To set the context, I'm currently building my own house which will end up about 200sqm.

We originally had an architect who only viewed u-value as the measure of the function of the fabric of the house and thus despite my requests, designed our house to effectively be wrapped in a plastic bag and then insulated purely with lightweight insulation materials (e.g. PIR) and very little consideration given to ventilation other than to open the windows every day. I dismissed this approach and instead went on an 18 month journey learning about alternative methods of building a house that provide not just good levels of insulation but a comfortable living environment that works with the seasons.

As a result of this research, I gave the architect the necessary information to re-design the fabric of our house using natural materials. This was supposed to be entirely wood fibre, but due to problems I had with the supplier and manufacturer being rubbish at both sales and customer service, I modified the designs again. In the end I chose to use Thermafleece cosywool through our first floor timer frame construction and rendered woodfibre EWI of the ground floor existing masonry walls.

The main priorities for this is that I don't believe wrapping our houses in plastic so they don't interact with the outside environment in a healthy moisture exchange is a good way to build houses. In fact, I think it's a bit mad. From experience, I also know how a 'breathable' house fabric works and the living environment it provides thanks to my mother's house in the middle of Sweden which was built in the mid 1800s. It is insulated entirely using sawdust and it provides a lovely internal environment all year round, even when I was there a couple of years ago during the massive heatwave where temperatures exceeded 30 degrees c for much of the summer. I also have a garden office that is insulated with thermafleece and feels very much the same.

So I have this breathable fabric which I think is essential to deal with and buffer moisture variations within the house, but this is of course not enough as it's going to be fairly air-tight, so we obviously needed ventilation. This was totally overlooked by our architect and therefore something I've been grappling with since I started my build.

So after a long period of research, I've opted to reject mechanical ventilation in favour of natural ventilation. I'm aware this in going against the grain as it seems like the market is flush with MVHR solutions, yet when looking at the offering available for natural ventilation, they're quite limited, particularly when it comes to domestic products. Those that are available, often don't ustilise best design practise - e.g. with passive stack, the roof cowels aren't typically designed to work well with the wind unlike the commercial ones which are much larger and are designed accordingly. So there is obviously something missing here, unless I've totally missed it in my searches.

I'm aware of the proposed benefits of MVHR in terms of heat recovery and fresh air distribution around the house yet when I've looked and some research, despite these benefits, the balance still seems to lean towards natural solutions. ( I also have to say here that some of the most informative stuff I've read has come from books about ventilation published in the 1800s rather than more recent stuff).

I'm therefore interested in having a discussion about the selection of MVHR over natural ventilation, just because I'm really curious about this topic. This is especially because we're seing so many problems within houses most probably caused by poor ventilation design.

What are your thoughts?

Posted By: SimonDI'm aware of the proposed benefits of MVHR in terms of heat recovery and fresh air distribution around the house yet when I've looked and some research, despite these benefits, the balance still seems to lean towards natural solutions. ( I also have to say here that some of the most informative stuff I've read has come from books about ventilation published in the 1800s rather than more recent stuff).

I was with you up until this point, but you lost me here. Where have you looked that came down in favour of natural ventilation and how did they calculate the 'balance'? It's clear you won't find anything about modern MVHR systems or modern thinking about ventilation in books written in the 1800s so I can't see how they are relevant at all.

When I looked, I started from the premise of building an airtight house, which means you need some means of guaranteeing sufficient ventilation and I found the most useful publications about the rate of ventilation were Swedish research into the subject. It was clear that purely natural ventilation (i.e. windows and other openings) could not do the job, which left mechanical or passive ventilation systems. Certainly at the time passive systems seemed a bit dubious to me* about how they could work in all weather conditions and still recover a reasonable amount of heat. Of the mechanical systems MVHR seemed clearly better than PIV or MEV or variations. Then it just became a task of choosing a particular system.

* I remember there's one video where somebody demonstrating a passive system paints an arrow on the floor and confesses there's one problem with his system - once the air is flowing it will keep moving, but something needs to tell it to start moving in the first place! The arrow is to tell it which way to go.

Hello SimonD.

You refer to 'breathable' construction (i.e. water-vapour permeability), but is your construction designed to be air-tight? Using wood-fibre internally I would expect the parge-coat/adhesive coat/WF/plaster coats to be sufficiently air-tight. What is the a/t layer over your sheep's wool?

I am with the others re mechanical ventilation. The decision to move from MV to MVHR can be cone as a cost-benefit calc. I believe one 'Low Energy' practice used decentralised MV without HR (and with trickle vents where the logistics and cost spoke against MVHR. The losses via not having HR are just fed into the calculations, but what is probably not able to be calculated is the effect of cross-draughts from the trickle vents.

I am in the process of improving airtightness and converting our house from passive to active (MVHR) ventilation, so I am probably biased, but the benefits of controlled ventilation over passive ventilation are in my opinion:
* like djh says, with active ventilation you have the option to design a whole house ventilation plan instead of just hoping that the air will move in the preferred direction. With active ventilation, every room, hallway, corridor, and even things like the understairs cupboard are designed to see some airflow, avoiding the build up of moisture etc. This is very hard to achieve with passive ventilation.
* decoupling the ventilation rate from the outside conditions, in particular wind speed & direction
* The ability to boost ventilation when required: when cooking, showering etc.
* Having the option of highly efficient heat recovery
* Having the option of filtering the incoming air to a high standard to combat allergies, hay fever and such like.

In the 1800s they had very different boundary conditions to design the ventilation against, one of the main ones being the presence of open flames inside the house: stoves, cookers etc. I really do not see the benefit of going back to the systems they had then when so much better, cleaner and healthier options are available nowadays.

@Ed Davies, I *think* Passivent once had a 'hybrid'? Don't know if they still do.

As others have said, to control the ventilation in order to match the required ventilation, MVHR is the way to do. Some MVHR systems are more efficient and economical than others, but all will be better (more controlled, less wasteful of heat) than natural ventilation.

For the avoidance of doubt, note that 'breathable' in the context of building fabric means 'vapour permeable', not wind-permeable - which I think is understood in this case, but is potentially misleading.

BTW, there is still a question mark in my mind over the use of sheep's wool insulation in general, following past problems with it being eaten by moths. For one related discussion (there are others) see http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=464&page=1
Personally I'd much rather use an alternative - maybe hemp or cork.

Ventilation guru Peter Rickaby has reservations about MVHR
https://www.linkedin.com/pulse/insulation-without-ventilation-monique-rosmarin-simons
considering it often a counter productive step too far.

Posted By: fostertomreservations about MVHR

From reports I've read on both new build and retrofit, there can be problems.

Users are probably the biggest problem - not understanding what MVHR is or how to use it / not cleaning or replacing filters / and turning it off to save money (particularly when users are short of cash on benefits).

Turning it off because of noise is another issue, which normally falls into the poor specification, design, installation & commissioning category, where problems do also occur.

But if properly installed, understood and used, very few problems.

Posted By: jamesingramI'd have thought if you want an airtight house so you can take maximum benefit from your high leveels of insulation
MHRV is essential , if not how will you fully control sufficient air changes ?
Seems not choosing MHRV you end up just having another system that ventilates but just loses the HR bit so what have you gained by that?

Posted By: djh
It's clear you won't find anything about modern MVHR systems or modern thinking about ventilation in books written in the 1800s so I can't see how they are relevant at all.

It was clear that purely natural ventilation (i.e. windows and other openings) could not do the job, which left mechanical or passive ventilation systems. Certainly at the time passive systems seemed a bit dubious to me* about how they could work in all weather conditions and still recover a reasonable amount of heat.

Posted By: bhommelsThis is very hard to achieve with passive ventilation.
* decoupling the ventilation rate from the outside conditions, in particular wind speed & direction
* The ability to boost ventilation when required: when cooking, showering etc.
* Having the option of highly efficient heat recovery
* Having the option of filtering the incoming air to a high standard to combat allergies, hay fever and such like.

In the 1800s they had very different boundary conditions to design the ventilation against, one of the main ones being the presence of open flames inside the house: stoves, cookers etc. I really do not see the benefit of going back to the systems they had then when so much better, cleaner and healthier options are available nowadays.

Posted By: tonyNatural ventilation is subject to how windy it outside, generally ventilates better when windy and does very little when not windy, typically trickle vents do nothing on calm days and way to much if open or leaky when it is windy.

Controllable ventilation is what you need

I probably have to clarify here that when I refer to natural ventilation, I'm including passive ventilation which includes trickle vents and passive stack ventilation systems.

I'm also very aware of the argument that MVHR provide more control, but in real world tests, that is questionable in comparison to natural insulation. Natural ventilation can be infinitely controllable, even an open passive stack as it's flow is not just controlled by it's duct diameter and the wind, but the flow of fresh air into the building - this has been shown in pretty extensive testing conducted in the UK during the 1990s I believe. If you're looking to provide an internal house environment that allows variation of temperature between rooms, this is readily achievable with natural insulation strategies.

In terms of the 1800s literature I've read, there is quite a lot of it that demonstrates a better understanding of ventilation in both domestic and non-domestic settings than appears in common construction practise today. Interestingly, and not entirely relevant to domestic installations is the importance of ventilation in classrooms and proposals for certain types of systems, some of which quite closely resemble the systems being designed and sold by Passivent and Ventive (who sell passive ventilation with heat recovery). A lot of this wisdom seems to have been forgotten, including where it comes to the ever increasingly important issue of condensation. Now, this doesn't mean I'm proposing to use the systems proposed lock stock and barrel, but to read an investigation into passive stack ventilation that gives guidance on how to design the cowels appropriately to prevent blow-back etc. was very interesting. It certainly helped me to gain a better understanding of the role ventilation plays in the design of a more comfortable and healthy lived environment.

My other point, which I remain uncomfortable about is the focus on recovering heat. This to me it is similar to our focus on U-value. I think our perspectives need to include overall energy demand and use.

Research from Sweden is usually pretty good, but I would also be cautious about the findings' relevance to the UK due to climatic variations, depending on where in Sweden the studies were conducted. However, UK research has shown that passive ventilation systems can provide all the ventilation requirements of a home, if it's been designed properly.

Here's a link to some interesting research I dug up in my quest to make my decision on natural ventilation. https://www.researchgate.net/publication/269515295_A_Natural_Ventilation_Alternative_to_the_Passivhaus_Standard_for_a_Mild_Maritime_Climate

For those that aren't interested in reading the paper, I'll quote part of the conclusion:

"While the Passivhaus model is a robust model for ultra-low energy building in cold continental
climates, the post-occupancy evaluation of a building in the mild southern UK climate designed to
Passivhaus standards was shown to perform more efficiently and without compromising comfort
without MVHR. This study also identified that research in field of thermal comfort supports the
premise that thermal comfort can be achieved with non-uniform temperatures of 16–21 °C in a
building as well as the uniform temperature of 20–21 °C as required by the Passivhaus model and that
indeed variable temperatures are preferred by certain individuals. Furthermore, naturally ventilated
buildings allow for user control which is also understood to enhance user comfort. Therefore, a
naturally ventilated building in a mild climate is as appropriate a building solution, if not more
appropriate, than a building solution with MVHR."

In terms of boosting ventilation during peak 'requirements' studies again show that whilst ventilation rates is not immediately boosted during periods of high moisture, there is a slight uptick in rates through passive stack due to bouyancy changes in the air. But more importantly, over time, there is little difference, which certainly doesn't allow internal moisture, for example, to build to unsatisfactory levels.

A very good book I read on this that includes a lot of salient research in the topic is Handbook of Domestic Ventilation by Rodger Edwards.

Posted By: jms452Simon D,

What requirements do you have that have led you to reject mechanical ventilation?

There are a number of things including, for example:
- capital input for purchase and installation
- ongoing running & maintenance costs
- applicability to our warmer climate
- we wanted full control of variable temperatures throughout the house - e.g warm living room, very cool bedroom etc.
- I have questions about the wider environmental costs of running MVHR beyond just the house heat recovery
- I also have questions over whether it is actually a better solution than using what's naturally available to us

Posted By: Nick ParsonsHello SimonD.

You refer to 'breathable' construction (i.e. water-vapour permeability), but is your construction designed to be air-tight? Using wood-fibre internally I would expect the parge-coat/adhesive coat/WF/plaster coats to be sufficiently air-tight. What is the a/t layer over your sheep's wool?

Posted By: Mike1For the avoidance of doubt, note that 'breathable' in the context of building fabric means 'vapour permeable', not wind-permeable - which I think is understood in this case, but is potentially misleading.

My walls, roof and floor are both vapour permeable and hygroscopic so do function differently compared to 'just' a vapour permeable system. (for those not familiar with this, the hygroscopic behaviour is the ability of the insulation fibres to absord a certain amount of moisture and later release this into the atmosphere, this can either be towards the internal or external side of the building fabric depending on the relative moisture content of the atmosphere.

It is debatable whether my walls constitute 'air-tight' as I'm using osb sheathing as the vapour barrier and not supplementing this with an air-tight membrane. OSB can be good in terms of air-tightness but it's variable. I'm not going for Passivhaus air-tightness but I am detailing the building fabric pretty meticulously but this is more to ensure a complete installation of the insulation without open space. Externally the timber building is wrapped in a breather membrane and the woodfibre ewi is coated with a lime render.

Posted By: fostertomVentilation guru Peter Rickaby has reservations about MVHR
https://www.linkedin.com/pulse/insulation-without-ventilation-monique-rosmarin-simons" rel="nofollow" >https://www.linkedin.com/pulse/insulation-without-ventilation-monique-rosmarin-simons
considering it often a counter productive step too far.

That's interesting, thanks. The above linked article discusses occupant behaviour in relating to indoor air quality and simple solutions to allowing those occupants to address this - in part involving education about ventilation.

Posted By: Mike1

Posted By: fostertomreservations about MVHR

From reports I've read on both new build and retrofit, there can be problems.

Users are probably the biggest problem - not understanding what MVHR is or how to use it / not cleaning or replacing filters / and turning it off to save money (particularly when users are short of cash on benefits).

Turning it off because of noise is another issue, which normally falls into the poor specification, design, installation & commissioning category, where problems do also occur.

But if properly installed, understood and used, very few problems.

Agreed!

Posted By: PeterStarck

Posted By: SimonDIt is insulated entirely using sawdust and it provides a lovely internal environment all year round, even when I was there a couple of years ago during the massive heatwave where temperatures exceeded 30 degrees c for much of the summer.

When we were starting on our self build journey over ten years ago we looked at Baufritz houses which used wood shavings as the insulation material and whose ethos is to build naturally. We were only put off by the price.

It's a shame that. In Europe, including Sweden it's been possible for some time to have wood fibre insulation pumped into the walls in a similar fashion to the recycled newspaper cellulose stuff over here (can't rement the company who does this in the UK?) I wonder how long it will take for this to come over to our shores as it would make a lot of sense and probably reduce costs.

There is clearly a preference for MVHR and I'm swimming against the tide here in my decision to go for natural ventilation. If I was building up in Scotland or back in Sweden, I'd most certainly be taking a different path, but I think for my location and house design, I'm still firmly in favour of the natural remedy. This discussion has really helped me to clarify this as I've revisited my plrior research and also found some new references. But at the end of the day, time will tell as I'm also kind of seeing this as a bit of an experiment with the house.

Thanks for the input so far and if anything else comes up on this topic, I'd still really like to be made aware of it

Thanks for the mention of Rodger Edwards book - I see it's available as a PDF if you search for it. I'll try to read it.

I have a question and a suggestion for you

The question is how you will arrange to have different temperatures in different rooms in a controlled way?

For myself for example in the heating season all I care about is getting a measured amount of energy into the house during the night (E7) and then the heat distributes itself around pretty evenly by itself. Rooms with windows to the south tend to be a bit warmer than the one bedroom that is on the north side of the house and only has a west-facing wndow (or really I suppose, that one bedroom tends to be a bit cooler than the other rooms is a better way to describe it.)

The suggestion is to consider what you will do if your ventilation strategy isn't completely successful. You may find it worthwhile to allocate some space for equipment that you don't fit and/or run some ducts in while you are building the house in case of future need, for example.

Posted By: SimonDOSB can be good in terms of air-tightness but it's variable

Yes, it's very brand-dependant, though even within brands there can be significant variability. Some are pretty much air-tight if joints are sealed, others very leaky. Personally I'd use it wrapped with an air-barrier membrane, unless you find a brand with a suitable BBA certificate.

Posted By: SimonDI'm also very aware of the argument that MVHR provide more control, but in real world tests, that is questionable in comparison to natural [ventilation]. Natural ventilation can be infinitely controllable, even an open passive stack as it's flow is not just controlled by it's duct diameter and the wind, but the flow of fresh air into the building - this has been shown in pretty extensive testing conducted in the UK during the 1990s I believe.

I certainly don't recall any 'extensive testing' in the UK, nor much independent scientific analysis of air quality and the like, however passive ventilation did rise in prominence in the UK in the 1990s (before fading away again) and there were a few stories & reports published. I followed the developments closely at the time, including visiting BedZED, but it was pretty niche even then.

It can be designed to provide sufficient airflow, given ducts of sufficient size, but most of the time the building will either be over-ventilated or under-ventilated - it isn't really possible to control it adequately without electro-mechanical controls, not least due to its reliance on human behaviour to make adjustments. Though I seem to recall that some used humidity sensors to add some automated control.

But with the ever-increasing emphasis on cutting energy use, throwing away heated air in winter really doesn't cut it any more, when it can be readily and efficiently recovered using MHVR. And cutting trickle vents into triple-glazed windows - even double glazed ones - is rather mad.

Posted By: SimonDResearch from Sweden is usually pretty good, but I would also be cautious about the findings' relevance to the UK due to climatic variations, depending on where in Sweden the studies were conducted.

Hmm. The UK does have plenty of experience of trickle-vents, but passive stack ventilation was mainly used in Eastern Europe and Scandanavia, before the UK's limited experiments in the 90s, so for similar reasons I'd be very cautious about your findings, particularly if they're not by independent scientists. MVHR has been far more widely used, tested and documented in the UK, even if more research would be welcome...

+1 to all WiA says. I'd add:

The ventilation rate you should use depends not only on current conditions but also on forecast conditions. E.g., if it's warmer and drier outside now than it's likely to be in the next few days it's worth “over ventilating” to get the internal RH lower than you might normally aim for with the intention of drying out any internal moisture buffering thereby reducing the amount of ventilation which will be needed in colder and damper times.

This is also a form of energy storage in that when it's colder and damper and the internal RH is allowed to rise moisture will be absorbed by the building fabric so releasing some latent heat. I don't have enough understanding or data to work out if that's actually a useful amount though previous back-of-the-envelope calculations on seasonal effects (in which DJH pointed out a fatal sign error [¹]) indicate it might not be completely insignificant on Passivehaus-scales of energy use. Maybe a big box of cat litter somewhere [²] would help.

[¹] I was assuming damper (higher-RH) conditions indoors in winter but DJH pointed out that normally the indoor RH is lower in winter.

[²] Somewhere in good contact with the indoor atmosphere but not somewhere the cat can use it.

Have you ever tried to give a cat a bath? They are hydrophobic.

To use humidity swing to buffer heat, you need to swing the humidity in the room high to condense and low to release humidity and (minus) latent heat from the fabric. Swinging the humidity low requires over-ventilation which might waste more heat than it stores?

It seems difficult to get MHRVs to condense anything like the bucketful of water we generate each day, so the latent heat of the water vapour is mostly lost. The difficulty is that the incoming air, if balanced, doesn't have heat capacity to match the latent heat capacity of the outgoing damp air.

Our old damp house is drier in winter, presumably it is drying to the inside, so we must be overventilating with cold air that has low abs humidity.

Ed mentioned ventilating on warm days, but cold crisp days seem the best for drying, if you can recover most of the heat lost.

Excuse my being wholly unconvinced by using ventilation to control dust mites... But maybe there could be an interesting system with a dust-mite-o-meter sensor in each room to adjust the airflow? Perhaps using a cat moving between rooms to buffer the dust mites.

Posted By: WillInAberdeenThe design rules seem to end up with massively over-sized systems, so people end up using only 20% of the capacity they bought.

They shouldn't be massively oversized, but the do come in a limited number of capacity bands - no doubt to keep the cost down - and need to be configured to suit the particular building. In use they should run at a fairly low fan speed in normal operation, with high fan speed only for maximum occupancy / humidity control.

The 200 m3/h unit I'm fitting next year will bet set to run at 45m³/hr when unoccupied, 54m³/hr in normal use, 120m³/hr in enhanced use (boost / 'visitor mode') and 180m³/hr on maximum ('party mode').