What's the purpose of the lower DPM? Get the BCO to explain exactly why he requires two DPMs (i.e. which part of which reg does he think requires it). What does the structural engineer who designed the system have to say about the proposed change to his design?

Posted By: TullichHis letter says "1200 gauge dpm below the insulation providing protection from the blinded hardcore layer as per the manufacturers instructions". Neither the architect or SE have drawn a DPM in this location though....

The key here is the BCOs get out clause 'as per the manufacturers instructions'. Unless you can find 'manufactures instructions' detailing both then there is no need. DPM specs that i have seen for EPS have either the DPM above OR below the EPS but not both. I could see a need if the below slab insulation was high density rockwool or alike but not with EPS.

Posted By: Tullichthe manufacturers instructions

Who is the manufacturer he is referring to?

Rereading your original post, I'm not clear how the ringbeam is insulated? Or what is the construction above it?

Posted By: Peter_in_HungaryAlmost certainly no particular manufacturer, just a get out phrase used when he is not sure what exactly to put - doing / demanding what the manufacturer specifies is almost never wrong, even if you don't know what you are talking about!

Well, it's a custom design, so there is no manufacturer AFAIK. And there's no manufacturer's details that require a DPM between a blinding layer and EPS that I'm aware of. In fact, the suggestions I've seen are exactly the opposite - to use a layer of EPS between the DPM and the blinding/hardcore to protect the DPM!

So asking the BCO to be specific seems like a plan to me, and a most curious use by him if he didn't have any manufacturer in mind.

My thoughts on this are (from the BSO's position)...

- you are creating a void below the DPM, albeit sheets of EPS put into it, but water "could" make it's way to that point, and may stick around or free drain away, we don't actually know.

- if water gets into that area of EPS, what happens to that insulation both over the short term and the longer term, structurally and insulatively?

- if you are the designer of this system, then you MUST have the answers to these questions. The BSO has every right to ask you to prove your design (that's his job).

In any case, before you can try to answer his question/concern, you need to be clear exactly what his concern is. The warrant responses don't always make it clear, referring more to the specific part of the BRegs in question. It can feel like the BSO is "having a go" at your design, but I think he has every right to make sure your design stands up.

I sometimes get P'd off at the BSO's, when they come back with these sorts of questions, but usually on reflection, I recognise I'm P'd off at myself for not providing proof of the detail's robustness or suitability. When I've done that, it goes passed no problem.

By the way, on my builds, I do put a DPM under the EPS, AND a second one to hold the water in the concrete, and not let it run into the EPS joints, especially if the ground conditions are a bit suspect. That's my call, a little bit of very cheap insurance. I've built plenty of houses this way, and I'm out on site as well as in the office designing, so I see how things can be done on site, and it's not always as the drawings might suggest, so some redundancy can be a good thing.