My plot is in an area where there is some radon. My Segal-like construction will have a 400 mm void under the floor structure which originally was going to be completely open but more recently I've thought to put some boards round the side just to stop wind blown rubbish and moderate sized animals, etc, accumulating under the house [¹].
From a practical point of view I think the combination of this very-well-ventilated solum undercroft and the roughly Passivhaus levels of airtightness above will mean that there will be no radon issues. The BCO is not averse to this argument.
However, that doesn't match any of the drawings in the BRE document the BCO has to hand which have pictures of cavity walls and the like. So what I'm wondering is if there is any documentation or case history I can point to which says that this sort of construction will be OK. Nick? Anybody?
CAT must have come across this so I might try them later. Findhorn who have a number of similar houses isn't shown as a radon area on the maps.
[¹] Edit to add: another reason for the boards is to substantially reduce wind blown rain hitting directly on the bottom of the floor. Any residual drops which get through will dry out very quickly (particularly in the sorts of wind when this could happen).
Books 
Comments
For the cavity walls etc., presumably there's a formula (minimum ventilation area per solum area?) that's met using air bricks? , Would equivalent ventilation in the boarding satisfy the BCO, e.g. slight spacing of the boards, or mesh covered "air brick" sized gaps?
I was assuming the cavity wall & suspended floor solution was to ventilate the solum not seal it off from the ground (if I've understood correctly).
If your boarded solum is as I imagine it will be little less well ventilated than a car port! It's hard to imagine any gas build up in there, let alone ingress through your undoubtedly well detailed floor. However presumably the BCO would like to see some sort of precedent, hence the topic?
Existing buildings with radon levels up to 850 Bq/m3 that have a suspended type of ground floor
construction may have an easily remedied situation. The air space under the floor effectively disconnects
it from the ground and offers the opportunity of intercepting the rising soil gas and removing it before it
can enter the building. The under floor ventilation normally provided to remove rising ground moisture
can usually be increased to allow a strong undercurrent of outside air to constantly replace the soil gas
as it emerges from the ground. The free area in old brick vents may be increased through replacement
with modern vents or it may be necessary to increase the number of vents on the leeward side of the
building
Lots more useful information in the doc at http://www.environ.ie/en/DevelopmentHousing/BuildingStandards/PublicationsDocuments/FileDownLoad,1656,en.pdf
Hope it helps.
http://www.ukradon.org/information/reducelevels_naturalfloorvent
I'm sure your boards can manage (& probably exceed) "Fit plastic vents to all outside walls every 1.5 to 2.0m"?
The page is "Public Health England" so not Scottish, but at least "official" and there are two downloads (which I've not looked at) at the bottom of the page, one of them apparently BRE.
Any help?
Even if you close off the void completely there is a high probability of them getting in over time. From memory the top suggestion for your application was a thick layer of broken glass under the entire house.
The BIL closed off his void to birds and larger animals by making up cylinders out of chicken wire and stuffing them in around the perimeter. So the void is perfectly ventilated and they are easy to remove if anything needs to be inspected.
Glass and chicken wire might be the perfect combination to keep it well ventilated and free of animals. Although you might want to use insect mesh if you are afraid of wasps nesting or the like.
The problem is the distinction between fairly specific instructions for new build vs various mitigation options for existing houses - which is basically about trying things and then testing the resulting Bq/m³ over three months. E.g., that Irish document is mostly about fixing existing houses which are found to be a problem and references the Technical Guidance for new build (TGD-C).
TGD-C is better than the equivalent Scottish document [¹] in that it spells out the measures required rather than just referencing a BRE document: BR 376 - which I'm pretty sure is the one my BCO showed me. However, TGD-C is still a bit limited:
“High Radon Areas: measures should be taken to protect the building from Radon in the ground. For example, in the case of a non-complex building of normal design and construction, a fully sealed membrane of low
permeability over the entire footprint of the building and a potential means of extracting Radon from the substructure such as a standby Radon sump or sumps with connecting pipework or other appropriate certified systems should be provided.”
Can't guess why they think “radon” needs an initial cap but, more significantly, they say nothing about whether Segal style is considered “normal design and construction” and if not, what they'd suggest for one.
[¹] http://www.scotland.gov.uk/resource/buildingstandards/2013Domestic/chunks/ch04s03.html
#5 Mesh - 4mm Aperture - 1mm- Wire Diameter - SS304 Grade- e - Woven Wire Mesh
from
http://www.themeshcompany.com/
over our air bricks to stop mice squeezing through the holes. It worked.
Many places offer that sort of thing, but the above company offered substantially the best carriage deal for delivery to an IV postcode. They also offered pre-cut pieces, which was both convenient (A5 was just right) & meant the mesh would go in a padded bag which probably brought the delivery cost even lower.
Nick
Ed
Ed.
Would still be interesting in general to know if a near-open undercroft would be considered sufficient mitigation for radon in general.
If the neighbouring house had been considered too far away or whatever then it would be possible to get a report from the British Geological Survey. That's a little more expensive at £44 + VAT. They also do cheaper reports via their iGeology app (Android and iOS). I tried that - it's mildly interesting anyway - but couldn't figure out how to get the report through it. Maybe you need some sort of account enabled or something but I would expect options to be present but be greyed out or give you a message or something. Just for curiosity, really, anybody know about that?
If you have a reasonably airtight floor and reasonably ventilated house, I can't see the problem. All the guidelines are based on the assumption that floors are leaky as hell.
Back-of-envelope, order-of-magnitude estimate:
let rate of release of radon into undercroft = r (in Bq/hr)
let ventilation into undercroft = u (in m3/hr)
then equilibrium Rn concentration in undercroft = r/u (in Bq/m3)
Now suppose the airflow from undercroft into house is v (in m3/hr). If you have any kind of airtight envelope this could be a very small number indeed (certainly too small to throw out the previous calculation).
And suppose the rate of ventilation of the house is w (in m3/hr).
Then (leaving out all the stuff about mixing) the equilibrium Rn concentration in the house will be roughly:
v*r/u*w
One thing that jumps out from this formula is: ventilation of the house, and ventilation of the undercroft, have multiplicative effects.
Another thing: if the undercroft had absolutely no outside ventilation, the equilibrium Rn concentration in the house would be r/w. Ventilating the undercroft improves things by a factor of v/u.
So if you can arrange that a cubic metre of air flows through the undercroft for every litre of air that penetrates the floor, you will reduce the radon concentration in the house by a factor of 1000. I am sure that you could greatly exceed that factor with reasonable design and workmanship.
A third way to look at this: if you can make an airtight floor which lets through, say, 1% as much air as the 'normal' floor construction of the radon guidelines, then you could make the vents to the undercroft 100 times smaller.
By the way, the docs tend to talk about 'radon tightness' as a separate issue from airtightness, but I see no reason why radon should go through a membrane faster than nitrogen. Both are chemically pretty inert, and in fact radon would be expected to diffuse slower because the molecules are heavier and slower-moving.
Let me know if you think I have oversimplified the physics!
Radon is a noble gas and therefore (I assume) monatomic. The single atom molecules are still pretty heavy but they'll also be small, won't they and have fewer degrees of freedom (getting just beyond my knowledge here) so it's not obvious to me which will diffuse slowest.
Not sure what the insistence on 1200 gauge rather than 1000 gauge is about. The odd page from the BRE document I have to hand says:
“The minimum acceptable thickness for a moisture-proof membrane is 100 gauge polyethylene. This needs to be increased to 1200 gauge polyethylene to satisfy the requirements for a radon-proof membrane.“ [Their emphasis].
Is it better diffusion resistance or merely resistance to damage by the concrete they're bothered about, I wonder?
Perhaps it's simpler to build what you think is sensible and then get a (certified/official?) measurement of the actual radon level inside the building (before/at completion?)
"Homes with crawlspaces can often be remediated by installing crawlspace vents, and/or installing a plastic barrier between the ground and the crawl space."
http://www.stat.columbia.edu/~radon/remediate.html
Might be worth asking http://www.northampton.ac.uk/research/geography-and-environmental-studies/natural-environment-research-group-nerg/radon-and-natural-radioactivity-research-group-(rnrrg)
The norm here is to have a fan running continuously to ensure that the crawl space is at a lower pressure than the house with an airtight layer in-between. Unfortunately a 6 year old house up the road from me went on fire last week due to a fault with the fan. That of course is in cases where the crawl space is enclosed by the outer walls or cladding.
Just for interest, the BGS iGeology app says I'm on “Berriedale Sandstone Formation - Siltstone, Mudstone And Sandstone. Sedimentary Bedrock formed approximately 392 to 398 million years ago in the Devonian Period. Local environment previously dominated by lakes and lagoons”
Therefore, any further discussion here is just for future reference for anybody who does find themselves with a radon problem Yes, or just a previous case that had been approved somewhere as working. Despite the referendum result, Scotland is still a different country with different rules. In particular, before you build you need a building warrant. It's probably not a big deal if you're churning out lots of similar houses but if you're new to the game and doing something a bit odd it's a right pain as you have to research stuff while not being in a position to actually place an order which doesn't motivate suppliers in the quite the same way. The worst effect, though, is that you have to show compliance up front on paper rather than being able to point to something and say “see, it works”. Dunno. In the UK and Ireland the norm is a three month test in the house. Haven't seen any mention of testing the ground before a build and some documentation to say that it's not possible to determine ahead of time. Exactly. In those cases the rules are pretty clear. There just don't seem to be any for a house with a near-open undercroft.