Posted By: tonyIt has nothing to with filling floor voids with insulation.

Ive also seen dry rot in the timber joists of floor voids where the owner of the house had blocked the ventilation pathways on one half of the house.

Posted By: gyrogearmaybe the solution is a good covering of lime on the floor of the void...

gg

I've been giving this some thought, but not sure if that would help or hinder. I suppose it depends on whether most of the vapour is coming up from the ground or down from the house.

Posted By: tonyA very dangerous and silly thing to do but very different from adding insulation

I agree with you that this is not the same, but is it completely different? If I block the air vents then I have a large unventilated void. If I then fill that void with EPS I have lots of tiny unventilated voids.

Obviously this is the same in EPS filled cavity walls, except that surface area of the ground is a higher ratio to the space and there is no wood in there to rot and feed the mould.

I keep relating this to my risky decision to fully fill my flat roof void, but my flat roof is south facing and at some times of the year the sun will be driving vapour in the other direction, while the uf void is cold all of the time.

I note the theory that the areas where the joists are is warmer, but the data from the research appears to show high RH in the upper levels of EPS.

What do you make of this Pelsmaker's research results Tony?

Tom,
From what I can gather, the RH sensors were mostly placed near the external wall, they say due to access restrictions, so it doesn't give a full picture of conditions throughout the void.

The sensor with the consistent high RH, was near top of joists and in beads. Another sensor is labelled as at top of joists, not in beads, which I would take to mean above the former. The latter had a consistently lower RH. I would take it from this that the moisture levels are not coming from the room above.

It does say somewhere something about further research required to better understand the impact of moisture sources.

What I would be interested to see is them taking one of the uninsulated floors that show a consistently low RH, and doing a full fill of EPS and monitoring the change in RH.

Posted By: fostertomAny RH gradient, presumably to 'very high' at subsoil, must begin in the room above.

Isn't possible that the high RH here is coming up from the subsoil?

On a couple of occasions I have inspected floor voids there they had water, from a high water table above the level of the oversite, yet the joist were in perfect condition and dry. The void was ventilated and the wall plates were on sleeper walls with EPC’s


I did an experiment where I filled a box with eps beads with four Temperature/humidity sensors dead centre, 1/4, half, 3/4 up and on top. Sat it in an ice bath about 25mm deep and fed ice in constantly

Humidity and temperature allowed me to calculate partial vapour temperatures - highest at the top where it was warm and lowest at the bottom. Moisture movement down into the ice

Repeated with cold water instead of ice, same result, lower gradient.

Posted By: tonyOn a couple of occasions I have inspected floor voids there they had water, from a high water table above the level of the oversite

According to previous statements, the floor needs to be 150 mm above the oversite, which must be above existing ground level. So how can that occur?

PS I'm glad those regs apparently didn't apply to our passive slab, which is level with the outside at one point in accordance with Part M. :) It's 200 mm or so above at the other side of the building, to be fair.

Posted By: fostertomActually RH 'going' anywhere makes no sense

It makes sense, but a very complicated sort of sense since it's a combination of many factors. Changing the temperature changes the RH without anything moving. Changing the absolute humidity changes the RH and does involve movement of water often as vapour by diffusion but it can also be as liquid by capillary or straightforward gravity flow followed by evaporation, which itself can also affect the temperature. And of course the wind or other air pressure difference can move both vapour and liquid.

Posted By: Kenny_MI meant, the water/vapour coming up from the subsoil to cause the high RH.

Yes, but you have to understand the physics first.

The current RH underneath my floor is 85%. The currently fairly warm air at a relatively low RH will be coming in the vents to a very cool area and the RH will rise, so there is every chance that the under floor RH will be above 85% over the whole of the summer, and so would be in the red according to Pelsmakers paper.

There is no such thing as "the RH under the floor". The RH depends on the temperature, so will be varying everywhere. You only care about the RH wherever you have mould-sensitive construction materials.

One thing to consider maybe, is to create a conditioned void, if you have got access: entails blowing (drier) house air through the crawlspace, and ejecting it to exterior.

One recommendation is to install continuously operated mechanical exhaust ventilation at a rate equal to 1 cfm for each 50 square feet of crawl space floor area. (I do somewhat more...).

Entails first insulating the crawl periphery walls, and treating the floor (either encapsulation or limecrete slab etc.)

gg

Posted By: gyrogear: “…entails blowing (drier) house air…”

Typically house air is damper than outdoor air in that it'll have more water vapour per kg or m³, though it'll usually have a lower RH because it's warmer. The extra water comes from people breathing and things drying out.

E.g., in my study now it's 20°C with an RH of 65%. Looking at this psychrometric chart: https://en.wikipedia.org/wiki/Psychrometrics#/media/File:PsychrometricChart.SeaLevel.SI.svg that shows an absolute humidity of about 9g of water vapour per kg of dry air. Run up green 20°C line to the point half way between the red 60% and 70% then parallel to the blue lines (horizontal) to the scale on the right-hand axis.

50 minutes ago Wick Airport, about 20km ago, was reporting 100% humidity and temperature 10°C which, using the chart in a similar way corresponds to about 8g/kg humidity ratio.

100% RH is a bit extreme, so often the outdoor air is drier still, but overall those are quite typical conditions for the UK.

Posted By: djh

Posted By: Kenny_MHow can there be "no such thing as RH under the floor"?

I didn't say that. You omitted the vital word 'the'. RH is a quantity that varies everywhere in space. There is no single value that you can call "the RH under your floor" since that volume has air currents and varying temperatures as well as varying moisture sources, so it doesn't make sense to talk about a single number for the whole space. Just as it doesn't make sense to talk about the RH being the same everywhere in a room. It will be higer in corners and behind furniture etc. It does generally make sense to talk about the RH in a room, since air currents are small, the temperature is fairly uniform, and we know to restrict the meaning to the central space. But you can't make the same assumptions underfloor.

I don't really see what difference "the" makes. Of course measurements of anything, whether it be RH, temp, light, air quality, they all vary at different points in space, but its a pretty normal generalisation to give a value for a space. My initial statement was "The current RH underneath my floor is 85%", in the context that I was talking about the space under a single room. I don't know if that is where the confusion has arisen. Of course I can't say that the RH under the floor over the the whole of the house is x, because this will be made up of different spaces with walls in between, but its not unreasonable to summarise the RH for an unrestricted air space of about 5m2.

I feel that I already have a sufficient understanding of absolute and relative humidity, and how the latter varies with temperature and pressure. Its this variation that I am pointing to as the source of the complexity. In a single unrestricted air space, the measured RH is not going to change significantly as the air is constantly circulating. When you fill that space with EPS the air flow is deliberately being restricted, as is the point of that type of insulation, so it must follow that the RH could be more significantly different at different points in the same space.

I went through a lot of this before when discussing my flat roof space and I previously posted research that indicated that its often better to fully fill (with insulation) and seal poorly ventilated roof spaces, especially if they are south facing.

A lot of the same must apply to ventilated floor spaces and insulation, except that the south facing aspect will probably have less of an impact. In the climate I live in, the RH of the air outside is usually quite high for most of the year. In the winter, the fact that the temp in the house is higher, means that ventilation tends to lower the RH in the house. In the subfloor void, in the winter the temp is not as significantly different, so the RH might only be slightly lower under the floor than the outside ear (not withstanding any other factors). In the summer, the temp in the void will most often be lower, so the RH is probably slightly higher than the outside air.

It might be that fully filling the void with EPS would reduce the RH, and reduce the seasonal variations, especially with a vapour barrier to the interior. In Pelsmakers paper the RH is high at a number of measurement points in the EPS installation, and that could be a problem, but what we don't know is what the RH measurements were for the year before the EPS was installed. Maybe its better, maybe it is worse. That to me is the main flaw with that piece of research. In some ways I find it a bit disappointing that publicly funded research is allowed to be designed in this way, with no requirement to follow up on the outstanding questions.

In my specific situation, I know that although the RH is high under there right now, its probably lower when the outside air is colder. I presume that because there is no sign of mould or deterioration in the timber that I believe has been there for at least 30 years. In any case, there are currently no mould or timber decay problems and adding EPS may or may not change that. Its a gamble, if I was to look at this as a business decision the relatively small saving wouldn't justify the risk, although if I was only in this to save money I wouldn't be doing any of these things, with the exception of maybe loft insulation and draught proofing.

Strange to have dpm on the cold side of insulation, it is acting as a vapour barrier and could in some circumstances ‘sweat’

This thread is incredible - I am seriously impressed by the level of analysis that's gone on.
The URL for the Pelsmakers study seems to have changed but available (open access) https://doi.org/10.1016/j.enbuild.2017.07.085

I'm interested to know what you did at the end, Kenny? Did you chuck in the EPS beads? And has your house fallen down yet? Love the idea of putting in nails to test conductance...

Am seriously considering the same thing - as it is we've got not enough cross ventilation in any case. I'd just be chucking them on subsoil above a 300mm-ish gap so very interested to hear how you got on.

Direction of travel also relevant as we have MVHR retrofit so presumably can be a little less worried about internal moisture...

I suppose one variable is how tricky are they to remove once they're down... I mean, it's a LOT of beads!

I presented 4 Passivhaus renovation projects at the first UK Passive House Conference, think it was 2010, 2 houses had 400mm pumped EPS under the floor, Sofie came to chat afterwards, said she's like to do her PhD on it.
My own 1880's house is done 15 years now without issues. I've a thermocouple and 2 steel rods for ground humidity testing beneath the insulation, the temperature's constantly 14 degrees, warmer than before so less likely for condensation to occur and also drier which proves that. We modeled it in Therm and added some drylining, because heatloss was focused to the joist ends in the wall, they were warmer than before the floor was pumped, so less likely to decay. I also did my neighbours 1880's house and removed some 5 years later to fit a wine store, the earth beneath was drier than before, the previous rising damp issues were gone, so insulating one side of a rising wall heats it and drives out damp, a 2 degree difference is sufficient to reverse capillary pull according to the Scandinavians. I also checked the electric cables and saw no sign of brittleness.
I purchased a foam concrete machine recently and am quite impressed with it, I thought it could also be used for insulating floor voids, satisfy the regulations easier and remove the need to modify cables.

Posted By: djhtony said: "Strange to have dpm on the cold side of insulation, it is acting as a vapour barrier and could in some circumstances ‘sweat’"
It's normal to have the DPM underneath most of the insulation in a passive slab, for example. Ours has one layer (of 100 mm EPS) underneath and three on top.

A Passive Slab'S DPM's usually on the warm side of the Dew Point, 100mm EPS above the DPM and 200mm EPS below.

for any of you that are mad enough you could shred your own polystyrene https://www.youtube.com/watch?v=TdpaMzGhSp4&t=87s