems are of two types.

  1. Those with one opening have a heat absorbing element. They take room air and expel it through the element. Then after a short interval the air direction changes and outside air is directed through the same element, absorbing heat as it goes. The cycle then starts at the beginning again.

  2. Those with two openings to the outside world work just like a whole house system except that there are no ducts to other rooms. By that I mean that they simultaneously take in fresh air from one opening and expel stale air from the other. There is a conventional heat exchanger between the two air flows. The incoming air is directed away from the unit, optionally with a short length of narrow ducting (narrow as it only needs to carry one room’s worth of air). If using ducting you could potentially direct the fresh air to an adjacent room, thus ventilating 2 rooms.


Our home has three single room heat recovery and ventilation units. None of these run 24/7 and all have humidistat control. These are located in the wet areas of kitchen and shower rooms and have been in place since 2018. For the kitchen we use the Envirovent heat sava, I would suggest a larger heat recovery unit to replace the cooker hood (150mm diameter unit) and get one of the quieter ones (trickle and boost functions). It will be quieter than the cooker hood on boost and much better for you.
Compare units on noise output on boost rather than trickle. Works very well and we also monitor humidity independently so know when we need to use.
Hope this helps


Where linking a cooker hood to a heat recovery ventilation system check compatibility with the manufacturer. Grease in the air will clog most heat exchangers. The normal suggestion is to have a recycling cooker hood to degrease the air and a separate ventilation unit.

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Retrofitted MVHR doesn’t have to be space-hungry. Most of our (insulated) ducting is in the (cold) loft and some in the ground floor ceiling. The drops are in the corners of two bedrooms and the landing. The small (Titon) fan unit is also in the loft. Wet washing normally hangs outside, unless it’s raining, in which case it hangs over the bath, where there’s an extractor with humidistat boost. We do have a cooker hood extractor too.


As I understand it, MVHR ducting in an unconditioned loft, even if the ducting is insulated, is regarded by the building science community in the US as to be avoided at all costs due to inevitable heat losses and leakage. Just sayin’. Maybe our conditions are different here in the UK. If thinking of putting it there, it might be best to speak to a knowledgeable building science practitioner.

With insulation the losses shouldn’t be great. The issue arises where there is auxiliary heating in addition to heat recovery. The higher Δt the higher the heat loss.

Which? has a number of Best Buy heat pump tumble driers under £500.
PDF print of brief comparison attached.
Heat-pump tumble dryer - Which under £500.pdf (562.6 KB)

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Instead of a tumble drier I have a drying room (aka bathroom) where I hang the washing with a dehumidifier. Cheap, low energy and more flexible than a metal box under a worktop.

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What dehumidifier do you use Tim ?
Have you pinpointed the likely savings between the dehumidifier and a dryer?

Thanks for sharing :+1: