Guy Martin’s House Without Bills

[SimonD]

3 hours ago, Iceverge said:

What is your occupancy per m3 of internal volume.  If it's low enough the volume of internal air can buffer periods of over and under ventilation.

 We are 4 plus dog in a volume of above 900m3. But as I mentioned above, ventilation requirements change dramatically once a hydrophilic fabric is introduced into the equation. So, to quote from an earlier study I read when deciding on my design:

 

Quote

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. 

https://web.ornl.gov/sci/buildings/conf-archive/2004 B9 papers/002_Simonson.pdf

 

Now, it does acknowledge that consideration regarding air polutants is probably separate, but other studies using MVHR show similar reductions on ventilation requirements simply because moisture drive such a significant proportion of those ventilation requirements.

 

18 hours ago, saveasteading said:

It looks as if we have a club of three at present, and some others dithering or not committing.

 

Yes, indeed. At some point when I actually find some spare time, I might draw together my collection of research into this and building physics just so there more readily available reference.

[Iceverge]

Interesting study that. 

 

Undoubtedly there's marked humidity buffering effect with the hygroscopic materials.  

 

The test was done in the presence of controlled mechanical ventilation. Had this been turned off I suspect RH would have climbed higher than desirable in both cases. 

 

Passively/manually ventilating can work with the proviso you have one of the following. 

 

1. Heat driven stack effect.

2. Very large internal volumes. 

3. Diligent occupants. 

 

Hygroscopic materials will undoubtedly help smooth out the worst RH extremes with the latter two. 

 

However for almost all owners I would think mechanical ventilation would still provide much cheaper and more consistent control over IAQ. 

 

 

 

 

 

 

[Nickfromwales]

49 minutes ago, Iceverge said:

Diligent occupants.

...or "home slaves".

 

MVHR just ticks over and you go about life. Just why would you go for anything that needed any such diligence aka compromise on lifestyle.

[MikeGrahamT21]

9 hours ago, Nickfromwales said:

...or "home slaves".

 

MVHR just ticks over and you go about life. Just why would you go for anything that needed any such diligence aka compromise on lifestyle.

And the benefit of air filtering too

[Iceverge]

Or a cheapo dMEV fan with humidistat if MVHR isn't in budget or feasible. 

[JohnMo]

1 hour ago, Iceverge said:

cheapo dMEV fan

Not all dMEV fans are equal, some are quite noisy, I installed one in the summer house, next day removed it as it made racket and installed Greenwood CV2, totally silent.

[TheMitchells]

On 15/02/2026 at 17:18, SimonD said:

 

Yes, indeed. At some point when I actually find some spare time, I might draw together my collection of research into this and building physics just so there more readily available reference.

I'd be interested in reading that - though I would need you to keep it simple!

[sgt_woulds]

I planned my house upgrades around using the Glidevale I-PSV system, combined with natural internal insulations for humidity buffering.

 

(Windows ready for inteligent trickle vents, undercut doors, and additinal vents between rooms and a central starircase / stack to ridge ventilation).

 

Benefits - zero power use, nothing to go wrong, and zero maintenance.  Occupants are the 'smart' part of the system and we are perfectly capable of sailing the ship as required

 

But now my other half is insisting on an MVHR as she 'doesn't believe' passive stack will work... 

 

 

 

 

[torre]

10 minutes ago, sgt_woulds said:

I-PSV

 

I'd be interested in how this complies with building regs minimum extraction rates etc or if you supplement with extract fans?

 

It also looks very humidity focused, MVHR seems like it'll do a better job of reducing CO2 levels.

[JohnMo]

15 minutes ago, sgt_woulds said:

Glidevale I-PSV system

In Scotland mechanical ventilation become mandatory when airtightness gets better than 5m3/m2. So with a decent airtightness I would worry (like the other half) about piss poor ventilation.  For the sake of a small cost in running install dMEV fans or a MEV system

[sgt_woulds]

Always a  worry when Governments mandate a solution.  You'd hope they would learn from past mistakes...

 

 

Building regs should set standards and highlight agreed solutions, but not prohibit alternatives measures where effectiveness can be evidenced.

 

As to MVHR, I prefer long term ruggedness over perfection.  Currently our house is manually ventilated and we have dumb window vents.  CO2 is not an issue but moisture can be.  I'll add the new moisture sensitive vents  and ridge vents just as soon as I finish all the other building works.  Only been at it for 12 years so far...

 

@SimonD how did calculate your ventilation and sorbative insulation efectiveness?

[Mike]

53 minutes ago, sgt_woulds said:

my other half is insisting on an MVHR as she 'doesn't believe' passive stack will work

She's right. Passive stack ventilation is driven by the difference between indoor and outdoor temperatures. In winter that difference is big so it tends to over-ventilate; in theory you could devise controls to restrict that but it wouldn't be easy. On a hot summer day the indoor-outdoor difference can be small, resulting in very little stack effect, so very little ventilation. In fact the outside temperature may exceed the internal temperature, causing the stack to operate in reverse and draw in warmer air from the top down.

 

MVHR is a more reliable choice and offers multiple other benefits as discussed in other threads.

[SteamyTea]

3 hours ago, Mike said:

In fact the outside temperature may exceed the internal temperature, causing the stack to operate in reverse and draw in warmer air from the top down.

I thought it worked by buoyancy, so hot are at the top, which will be lower density, will stay at the top.

Probably find that any hot air that is drawn downwards is caused by a venturi effect caused by a through draft at low levels i.e. leaving windows and doors open.

3 hours ago, sgt_woulds said:

CO2 is not an issue but moisture can be

I think you are correct here, people get very hungry up on CO₂ numbers but I think there are more problems, both physical and emotional, caused by high RH.

There is a good correlation between RH and CO₂ in occupied buildings anyway, unoccupied is a different story.

[SimonD]

6 hours ago, sgt_woulds said:

@SimonD how did calculate your ventilation and sorbative insulation efectiveness?

 

The basis for my calculations came from the Handbook of Domestic Ventilation by Rodger Edwards. It's a book that's getting a bit long in the tooth and could really do with another edition, but the fundamentals, including various formulas are there. Its section on PSV is very comprehensive and shows that it works, and if facts works better in air-tight houses because they have more control - e.g. studies in Denmark show a mean ACH of about 0.45 with PSV with no detrimental effect on air quality.

 

In terms of understanding implications of hygroscopic materials, I did a lot of research looking at studies across Europe and North American to gain an understanding of how these materials reduce ventilation rates due to buffering moisture load - so, for example in cases of PSV, with a house that does not have moisture buffering peak RH can reach 75% for short periods (but not periods long enough to cause issues with condensation etc, in the building) and with moisture buffering, this can easily be reduced to peak loads of 60-65% during peak moisture load, like showers, baths and cooking etc. However, the area is still pretty poorly research tbh, so there is some guesswork required and a careful attention to detail. I have made sure I choose materials through the whole of the fabric, including paints - so everything is finished with clay paint. I also use gypsum plaster as opposed to lime because gypsum is both vapour permeable and hygroscopic whereas lime is only vapour permeable - which is why lime can survive damp stone properties and gypsum can't, but gypsum is excellent in a newly built of deep retrofit property (but it's also been used in Italy for a good couple of thousand years because of its moisture buffering properties). We also have clay bricks on the ground floor, which are brilliant for moisture buffering. 

 

2 hours ago, SteamyTea said:

I thought it worked by buoyancy, so hot are at the top, which will be lower density, will stay at the top.

Probably find that any hot air that is drawn downwards is caused by a venturi effect caused by a through draft at low levels i.e. leaving windows and doors open.

 

Yes, it's bouyancy and also wind - if the PSV is designed and installed properly it should benefit from a negative pressure zone above the roof. It's actually been shown that if ventilation at the bottom of the stack is increased, the flow through the PSV increases, which is one reason why PSV performs better in airtight buildings that have controllable ventilators - in leaky houses you can easily get over-ventilation with PSV.