Mike 1
So far as I'm aware, that has only been reported in older / lower spec timber frame buildings - for example those that rely on an external building paper wind barrier for airtightness. I don't recall reading any reports of significant variation when a fully taped dedicated vapour barrier has been correctly installed.

I’ll link to a study made in Sweden from 2015 that looks at 3 buildings, 2 of which are timber frame, one of which is concrete form. All pretty well detailed. The timber frame buildings in the study showed that airtightness does vary across the year, correlating with RH – the higher the RH the tighter the building. The concrete building just saw an increase over time.
Of course, this could have to do with poor onsite detailing, but it is also surmised in the study that movement of the frame due to RH variation across the year may lead to crimping and such of the airtight layer – thus leakage potentially caused over time.
The suggestion made in the report is that airtightness testing should be conducted several times following initial construction.
Here’s the link to the study and unless you read Swedish, you may need google translate:
https://www.chalmers.se/en/projects/Pages/LufttQthetens-variation-Qver-Qret.aspx

djh wrote:
It'd be nice if you provided links to these studies you mention so other people can refer to them.

Indeed, the particular one I mentioned I can’t find but IIRC it was either a German or Dutch paper. However the PHD thesis I’ve linked to below mentions some similar behaviour with occupiers ignoring the passivehaus manual and leaving their bedroom windows open at night in the winter.

djh
I'm not sure it would make much difference. Yes, sure, the humidity in a hygroscopic house will behave differently to the humidity in a 'plastic' house but hygroscopicity doesn't affect other pollutants.

SimonD
I understand that, my point was more to do with how human perception of and relationship with the environment will affect behaviour, consciously or unconsciously, and thus impact on overall energy consumption of the house.

djh
I'm not sure I understand that, so again a link to some explanatory material would help.

This was more a propositional statement but I’ll quote from a PHD thesis I read (link below) that deals with some of this:

“The importance of occupants’ everyday practices was highlighted by the literature on ‘practices and behaviours and domestic energy consumption’(Hargreaves et al., 2013; Firth et al., 2008; Owens & Driffill, 2008). Research exploring occupants’ practices and the possible effects on energy consumption show that there is a significant gap between the predicted and the actual energy performance of buildings, since occupants’ practices can vary to a great extent (Fabi et al., 2012; Socolow, 1978). Evidence shows that intended energy reduction outcomes in buildings may not be achieved as initially planned and in some cases any attained energy savings are short lived (Van Dam et al., 2010). This performance gap in energy consumption suggests that any possible benefit from the use of domestic technology solutions may be significantly reduced, or even invalidated as a consequence of occupants’ practices (Pilkington et al., 2011).”

Link: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.761567

My particular interest is how not necessarily how occupants’ everyday practices affect energy consumption of a building as I think given the research, that is a given, but rather how our relationship with both the building and immediate natural environment affect those practices, some of which we may not be aware of.

And for me, the approach to simply provide a technical solution is inadequate. The same PHD suggests something similar:

“.. research on MVHR has already demonstrated that these systems are failing to perform as initially intended (Balvers et al., 2012). Other studies show that the ability of these systems to provide adequate indoor air quality depends on many factors such as the existence of user friendly controls (Stevenson et al. 2013) maintenance of the system (Lowe & Johnston, 1997), performance (Kurnitski et al. 2007) and operation (Park & Kim, 2012; Balvers et al., 2008; Stevenson & Rijal, 2008). Many of these factors place the MVHR user as a key determinant of the effectiveness of the system, contradicting some assumptions that domestic technologies will produce the expected results if they are installed and functioning well (Shove & Guy, 2000).

To link this back to my original discussion about ventilation, this same PHD cites a German study of ventilation (which I don’t have access to) where it mentions:

“a study comparing the performance of ventilation systems in energy-efficient homes in 38
Germany (Maier et al., 2009) identified a strong relationship between indoor CO2 concentrations and window opening behaviours.”

SimonD
Now, I'm referring to a tendency to design & build houses in the same way regardless of the environment within which they are going to be placed and how they're likely to be used

djh
It depends what you're talking about. Obviously designers tend to use techniques and materials that builders and regulators are familiar with, and reuse parts of designs where they can to a greater extent than I and many others would prefer (as a matter of cost-reduction). But all the planning applications I've seen recently of any scale have talked about orientation with regard to solar input and street design and so forth. Some are better designed than others but that is inevitable.

Okay, let’s link that to my propositions about behaviour and relationship with our environment. If CO2 concentrations are related to window opening behaviours (it is clearly a pre or non-conscious sensory behaviour because of CO2s characteristics) , then utilising building techniques that naturally reduce indoor CO2 levels, may then have a significant effect, over the life-time of the building, on its overall energy use (and many other things).

I’ll quote from another study on this particular point:

“Simonson et al. (2004a) found that a permeable, hygroscopic building envelope primarily made of wood can affect the concentration of CO2, SF6 and water vapour. Such a building envelope can reduce significantly the CO2 concentration in low ventilation conditions, and still moderately when mechanical ventilation is provided. This building envelope reduced significantly the moisture fluctuations in a room in both well-ventilated and poorly ventilated conditions.

Simulations of hygroscopic building materials also showed that they are effective in reducing peak humidity levels and to increase the minimum humidity level in winter. [11]. This study concluded that additional research was needed to determine appropriate hygroscopic and permeable materials for different climates and buildings.”

https://core.ac.uk/reader/80323150

Sensibly this paper suggests adapting to climate and buildings not assuming all materials will work equally well in all climates, even microclimates, I suspect.

So this also suggests to me that the typical approach to MVHR installations regardless of the building fabric overlooks the nature of the building itself.

For example, this Canadian paper, mentions how hygroscopic materials have an effect on HVAC systems leading to systems being oversized. https://web.ornl.gov/sci/buildings/conf-archive/2004%20B9%20papers/002_Simonson.pdf (this paper also covers differences between geographical location and thus climate)
Overall, I’d hazard a guess that whilst builders, developer and governments like to use what they’re familiar with change of habitat is a human - social and cultural - issue, rather than a technical one.

SimonD
airtightness in a timber frame building is likely to vary across the seasons, with airtightness reducing during the winter months

djh
I don't see why that would be the case, given that the timber frame doesn't contribute to the airtightness?

If you have the time and inclination, the study I linked to above looks at this.

djh
This all depends on the building techniques that are used and again its clear there are preferences to use techniques and materials that require less effort and thus cost, because cost is important and so is reliability.

I’m not so sure I agree with the implicit assumption I read into here. I don’t think it has to require more effort and cost to adapt building techniques and materials. In many instances, it could yield a reduction – I’ve found the construction industry to be one of the most inefficient and wasteful industries I’ve come across in my life.

I don’t mean all of this to become some evangelical rant, more that when it comes to ventilation, we’ve got a lot more to understand in terms of how it sits with the whole house design and those living within it, the energy actually consumed, health and comfort.

Lufttäthetens variation över året

House 1
124 m² (built in 2004). Insulated concrete slab on the ground and light wooden stud walls with mineral wool insulation. Air and vapor barrier is a 0.2 mm polyethylene foil. Exterior walls are built as follows from the outside in: 22 mm spontaneous wood panel, 22 mm spacer, windboard, 45 mm wooden joists and mineral wool, 145 mm wooden joists and mineral wool, 0.2 mm plastic foil and 13 mm plasterboard. However, we have not been able to take see detailed drawings of how the air and vapor tightness should be performed and we have not had opportunity to investigate this on site.

Some of the major leaks we have noticed are above windows in the bay window in the kitchen, attachment of the carport roof at the stairs to the 2nd floor and attic hatch. Furthermore, several were noted at floor angles and at window sills. The house is at most 10% more leaky in winter than in summer (10% higher q50).

House 2
179 m² (built in 1993). Insulated concrete slab on the ground with underfloor heating coils and light wooden stud walls with mineral wool insulation (prefabricated). The air and vapor barrier is 0.15 mm polyethylene film. Exterior walls are built as follows from the outside in: 15 + 21 mm lid-wood panel, distance batten, 13 mm asphabo board, 240 mm wooden joists and mineral wool, 0.15 mm plastic foil and 9 mm byggboard. We have seen several drawings and construction details but not had opportunity to examine construction details on site.

The largest single leak is found at the shaft for the chimney, which turns out to be both a leak from the underside of the mezzanine floor in the boiler room on the 1st floor, as cooling of the mezzanine floor, leakage in the floor angle and in the closet in the living room on the 2nd floor, cooling of the shaft wall on the 2nd floor and leakage in the roof angle in the living room and walk-in closet on the 2nd floor. Other types of leakage are mainly leakage in window sills, at / out electrical installations, roof angles, floor angles, around ventilation devices and at attic doors. [The house is at most 8% more leaky in winter than in summer (8% higher q50)].

Apartment
79 m² (built during 2013-2014) with a concrete frame. Exterior walls is built as follows from the outside in: plaster, 120 mm cellular insulation, 150 mm reinforced concrete, 120 mm cellular insulation, steel (studs?) and inner plaster. The 150mm gap between the insulation blocks is filled with insitu reinforced concrete. The air tightness for exterior walls is in the concrete and in various connection details such as window fasteners. We have not had the opportunity to examine construction details on site.

We noted at measurement five in October was that the sealant at the inside window lining has begun to crack, which has led to local air leakage. Some of the major leaks we have noticed are the connection between the window frame and window frame, floor angle (can be the connection in the mezzanine floor in the outer wall and internal leakage within the building) and pipe shaft in the bathroom (internal leakage within the building). Furthermore, several air leaks were noted at floor angles and at windows between arches and frame. [It has] slightly deteriorated density over time. There has been a 6% decrease in concrete density(?) during the scarce year that the measurements are in progress. There is no correlation with relative moisture inside.

General
In wooden houses, there is a correlation between relative humidity inside the house and that of the building air tightness (Wahlgren, 2014). For [House 1] it is only when using instantaneous values ​​of the relative humidity indoors that the conformity becomes something worse. For [House 2] the best agreement with relative humidity indoors is obtained for averaging over two and three weeks. For both [houses] there is a poor correlation with temperatures inside and outside.

A probable reason why the air density varies with the season in [Houses 1 & 2] is that there are relatively "superficial" air leaks (near the inside) that change with prevailing relative humidity in the air indoors and outdoors. An example of movements that cause leaks can be that a wooden bar, beam or board material in the construction near the hot side swells or shrinks against the plastic foil depending on how it relative humidity indoors is. If the shrinking material has the function of squeezing the plastic foil, this can create an air leak. It can also be the case that themselves the disc is the airtight layer (sometimes together with the surface treatment), which makes it leaks are also created in this case when the disc shrinks. However, it is not possible to exclude others reasons...

To get a good and durable air tightness, the right products and solutions must be used. In a report from SP, Ylmén et al. (2012), it was found i.a. to show construction products is worse than others regarding age resistance and that some products were not very compatible with each other. An example is different dimensional stabilities of splicing tapes and foil products after accelerating aging testing. This caused air leaks through channels formed where tape and foil have contracted to varying degrees. Whether this affects the annual variation of the air tightness over the climate screen is uncertain.

If the leakage varies by one order of magnitude as in the wooden houses in question this gives a small impact on energy use in most cases. [As a result of climate change] one should make sure to have a good one
safety margin in terms of airtightness.

One factor that has not been investigated in the project is how the building's ventilation system affects the airtightnesses. Different ventilation systems provide different pressure images in the building and thus different direction and speed of air flows through the climate shell. The air flows, in turn, affect the building and can be both moisturising and dehydrating. A building with natural ventilation could have a lower part of the building being dried out and an upper part of the building where the climate shell is moistened.

An interesting RH link, though not very significant in impact (compared to studies of older homes that I've read), and not sure what 'disc' refers to, but the overall message is that airtightness is important, most leaks are at junctions, so get your details right.

Posted By: Mike1An interesting RH link, though not very significant in impact (compared to studies of older homes that I've read), and not sure what 'disc' refers to, but the overall message is that airtightness is important, most leaks are at junctions, so get your details right.

Thanks for the translation, Mike. The study seems to be of three unremarkable houses - it's a pity they don't state the actual airtightness numbers as well as the percentage change. But they don't seem to have been built with any special care and the failings all seem to be at junctions as you say. I was going to suspect that there may have been no gaskets fitted under the sole plates, or problems with their fitting, but it seems like the problems were much more basic than that. I don't think the information tells us anything new, just reinforces the importance of details as you say.

I found the abstract for one of his other papers 'Moderating Indoor Conditions with Hygroscopic Building Materials and Outdoor Ventilation' ASHRAE Transactions . 2004, Vol. 110 Issue 2, p804-819. It says: 'In general, increasing the ventilation has a stronger impact on the average indoor conditions than applying hygroscopic materials, but the impacts of ventilation and hygroscopic materials can be similar during certain operating conditions.' which seems reasonable to me. The other paper about numerical modelling that predicts the transfer of water vapor, CO2, and SF6 between the building envelope and air seems a little strange. Why would SF6 be considered?

Posted By: djhThe other paper about numerical modelling that predicts the transfer of water vapor, CO2, and SF6 between the building envelope and air seems a little strange. Why would SF6 be considered?

Good question., The only thing that I can spot in a quick Google search is a report from 2001 mentioning that that SF6 was used in double glazing in some countries - Mainly outside Europe, but Germany gets a mention (https://ec.europa.eu/clima/sites/clima/files/eccp/docs/eccp_wg_final_report_en.pdf)

Posted By: djh

Posted By: Mike1An interesting RH link, though not very significant in impact (compared to studies of older homes that I've read), and not sure what 'disc' refers to, but the overall message is that airtightness is important, most leaks are at junctions, so get your details right.

Thanks for the translation, Mike. The study seems to be of three unremarkable houses - it's a pity they don't state the actual airtightness numbers as well as the percentage change.

I found the abstract for one of his other papers 'Moderating Indoor Conditions with Hygroscopic Building Materials and Outdoor Ventilation' ASHRAE Transactions . 2004, Vol. 110 Issue 2, p804-819. It says: 'In general, increasing the ventilation has a stronger impact on the average indoor conditions than applying hygroscopic materials, but the impacts of ventilation and hygroscopic materials can be similar during certain operating conditions.' which seems reasonable to me. The other paper about numerical modelling that predicts the transfer of water vapor, CO2, and SF6 between the building envelope and air seems a little strange. Why would SF6 be considered?

The Swedish study does provide the numbers and percentage changes, they just don't seem to come across in the translation. If I get the time, I'll translate those figures but it's likely to be a few days. I'll also make an effort to modify the nonsensical parts of the google translate output

In terms of this hygroscopic/ventilation question, the conclusion from 'Potential for Hygroscopic Building Materials to Improve Indoor Comfort and Air Quality in the Canadian Climate 2004 ASHRAE' reaches a slighly different conclusion:

"At a ventilation rate of 0.5 ach, the peak humidity is typically reduced by 10% to 25% RH. The reduction is slightly greater in Toronto than in Vancouver and Saskatoon. At a ventilation rate of 1 ach, hygroscopic materials have a smaller effect on the indoor humidity, and the reduction in the peak humidity typically varies from 5% to 10% RH.
A comparison of the hygroscopic and nonhygroscopic materials using data from three-day test periods during humid weather in Toronto (June), cool weather in Vancouver (November), and cold weather in Saskatoon (January) further demonstrate the importance of hygroscopic materials. During the humid and cool weather, the hygroscopic materials improve the indoor conditions, but during the cold weather, the effect is minimal. An important result from the humid and cool test periods is that the comfort and air quality in a room with significant hygroscopic materials and a ventilation rate of 0.5 ach is nearly the same as that in a room with no hygroscopic materials and a ventilation rate of 1 ach. This indicates that buildings with a large amount of hygroscopic material may not need as much outdoor ventilation air as buildings with no hygroscopic material to provide a comparable thermal comfort and perceived air quality. However, the ventilation required to dilute pollutants is likely independent of the amount of hygroscopic material."

So for me the relation between ventilation/hygroscopicity is still up for debate

Posted By: djh

Posted By: SimonDI don’t mean all of this to become some evangelical rant, more that when it comes to ventilation, we’ve got a lot more to understand in terms of how it sits with the whole house design and those living within it, the energy actually consumed, health and comfort.

Me neither, and I suspect we agree about many things but are just looking at them from different viewpoints.

I'm not convinced about humidity and hygroscopicity though. I think we understand humidity pretty well but I'm less convinced about our knowledge of hygroscopicity and how it interacts with other factors. We know enough to know that humidity can be dangerous, or just uncomfortable, and so the tendency is to simply design out those risks. Hence the prevalance of non-organic materials, for example, since they simply don't carry the same risks. I think you may find it interesting to read some of Tim Padfield's writing.

For myself, having chosen to build one of the most hygroscopic houses I'm aware of, I didn't feel that taking additional risks with the ventilation strategy was sensible. And I'm not convinced there's that much to be gained from the possible alternatives, TBH.

Yes, I think we do agree and are coming from it from different directions.

I'm fascinated by and I'm convinced by hygroscopicity in so far as it plays not just and important, but essential role within the whole system of a building as opposed to it being a solution, but I do think that our tendency to try to design out those risks often ends up detrimental, sometimes even over the very short term.

There is an ancient fable from China about how a god tried to tame the elements, damming up the water for irrigation, only to cause some of the most catastrophic floods that destroyed the farming lands, causing famine. Obviously the moral of the story is to work with the elements as opposed to against them

I appreciate your approach and decisions re your ventilation strategy. I'm using some fairly traditional and well tested building approaches in terms of timber frame etc. that I feel the freedom to explore the natural ventilation road and take on those inevitable experimental risks.

Thanks for the heads up on Tim Padfield. I've just read his presentation on Breathability (https://conservationphysics.org/breath/breathability.html) and chuckled at part of his conclusion:

"Finally, I want to emphasise that 'breathability' used as a universally good principle is as useless a way of evading understanding of the immediate situation as is 'sustainability' or 'ecological': words that lost their meaning as soon as they were adopted as buzzwords by right thinking people. Every situation has to be evaluated from a sound knowledge of physics."

Posted By: Mike1In addition, buildings built with hygroscopic material require their inhabitants to understand the nature of the building and how to maintain it. You wouldn't want them painting the walls with plastic paints, for example. And if it's difficult for people to understand what a MVHR unit does and how to operate and maintain it, despite being able to physically see it, how well will they cope with a building that looks like any other, but isn't?

I agree and find myself frustrated by, for example, the marketing literature of wood fibre insulation. Much is touted of their benefits, but there is nothing about considering the internal finish to get the most out of the products, thus people may just cover it all up in plastic paints. Maybe these companies are just concerned with the building fabric as opposed to the whole house, who knows.

The hygroscopic debate & state of research remind me very much of the state of knowledge of how to design low energy buildings a few decades ago. Experiments with south-facing glazing and Trombe walls, solar porches, DIY solar-thermal panels, 'thermal mass', earth sheltering, passive stack ventilation and the like, particularly in the USA, Canada and Scandinavia, with interesting but often not very practical reports, sometimes lacking in scientific content.

Trying to assemble a sensible collection of such ideas and technologies into a practical, affordable house that actually worked as expected in a particular climate was pretty challenging. That pretty much remained the case until the research by Bo Adamson and Wolfgang Feist eventually lead to the launch the Passive House Planning Package in 1998, which finally gave sufficiently predictable results.

There must be scope for some researchers to do likewise and build a reliable model for hygroscopicity that can be readily and reliably applied in the real world.

Guessing - yes 65% saving but in Canada near 100% as mostly hydro electric

Cost only 35% saving on fuel varying up to zero saving depending on how well designed

Capital cost can be high

I agree

Posted By: WillInAberdeenAFAICT they assumed that if a very-leaky building was fitted with MHRV, then a corresponding number of air leaks would be fixed at the same time, to keep exactly the same airflow through the building.

Sorry, I'm being dim but where do you see them making that assumption?

I saw that they do say that the airflow through an MVHR is independent of the leakiness of a building, which is generally pretty much true for a constant flow MVHR, but that seems to be something different.

Posted By: djhApologies for adding to an old discussion, but it seems as good a place as any ...

I've just come an interesting paper (not an academic, peer-reviewed paper but it says it's a paper) that makes the case for MVHR in buildings with any airtighness and recalculates their carbon emissions:

https://www.passivhaustrust.org.uk/UserFiles/File/research%20papers/MVHR/2020.04.27-The%20Case%20for%20MVHR-v7%20new%20cover.pdf" rel="nofollow" >https://www.passivhaustrust.org.uk/UserFiles/File/research%20papers/MVHR/2020.04.27-The%20Case%20for%20MVHR-v7%20new%20cover.pdf

It agrees with a lot of what people on here have found.

Passivhaus Trust should be ashamed of themselves for the poor standard of this paper. It's spurious at best and comes across as MVHR propaganda to support lazy building design.

Just to start:

Paper says:

"Natural ventilation relies on opening windows, wind and the stack effect backed up by intermittent extract
fans in bathrooms and kitchens. Wind speeds are variable and opening windows relies on the occupants.
There is good evidence to show that this arrangement simply does not deliver appropriate levels of
ventilation2."

1st reference to support this claim is "Ventilation and Indoor Air Quality in New
Homes" (https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/835208/Research_-_ventilation_and_indoor_air_quality.pdf)

But what does the research they cite actually say?

"Failure to meet indoor air quality indicators, in all cases, corresponded with failure
to meet the ADF ventilation recommendations."

And

"It is difficult to assess whether the relatively high pollutant levels identified and relatively
low air exchange rates are due to any inadequacies in the recommended ventilation
provisions in ADF due to nearly all, homes in the study not fully meeting these minimum
provisions set out in the guidance."

Which basically tells us that builders do not even comply with current Approved Document F for the provision of ventilation. Hardly an assessment of the effeciveness of natural ventilation, more of an endictment of UK building practise. Even worse is that the Passivhaus Trust tries to use this as evidence for MVHR against natural ventilation. You'd get similarly poor reviews for MVHR if you failed to install the heat exhanger or installed an air intake 1/4 the required size.

Anyone who has cared to look into natural ventilation strategies will know and understand that it needs to be designed into the building from the start and that studies of good standard have shown that natural ventilation can and does work effectively, especially when it comes to cooling, which is why so much research is going in to using natural ventilation rather than HVAC in commercial and office buildings. There are many other areas where natural ventilation stands very well up to MVHR.

The UK climate is also very good when it comes to environmental conditions that fascillitate function of natural ventilation systems, for most of the year.

No need to go any further....

@SimonD
I guess you're aware of this (mostly) passive "termite" ventilated school in northern Sweden? - https://www.youtube.com/watch?v=M5BnTi0L82w

Re, your mum's 170 year old timber house - there won't be any sawdust left now will there? Mice, bees, hornets, weevils, etc etc; and what's left will have all sunk to the bottom of the walls?

Posted By: WillInAberdeenBecause the slope of the lines in their charts are parallel, but the intercept for the MHRV line is lower than the natural ventilation line.

(If MHRV were added without also fixing leaks, then the building's total ventilation and hence energy usage would increase, so the MHRV intercept would be greater. If the MHRV flowrate were adjusted to account for the leak rate, the lines would not be parallel.)

Agree it is not very clear how this was done.

I think you're overlooking that in a 'normal' building without MVHR the ventilation is done not by infiltration (lack of airtightness) but by deliberate holes in the walls (ventilators) and by opening windows some of the time. The air exhange that results has to be heated by the assumed gas heater and I believe that accounts for the additional emissions. The deliberate holes in the walls do not exist and the opening of windows does not occur in houses with MVHR during the heating season.

So I suppose that, yes, they have assumed that some leaks have been fixed, because that is the case by deliberate building design. No trickle vents; no extractor fans; and windows that stay closed.

Posted By: djhI think you're overlooking that in a 'normal' building without MVHR the ventilation is done not by infiltration (lack of airtightness) but by deliberate holes in the walls (ventilators) and by opening windows some of the time. The air exhange that results has to be heated by the assumed gas heater and I believe that accounts for the additional emissions. The deliberate holes in the walls do not exist and the opening of windows does not occur in houses with MVHR during the heating season.

So I suppose that, yes, they have assumed that some leaks have been fixed, because that is the case by deliberate building design. No trickle vents; no extractor fans; and windows that stay closed.

That is the previous thinking, that infiltration had to be reduced to a low enough level, before MHRV is effective, such that windows/vents would be opened in winter. Then MHRV would replace the ventilation from the slot vents. It is thought that MHRV is not effective in accidentally-ventilated (IE leaky) buildings, because MHRV would then add extra to the air ingress from leaks, rather than replace it.

This document disputes that view, and concludes that "MVHR systems
result in lower carbon emissions for *all levels* of airtightness ... including retrofit."
(my emphasis)

While I admire their enthusiasm for MHRV, if the building already has sufficient (accidental) air ingress, then bolting on a MHRV will not reduce that. First you need to fix leaks.

Even in a super-airtight building, open windows are lower-carbon than running fans, whenever heat doesn't need recovering, such as outside the artificial-heating months. It is also better to avoid constant-flow 0.5ach MHRV systems, and instead vary the ventilation according to need, ie how dry the inside and outside air are on each day. That might depend on how many people are at home and what they are doing, and on the weather. Their case for MHRV would be stronger if they modelled such a natural/mechanical hybrid, rather than a purist MHRV-is-better-than-natural approach.

Posted By: djhSimon, I'm sorry I posted the link here nowhttp:///newforum/extensions/Vanillacons/smilies/standard/cry.gif" alt="" title="" >

When I read the document (and yes, I don't know what to call it either) it didn't even occur to me that it could be seen as attacking natural ventilation. I believe it simply set out to explain a particular error in the way MVHR is accounted for by Building Regulations that has resulted in them not being installed in some circumstances where they would have been appropriate.

I don't think there's any consideration of the merits of the better natural ventilation designs versus the usual rubbish of trickle vents etc with no thought to airflow at all. I think that's a completely separate subject and outside the scope of the document.

Oh, no worries.

I do agree on what the paper was trying to say about MVHR specifically. Always sensible to review these things from time to time.