Turning our Victorian terrace into an ecohome: part six – ventilation

| 19th November 2010
Installing the kitchen single unit HRU
Installing the kitchen single unit HRU

Installing the kitchen single heat-recovery unit

Better insulation means more warmth but also more mould and condensation. Sue Wheat's eco retrofit continues with a look at a super-efficient ventilation system that recycles heat while stamping out damp

The draught-proofing’s done, the internal-wall insulation’s in, the colder weather’s here and the house is holding the heat in nicely. Walking into our house after a day out at work it still feels warm, even without the heating on, which is a warm welcome indeed. The truth will be in the gas bills, but believe me, it’s like a new house in here.

But there’s a vital thing to factor in after the insulation’s done, without which your whole project can go horribly wrong. At the beginning of this ecorefurb journey, I’d had an eco audit from Parity Project, who has pointed out that if we were to improve the insulation we must make sure we had good ventilation, otherwise the house would become soggy with condensation, black with mould and full of moist, stale air – none of which would make for a good ecohouse vibe.

Good ventilation was patently something we were lacking before our ecorefurb started, as every morning in winter all the windows streamed with condensation and our son’s room – with two outside walls – had constant condensation on the walls and thick mould behind the cupboards and around the windows that just kept coming back no matter what we did. His room was next-door to the bathroom, so the moisture from four people showering every morning undoubtedly wasn’t helping. We also dry clothes on the bedroom radiators, so the moisture was evaporating from the washing and condensing on the cold walls and windows (see how to diagnose various damp problems if you have them).

There are various ways of ventilating a house, the simplest being to open a few windows or have air bricks or vents in the walls, but this would mean losing the precious warm air that the insulation has kept in. In addition, they don’t move air around, so although our son had a large air brick in his room it didn’t stop the damp; it just made his room cold and caused a thick growth of unhealthy mould to grow behind the wardrobe. Nice.

Heat-recovery units

Parity Projects told me about heat-recovery units (HRUs), which are ventilation systems that rather ingeniously capture the heat in the air before it goes out of the ventilation system, and put it back into the room. Depending on the system, they can capture up to 91 per cent of the heat (those are the super-efficient systems used in modern houses), which means you save on heating the new air coming in and don’t have a cold draught.

As it happened, a friend of mine had had some HRUs put in her house last year and was very pleased, as they’d helped combat her asthma. She has them in the bathroom, which is an extension off the kitchen, as well as the lounge and her bedroom. What struck me when I entered the bathroom was how white the wall tiles were. Ooohh... how do you keep your bathroom tile grout so white? Ours goes all mouldy,’ I asked, TV-housewife style. I don’t do anything,’ was her response. The ventilation unit stops it happening.’


I was sold. Not having condensation on the windows or black bedroom walls would be great. Curbing asthma is brilliant. But finding something that means not having to scrub the bathroom with a toothbrush ever again could quite easily be the most popular invention every known to woman.

The company she used – Abbott Damp Proofing Ltd – has offices in the East Midlands and Enfield, North London, which happens to be local to us, so I got them in quickly. The owner, Dave Prince, surveyed our house and recommended a Kair single heat-recovery unit in the kitchen  – because of cooking, the main source of downstairs moisture. And a three-room ‘Apartment’ Kair system for upstairs, ducted from a main unit in the loft into the rooms via small, white vents in the ceilings. This would extract air from the dampest two rooms (bathroom and small bedroom) and supply fresh air to the other bedroom, which then gets circulated.

Each unit runs constantly on trickle vent, maintaining ventilation to the house 24 hours a day, but when the moisture in the air builds up the humidity sensor switches the unit to boost. This is usually set at 55 per cent relative humidity – the point mould growth dies off and dust mite activity stops, yielding health benefits for allergy and asthma-sufferers, and indeed anyone. The precise definition of relative humidity is ‘the amount of moisture in the air as a percentage of what the air can hold at that particular temperature’.

This constant push and pull of ventilation creates airflow through all of the upstairs rooms. I asked whether it worked when the doors are closed at night, to which Dave responded that air has to keep flowing into rooms somehow otherwise we’d suffocate in our sleep. Good point.

The ducted system recovers less of the heat than a single unit (65 per cent versus 86 per cent. With the ducted system the air loses some heat on the way to the unit (but most ventilation systems lose all the heat, so it’s still a big benefit). It also has the advantage of being silent in the rooms as the machine is in the loft. If you did just want a single unit in a bedroom, a light-sensor could be put on the unit so it doesn’t disturb you in the middle of the night.

Draught-proof of the pudding

Last week the system was installed – two days’ work for one of Abbott’s installers to fit both upstairs and downstairs systems, but minimal disruption to us. In the loft, two holes were drilled in the outside wall and covered neatly with plastic ventilation caps outside. The main heat recovery unit is actually very small – about the size of a shoebox. This was placed in the middle of the loft and a 10cm diameter flexible ducting from the unit to the outside walls (this ducting is light and easily moved if you have to lag the loft or do any building work). A heat-exchanger in the main unit captures the heat from the outgoing air and transfers it to the incoming air before it’s sent through the vents to each room.

In the kitchen a hole was drilled in the outside wall and the single unit – which both extracts and supplies – is fitted into it, with a small sensor box attached to the wall a few feet away. The electric cabling can be fitted into the wall and replastered over, or held neatly on the wall with electrical mini-trunking. Then the units are switched on, the humidity sensors set and they’re left to do their work automatically (they have very low energy use, even at boost).

And the cost? The upstairs system cost £1,200 + VAT including installation, and the kitchen system £649 + VAT including installation. So the whole system was around £2,000. Having lived through eight years of damp I knew it was an investment we had to make – I really didn’t want to see our newly insulated, skimmed and painted walls covered in black mould in a few weeks, and I’m sure it’s not good parenting either. In fact, it was something we should have done years ago, even before the wall insulation was put in, and saved ourselves the decorating bills.

Amazingly, we went from having streaming condensation on the windows every morning to dry windows overnight. In the kitchen the single unit boosts whenever we’re cooking, preventing moisture migrating into the lounge, so the whole house feels much less damp and much more healthy.

I’ve got the science to prove it too, having measured the humidity in the house with a rather exciting moisture meter. Before the insulation and the heat-recovery units, the relative humidity on a cold, wet morning upstairs (which produced lots of condensation on the walls and windows) was 76 per cent at 17C air temperature (40-60 per cent is considered healthy and comfortable), and the condensation readings went off the scale (showing the surface temperature was lower than the point at which condensation forms – the ‘due point’ – creating a sodden wall with 30 per cent moisture content). Unsurprisingly this was all categorised as ‘wet’ and set the machine off in a moisture-beeping frenzy. After installation, relative humidity reduced to 54.9 per cent (at 17C on a similar cold day) and the moisture content in the wall was 9.1 per cent, which registered – thankfully – as ‘dry’.

It’s too soon to know whether I can ditch the tile-cleaning toothbrush, but it’s certainly looking hopeful.

Sue Wheat is a freelance journalist

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Turning our Victorian terrace into an ecohome: part one
One family's journey to turn a draughty, turn-of-the-century terrace into a snug paragon of eco efficiency
Turning our Victorian terrace into an ecohome: part two - insulation
Sue has a brief love affair with solar panels, but realises that if she shies away from the thorny issue of solid wall insulation, she's throwing good money after bad
Turning our Victorian terrace into an ecohome: part three - floors
Insulating the floor can be fiddly, messy and costly. Luckily, there are simple, cheap options too as Sue discovers
Turning our Victorian terrace into an ecohome: part four - ecogadgets
Sue immerses herself in the addictive world of ecogadgetry, including water-powered alarm clocks and a crash course in cooking with blankets...
Turning our Victorian terrace into an ecohome: part five
Research over, it's crunch time in Sue's house, as materials are delivered, builders are educated in new eco techniques and wall insulation is fitted

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