The architects have said they are concerned about over air tightening our proposed new build. I think this was as a result of us saying we'd rather not have MVHR, favouring opening a window if we need fresh air and the fact we live in the country, so door and windows do tend to get opened (how else do you hear the bird song and smell the country smells if you're hermetically sealed in a box?).

So being a complete novice at air tightness and having Googles the subject and been baffled by the huge amount of information, I was wondering if someone here could synthesise the requirements. We realise there will be a spread, starting with minimum building regs at one end and passive house at the other, but where is that happy mid point - you know that point just before the requirement for MVHR kicks in?

Thanks in advance.

Go for max airtightness, then open windows as nec - you won't suffocate! Deliberately leaving things leaky will just leave you with draughts esp when the wind blows, and still won't provide enough ventilation when it's still and heavy outside.

Also, leaving well insulated structures with small all-over leakiness (as distinct from major leaks around windows etc) is a major cause of moisture transport leading to interstitial condensation. This latter is a new finding of Fraunhofer research (the authors of WUFI) - water vapour transport by air leakage turns out to be at least as big an interstitial condensation risk, as is water vapour diffusion, which has been the traditional focus.

The point is, it's only worth having MHRV once U-values are down to the 0.12 -0.15 range - doesn't mean you have to. The other necessity is to be airtight.

Why should low U-values and/or airtightness make MHRV necessary? If no MHRV you just open windows - not going to die gasping! It's true that you'll use more energy that way, because none of the heat in the outgoing air is recovered.

It's also true that without MHRV you won't experience the delightful always-outdoor-fresh feeling that everyone reports, with MHRV. It continually removes all the many indoor toxins that we all live with and make us feel less than 100%.

An airtight building does avoid the condensation caused by air-exfiltration-transported moisture, that I mentioned. But you'll get that benefit from airtightness, with or without MHRV.

Our 300m^3 house with 2 adults 2 kids, all out during school hours, has a sentinel kinetic plus with humidity & CO2 control. It uses an average of 13W throughout the day according to one of those plug in power meters, left averaging over a few days.
The lowest power it uses is 9W(nobody there - this number really should be less), then 30W(excessive CO2),the highest power is 50W(lots of humidity due to a shower).

Best advice is get an efficient unit, and use CO2/humidity control - that way it won't keep going at excessive power when it's not needed because there's nobody there.

I think it needs to respond to CO2 quickly(<30mins), to keep everybody happy. The humidity feedback could be slower(days!), as most houses have significant humidity buffering - generally many days worth - so perhaps off grid you could only activate the humidity sensor when you have wind/pv/other renewable resource ?

My rule of thumb would be that the capital cost of off-grid production in winter would be about £40/W or perhaps a bit more so, taking RobL's average of 13 W, that would be about £520 worth of panels, etc. Bit more wear on the batteries, as well, of course.

You need to balance the cost of running the MVHR vs the cost of replacing the heat which would otherwise be lost from the building. If you're a pyromaniac with cheap or free wood and think you'll drop dead before you're ever too infirm to deal with moving and using the wood then the calculation is different from if you're trying to run the house purely on nuclear fusion.

Similar sort of calculation for the sewage treatment plant. Was going for Bio-rock but my house designer specced one with an air pump. Started writing an email saying to change and including the calculations of why and realized that actually generating the electricity would only cost a bit more than upgrading to a fanless plant. And that's over the winter months - spending the extra on PV to run the plant would then give more spare power in the brighter parts of the year.

Thanks for the answers and suggestions. This is just theoretical at the moment but that may change in the near future.

Jonti