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.

 

 

[sgt_woulds]

Thats the point.  I'm concentrating on airtightness, then introducing a controlled volume of fresh air, based on internal moisture levels.

 

As long as internal moisture levels are low, the vents remain closed so over-ventilation shouldn't be an issue.  

 

Empirically, I have noticed CO2 levels rising pretty much in line with moisture levels and occupancy so I'm not concerned about passing out from hypercapnia using moisture as the controlling method.

 

It would be nice to have some heat recovery and filtration, but since Ventive pulled their product from the domestic market this doesn't seeem to be commercially available.

[sgt_woulds]

Interesting comments on Gypsum plaster.  

 

For our woodfibre IWI, lime plasters are always specified because it is more breathable than Gypsum in the same situation.  The sorbative properties of woodfibres (and natural fibres in general), can release moisture quicker with direct lime plasters than than gypsum.  Some testing in Germany has shown woodfibre insulation to activily transport moisture from damp structures to the surface, so breathablity is critical.  

 

We have an alternative system using battens and boards with plasterboards and flexible woodfibre behind a moisture vapour variable membrane.  (M-V VL membrane restricts moisture entering the insulation, but still allows it to breath out effectively when contditions allow. (After the Gypsum has (slowly) released its moisture)

 

This works technically, but holistically it reduces the moisture buffering and moisture exchange rates and is not the best use of natural products.  (Essentially, building moisture buffering is almost wholly down the the plasterboard and skim).  

[Nickfromwales]

23 hours ago, sgt_woulds said:

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

 

She’s a keeper. You’ll be so much happier you went with MVHR once you live with it.

 

The cost of running the fan is near to nothing btw, as you’re on trickle most of the time so are using (if thought through properly) less than 50% fan power on average. 

[Nickfromwales]

21 hours ago, Mike said:

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.

Amen!🙏 

[Nickfromwales]

28 minutes ago, sgt_woulds said:

This works technically, but holistically it reduces the moisture buffering and moisture exchange rates and is not the best use of natural products.  (Essentially, building moisture buffering is almost wholly down the the plasterboard and skim).  

If moisture was routinely buffered any further into the fabric of a domestic residence, that’s just too far and has not been addressed or controlled properly.

 

Seasonally, perhaps this is an acceptable swing, but day to day it should be the immediate room interior surfaces at most.

 

I dislike this approach, and am not afraid to say it!

 

Grab the heat and moisture, use it to better facilitate heat recovery, and get the MVHR in and running.

 

Tin hat time. 

[saveasteading]

23 hours ago, sgt_woulds said:

doesn't believe' passive stack will work... 

I've got a woodburner. I can demonstrate passive stack by cleaning out the ash. It creates dust of course and it goes up the chimney. Spill some ash at the open door and it flows swiftly in and away. 

This is accompanied by a swift flow of rhe heated room air. 

Rate of flow about 15 seconds per metre through a 100mm duct.

Stack effect is real.

 

I've an open fire too. I stuff fibreglass up the chimney when there  will be no fire (for hokey effect only, not efficient heat).

I've had my head above the chimneypot while working on the tv aerial. There is a horrific flow of hot air constantly... that's central heating warmed air.

Stack effect is very real.

[Nickfromwales]

47 minutes ago, saveasteading said:

I've got a woodburner. I can demonstrate passive stack by cleaning out the ash. It creates dust of course and it goes up the chimney. Spill some ash at the open door and it flows swiftly in and away. 

This is accompanied by a swift flow of rhe heated room air. 

Rate of flow about 15 seconds per metre through a 100mm duct.

Stack effect is real.

 

I've an open fire too. I stuff fibreglass up the chimney when there  will be no fire (for hokey effect only, not efficient heat).

I've had my head above the chimneypot while working on the tv aerial. There is a horrific flow of hot air constantly... that's central heating warmed air.

Stack effect is very real.

If the air can get in, yes. 

[JohnMo]

1 hour ago, saveasteading said:

I've got a woodburner. I can demonstrate passive stack by cleaning out the ash. It creates dust of course and it goes up the chimney. Spill some ash at the open door and it flows swiftly in and away. 

This is accompanied by a swift flow of rhe heated room air. 

Rate of flow about 15 seconds per metre through a 100mm duct.

Stack effect is real.

 

I've an open fire too. I stuff fibreglass up the chimney when there  will be no fire (for hokey effect only, not efficient heat).

I've had my head above the chimneypot while working on the tv aerial. There is a horrific flow of hot air constantly... that's central heating warmed air.

Stack effect is very real.

Does your house leak air quite badly? Old houses don't need additional ventilation if the have the fire places still there, they move air at a high rate. Gets back to the point made by @SimonD that airtight houses have better control of passive stack ventilation than a leaky house, where flows can be very high

[saveasteading]

1 hour ago, JohnMo said:

house leak air quite badly

Oh yes. Weatherboard without membrane outside.

 

But if I put a floor to ridge pipe, or ceiling to ridge,  in a passivhaus it would draw air in through every gap and in at the vents.

 

I'm not saying it is ideal or controllable but it is real And I don't want anyone thinking it isn't.

 

In the new project with some vaulted ceilings we anticipate it getting hot ul high. We expect to retrofit a natural or fan vent, but either way it will be closable.

[sgt_woulds]

12 minutes ago, saveasteading said:

OIn the new project with some vaulted ceilings we anticipate it getting hot ul high. We expect to retrofit a natural or fan vent, but either way it will be closable.

 

 

I remember working in a factory conversion many years ago.  They wanted the high vaulted ceiling look, but realised that in winter all the heated air would be above their head. 

 

They had a wierd looking single bladed fan to force warm air back down to occupent level...

 

 

[saveasteading]

9 minutes ago, sgt_woulds said:

fan to force warm air back down to occupant level...

 

fine in a warehouse. Punkah Fan is perhaps not a permissible term now. Also an option was a fan in a flexible tube down to ground level.

 

I've been up at the ridge of a warehouse in a cherry picker in mid summer and it must have been in the 40s. C