Totally agree Paul. Maybe we need to change the quote to "build maintainable tight, ventilate right"

So the building owner can easily maintain their airtight envelope.

Cheers

Steve

I think the motivation is to save energy - if passive systems can provide part of the solution, then they should do so. The aim is to provide proper ventilation for as little energy expenditure as possible. The reports I read state that passive systems cannot work all of the time whereas active systems can (by design). A combination of the two makes sense - use passive when the conditions dictate (so no energy expended) and turn on the active systems when required. Sounds like a win-win.

The main problem with any system that the user has control of is that users are wont to turn them off or forget to turn them on (this is true for both active and passive - though with passive it's usually the reverse situation that's the problem - they're "running" when they don't need to be or are providing too much ventilation (and heat loss)). As an aside, many offices have dummy thermostats which give people the feeling that they're controlling the heating/cooling but, in fact, do nothing - thus satisfying the urge to tinker with the controls without the drawback of the user setting them incorrectly.

Your link is interesting, but I'm always wary of studies that benefit a manufacturer of equipment. At least the Canadian studies are impartial. Also, just because something hasn't been shown to be inadequate (and I don't necessarily agree with that assertion either - I believe there are studies which do show this) doesn't mean the converse is true. It all depends what the success criteria are. MHRV does satisfy the requirements in Canada which is why such systems are mandated in the building codes (despite opposition from builders in some places who would rather take short cuts - but that's always the case with anything in the codes).

It's funny how people are happy to drop several Gs on stone countertops, but wince at the thought of spending a G or so on something that improves the energy efficiency and interior comfort of their dwelling. We humans are a funny lot.

Paul in Montreal.

Hi Paul,
I think the Building Research Establishment is impartial?

>just because something hasn't been shown to be inadequate (and I don't necessarily agree with that assertion either - I >believe there are studies which do show this) doesn't mean the converse is true

Eh? Too many negatives Paul, I don't think I can be bothered typing the obvious reply

Where are the reports showing passive ventilation systems are inadequate, i want to see them if you know of any.

Peter

Steve,

If you assume (say) 35% of a building's energy loss is through air leakages in normal UK housing stock, then take a passive ventilation system that improves leakage for all other areas apart from the designed ventilaion, the energy loss through ventilation will be less. I've no ideas of the figures, but given how leaky normal houses are, it might reduce energy loss to (say) 20%. OK, I'm plucking these figures from the air, but they don't seem unreasonable?

In that case, switching to MHVR will save around 17% of your heat loss - assuming that it continues to run at maximum manufacturer's efficiency with no drops. This also assumes that no-one breaks the air-tightness by opening windows or doors.

That means that something like 83% of your heat loss is occuring elsewhere. It also means that a passive system gets you half way there at much lower cost.

From a random supplier site, an MHVR system uses about 800kWh electricity in a year to run.

So, when specifying a house, assuming the owner doesn't have unlimited finances, should they specify MHVR, or concentrate on other areas which may have more direct results? The theory is one thing, but looking at what happens in real world systems, theoretical maxima are rarely, if ever reached.

So my feeling is that passive systems may be quite effective in real world applications.

You could look at it that 10% is wasted but 10% of what??? With no ventilation there would be no ventilation heat losses.

All this 90% recovery stuff is all very well but please put in in kW per year of something meaningful.

Passive stacks waste 100% of the heat in the air that goes out through them but this is very difficult to control -- too much goes out when it is windy. kW handier.

Air leakage is never irrelevant it may still be the biggest heat loss depending on how well or poorly insulated the building is

To be honest, Steve, I'm surprised you're not considering passive ventilation for your roofkrete houses - the material and manufacturing techniques would be almost perfect for designing in passive stacks, which would give you good healthy ventilation for free and with no maintenance and running costs.

In the mild UK climate, a 10% saving in overall energy costs only equates to a twenty or thirty pounds a year for a small house - which makes a system that has a four figure price tag look expensive. That expense is important to me, because I could be spending it on a bio-mass boiler, grey water system or one of a dozen other 'green' technologies.

I'm frustrated to be discussing this though - this topic started as being about breathing walls and has been hijacked by a discussion on ventilation involving Steve's pet project and Paul's view of how it's done in Canada. Neither of those seem remotely relevant to either moisture permeability in walls or most green building projects currently in the UK (including my own). It's disappointing that there seems to be an inability to fairly asses the relative advantages of different systems, and accept that the huge compromises that have to be made in real building projects can lead to different choices being made.

We are having to make major design decisions for our self build right now, and discussions like this aren't helping at all. Theory and promises of 100% efficiency at some point in the future are of no use to us when we are going to be building next year (touch wood). Nor is discussion of how Canada deals with a climate that is dramatically different to that of the UK (and likely to become more different if climate change predictions prove to be correct).

Hi Tuna,
Sorry to have been party to hi-jacking your discussion. I believe this happened because the issue of walls and breathability is intimately related to ventilation and air tightness.

This document is interesting

http://www.naturalinsulation.co.uk/cms_items/20060607164406.pdf

There are two buts:- it is a bit long and I have some concern about impartiality.

But it is very interesting what he is saying.

Peter

Posted By: TunaThat expense is important to me, because I could be spending it on a bio-mass boiler

With a PassivHaus standard you don’t need to have a biomass boiler producing health threatening fumes (we can’t all have one). You would not need an heating system at all, body heat and appliances would heat your house, with a contribution from solar of course.

Posted By: Paul in MontrealI think the motivation is to save energy - if passive systems can provide part of the solution, then they should do so. The aim is to provide proper ventilation for as little energy expenditure as possible. The reports I read state that passive systems cannot work all of the time whereas active systems can (by design). A combination of the two makes sense - use passive when the conditions dictate (so no energy expended) and turn on the active systems when required. Sounds like a win-win.

Thanks Paul in Montreal that is an obvious win win situation