Basements

[Mark Siddall]

A potential client is purchasing a plot on a hill. Current planning permission is for a 2.5 storey home that has an inhabited lower ground floor is partially cut into the hill i.e. half of that floor is formed by a basement. Whilst a re-design is on the cards it is unlikely that the basement can be designed out. The ambition is to achieve AECB Silver/Passivhaus.

Site conditions:
Permeability of the ground will influence matters – some rock below the site could be expected.
Hydrostatic head will influence opportunity for drainage, though topography and partial basement design could help.
Harmful gases: To be determined but radon is a possible risk (maps show 3-5% of homes are above the actionable level within the area.)
Ground water: Level not yet determined, though being on a hill I would like to think that it will not be a problem.

Types of tanking:
BS8102 has three use classes. Grade 3 is habitable spaces. Three types of waterproofing are described (barrier, structural integrity, drained cavity).
BS EN 1992 has four tightness classes. Type 3 is no leakage.

Guidance:
One option that guidance shows is the tanking on the warm side of the insulation; this helps to provide protection to the tanking membrane and allows hydrostatic pressure to be countered by the building structure.

Placing the insulation outside the tanking poses certain energy performance concerns as water could get behind the insulation and help conduct heat away from the home; learning to a risk of low surface temperatures, possible damage to finishes etc. On this basis placing the membrane on the outside of the insulation seems to be preferable, however, this then requires provision of additional membrane protection.

Moisture:
Ground temp say 10C. Are there any concerns about moisture drive and condensation in places that it is not desired?

Insulation:
Is it fair to say that the insulation will need to be XPS in order to achieve some degree of moisture resistance and avoid interstitial condensation within the construction? ….Tanking over some vapour permiable EPS would no doubt be cheaper but the vapour drive from inside to ground could be an issue?

Structure and thermal bridges:
….I find myself in a quandry: EWI/ETHICS on single leaf masonry?
Cavity wall construction is a familiar regional construction technology, also, having worked on a cavity wall Passivhaus (uncertified) it would be easy for me (and cost effective for the client) to simply extend my portfolio of details to include a cavity wall basement. On the other hand EWI seems so simple and straight forward that it could be a missed opportunity, though it would necessiate much more new detailing; thus incuring costs on the client.

Very early days but thought that it would be good to flush out some experience sooner rather than later.

Cheers,
Mark

[Tom Foster]

Wouldn't really effective french drain or even bridged-over open crawlspace around the perimeter solve many problems?
Graded bank not retaining wall.
Effective outside air not wet solid ground, outside a standard non-retaining wall construction.

That is, if you're not going for PAHS/AGS style ground-mass thermal coupling.

I always feel that conventional tanked + insulated retaining-wall basements miss both, alternative, tricks – and are ferociously expensive. Semi basements are standard practice, without fuss and agony, in Europe.

[Anonymous]

Mark

Have you considered insulated concrete forms for the basement?

[Anonymous]

Mark,

Interesting questions and I look forward to some interesting answers. One aspect of the site that you didn't mention is how it slopes, as in what is below it?

That is, it is easy to assume a French drain or somesuch but that in turn needs somewhere lower for the water to run. So if the slope were directly onto a road, for example, there might still be difficulty with drainage.

But having the external interface of part of the wall at 10 °C instead of below freezing seems like an opportunity rather than a problem. As Tom says, the continentals are more experienced than we are.

[Anonymous]

We Canadians have a bit of experience also when it comes to basements. Structural stability, low thermal bridging, air tight, dry and comfortable, uncomplicated and cost effective, icf's in a nutshell.

[Tom Foster]

Or if not ifcs, then all-timber, which is the answer for everything over there.

[Anonymous]

They could take a step backward and start using cavity construction but they like building things that will rot, burn and blow away fast so maybe they won't.

[Anonymous]

Mark,

Are you saying that there is a below ground system that is issue free? Have you used ICF's? EPS below ground is as good as or better than XPS (so waterproof) If you ever require some information, be it technical or case studies or a RIBA CPD on the system, let me know. I build all over the world with ICF's and in EVERY environment and the end product unsurpassable. But then again ICF's are an uncomplicated system and you do like complication here.

[Mark Siddall]

Jean-Marc,
I'm not suggesting that there is an issue free basement technology. Ideally I'd like to do is develop a way of assessing and comparing the risks so that the most appropriate solution my be sought. Believe me I'm not one for complication.

[Anonymous]

Mark,

Again I offer what little knowledge I have to help in your assessment.

[Mark Siddall]

Thanks to Nick Devlin for this link. (Some interesting scientific papers on basement design.)

http://www.ibp.fraunhofer.de/en/Publications/contributions_to_conferences.html#tabpanel-2

[Anonymous]

“No I hadn't got around to thinking about ICF. The energy/tanking/moisture issues still remain with that technology; though they may be reduced? I'll need to think on that one, though I must say that the ICF that I've seen uses EPS (so not waterproof.)”

We have used Nudura ICF (Jean-Marc's ) for our semi basement and house into a hillside of loose Yorkstone and some running water. As an added precautions we put a layer of bitumen on the outer face of the ICF and a 3 metre deep french drain.

I've never been sure if the bitumen was a good thing as it could trap any wet that did get in. However after 3 years I'm happy to say that the walls are still bone dry.

[Tom Foster]

Wow that's a rich vein of things to read Mark, which I wasn't aware of. Off to Dublin Thurs for a WUFI refresher, returning to catch Therm in Bristol on Fri!

[Anonymous]

I forgot to add that the next thing these scientific papers will be telling me is that a stick frame building and an icf building with the same u-values will perform the same.

[Mark Siddall]

An interesting summary of conventional basement design for the UK may be found here
http://www.basements.org.uk/
http://www.basements.org.uk/iimni/dlfiles/alan_tovey.pdf

(Perhaps not the greatest resource for low energy detailing however.)

Jean-Marc. Show me (and others) the data for IFC basements; case studies backed up with moisture and thermal measurements would be ideal. Thanks.

[Anonymous]

Here are a few.The pdf is a UK publication written by Alan Tovey of the basement centre.

http://archive.nrc-cnrc.gc.ca/obj/irc/doc/ctu-n36_eng.pdf

http://www.plastifab.com/TechnicalLibrary/docs/pibs/PIB%20268%20-%20EPS%20Insulation%20R-value%20Retention%20After%2015-Year%20Below-Grade%20Service.pdf

http://www.hpbmagazine.org/case-studies/office-institutional/cmta-office-building-louisville-ky-

http://www.hpbmagazine.org/case-studies/educational/richardsville-elementary-school-richardsville-ky-

ICF Design UK.pdf

[Anonymous]

Mark

I read with interest the news about Dartington Primary School in Devon today. It states that it is a Zero Carbon school but try as I might, I cannot find any data that backs this up. Have you or can you direct me to any data for this school or any others of it's type?

[Mark Siddall]

Passivhaus Basements for German speakers: http://passiv.de/en/05_service/03_literature/030307_basements.htm

Jean-Marc,
No I don't know about the school. Sorry.
(In my view zero carbon buildings is a fools game. Once energy efficiency has been maximised focus upon the efficiency of other valuable things instead – like transport and food. This is likely to be much more cost effective and have greater success at reducing environmental impact.)

[Anonymous]

There's an awful lot of games and an awful lot of fools out there. I am more of a practical and seeing is believing sort of fella and my personal conclusion is if I am to build new, no matter where and what climate, icf's will always be the choice. I have yet to see anything better and if I do, I will use it. For me the only piece's of paper that I want to see concerning any type of build are the fossil fuel and electric bills, they are the true indicators of a successful build. In God We Trust, all others bring DATA!

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