Comments/questions on details specific to CLP documentation

[Mark Siddall]

A cavity tray at dpc level would tackle the rain penetration risk.

Mark

[Daren Thomas]

In parts of the country with radon, a stepped cavity tray will have to be built in which will direct any rain on the internal face of the external skin to the outside. The only issue that this then creates is how to support the insulation clear of the concrete cavity fill.
Also, lightweight blocks below the floor slab can be 7N/mm², which hopefully overcomes any issues that the structural engineer would have.

Hope this helps.

[neill lewis]

Sorry to come into this discussion so late. 

I have never had a problem with using lightweight blocks in this sort of situation, though I have to say that I have never seen this arrangement as a ground floor, only intermediate floors.  I have to say I cant see the advantage of it.  The main argument for it seems to be avoidance of settlement: I have been specifying in-situ concrete with insulation either above or below for millenia and have never had complaints of settlement: if it occurs it must be minimal and it is unlikely that a dpm could be placed so that there it can't allow some movement.  Also if it properly lapped with the dpc and proper cavity trays installed, an airtight/radon-tight barrier should be possible. 

The suspended slab needs shuttering, as well as reinforcement, which adds to the cost and complexity. 

Reading through this thread and others on the board, it looks like we are forgetting the basic detailing we learned at college and over-complicating detailing.  A re-read of Chudley or Mitchell wouldn't come amiss. 

[Andy Simmonds]

Neill,

I am sure you are not really so old as to have been designing and specifying for that long...?

Whatever detail is used at this junction, long term protection of the membrane at this point (or other airtightness measure)  is very important when the airtightness of this (often very) long external junction is so critical to energy performance - over a long period of time (i.e. life of the building!).

Feel free to offer alternative details that simplify construction, yet can be shown to ensure long term airtightness.

The silver detail as illustrated, aims to emphasise the impact of site practice (folding, fitting membranes at wall corners etc), short and long term settlement, and general building movement on critical energy performance details.

All alternative details welcome!

[David OLIVIER]

The detail mentioned was first used by the Vales in Sheffield, Yorks. in a series of energy-efficient buildings c. 1986 onwards. It was designed to avoid

(1) air leakage and
(2) risk of partial collapse of ground floors into abandoned cellars on brownfield sites (apparently a serious local problem!)

Prof. Bob Lowe, among others, has tested a lot of buildings with a slab resting on the ground. The junction of slab and external wall usually shows considerable air leakage, even though in theory the DPM under the slab could act as an air barrier. This is due to the small gap at the base of the plaster esp. as the slab settles.

If the DPM was capable of maintaining its integrity, and acting as an air barrier, there's no reason why a slab on ground couldn't be used. In practice, though, surely the edge of such a membrane is likely to be damaged as the slab settles or damaged as the slab is poured.

David.

[neill lewis]

You're worrying me, David: how much settlement are talking about? 
I accept that the resultant gap doesn't have to be great to cause air leakage and the suggestion is that the settlement is quite small, therefore I would have thought that the membrane would be flexible enough to deal with some movement.  Otherwise some slack could be created in the installation - yes, I know, yet another quality control issue.

[Andy Simmonds]

perhaps an opportunity for a perimeter gasket product to seal dpm to walls (and at corners) performing at level suitable for longterm airtightness.

If I was only more entrepreneurial.....

[David OLIVIER]

Neill

I don't know exactly how much settlement would occur but filled ground probably settles even if it was compacted very vigorously. Suppose it drops a further 3-5 mm. That's a potentially significant air leak, unless the membrane remains intact *and* overlaps with the plaster. But it may well be invisible to building users. Do you think so?

By contrast, the membrane doesn't have to be totally intact to act as a  effective DPM. Broadly speaking, it's 99% effective even if 1% of its area is totally missing. Greater care is needed if the membrane is designed to act as the air barrier, not just as the DPM.

I don't recall from Bob Lowe's tests how much this particular leak increases the building air permeability. But if you're aiming for an air permeability of <1.5, not the UK's conventional 5-20, you can only tolerate a certain number of small to moderate air leaks.

So this air leak appears to be worth avoiding, even if it isn't a huge leak in the context of current practice. It's also quite likely to contribute disproportionately to thermal discomfort given its location - i.e., near peoples' feet.

David.