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).

Trouble is, the cavity wall solutions require 1200 gauge radon membrane and a concrete solum cover. Really don't think that's necessary in my case. Also, their idea of airtightness for a timber floor is using tongue and groove.

Posted By: skyewrightHowever presumably the BCO would like to see some sort of precedent, hence the topic?

Precisely.

Posted By: Ed DaviesHowever, 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.

A search found this about "Natural under-floor ventilation" with respect to Radon.

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?

Thanks both. Already been looking at http://www.ukradon.org. That Irish document is useful background.

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

Posted By: Ed DaviesThanks for the idea of insect mesh Crispy. Putting large areas of that, if not the whole, might avoid any controversy re ventilation.

Good idea. On a much smaller scale application I used

#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.

Thanks, so I take it you've never done a Segal build where radon's been discussed at all?

Posted By: Ed DaviesFrom a practical point of view I think the combination of this very-well-ventilatedsolumundercroft 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.

I would agree with you here. The only sure answer is to carry out a post construction indoor radon test, the kits are available online. I'd suggest you do the test when you've moved in and are living normally in the property. Then you can be sure your are radon free.

Ed, yes. And yes, the BCO is reasonably happy that that's right. All he wants is some sort of documentation or precedent to support the case. My assumption that there wouldn't be a problem is why I haven't looked for such earlier in the process.

Ed.

Posted By: Ed DaviesWould still be interesting in general to know if a near-open undercroft would be considered sufficient mitigation for radon in general.

(warning: long and boring post)

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!

Posted By: Ed DaviesWould just need that in a document from BRE!

Or presumably in a statement written by somebody with some appropriate letters after his/her name who you had paid enough to be covered by their professional indemnity?

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-%28rnrrg%29

As I say, in my case the BCO is happy with the radon report from the neighbouring property (£3.90) saying not a problem and, if he wasn't, I could get a BGS report for my plot (£44 + VAT) which would much more likely than not show the same thing.This is despite being in a brown square on the radon maps so finding that the immediate locality isn't a radon problem after all was a pleasant surprise - I'm assuming that the brown comes from houses at the bottom of the hill where, I guess, the glaciers or water in general has cut through to more glow-in-the-dark rock.

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

Posted By: djhOr presumably in a statement written by somebody with some appropriate letters after his/her name who you had paid enough to be covered by their professional indemnity?

Yes, or just a previous case that had been approved somewhere as working.

Posted By: djhPerhaps 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?)

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”.

Posted By: Chris P BaconPresumably there is a test method you can use to test for radon in the ground before you build? Perhaps one of those 10 day test kits placed in a hole in the ground with a sheet of something over the top?

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.

Posted By: Chris P BaconThat of course is in cases where the crawl space is enclosed by the outer walls or cladding.

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.

According to the rules, for building warrant purposes all this is required in the risk report, as described above. For your own peace of mind an indoor radon test would either confirm the radon levels were within satisfactory limits or not. You could also test the ventilated void space at the same time so as to have real data to put into the model suggested by ramdu.