Below DPC level detail advice needed for new build.

[ruggers]

Detached house build - I'm wanting to check, how wind driven rain will leave the cavity once it comes in through the facing bricks if outside finished ground level is 300mm higher than the bottom of the cavity. If the cavity fills by 75mm, it will breach the DPC under the beams. Ground is all clay soil and poor drainage until land drains are completed. Render doesn't let water in but bricks can be quite a lot.

 

I have my beam and block floor in and next stage is to complete the partial fill cavity walls up to DPC level. It's ready for outer leaf block & 3 course of Staffordshire blues, then I can get the outside to finished ground level, some drains in, stoned up clean ready for builder and scaffold. 

 

My plans were very generic for the DPC area, a few versions had been sent and none were suitable for B&B. I've attached a sketch of the buildup I decided to go with. 

[ruggers]

A stepped DPC in the cavity directing any water back out through weep vents won't work because it would divide the PIR insulation.

I'm aware that angled cavity trays need positioned over the telescopic vents. Would it be advisable to fit Type E horizontal cavity trays at outside DPC level right around except under window openings, some like the image? I calculate 40 required at 900mm, £280. Might seem like over thinking but where else can the water go. It doesn't move on site summer or winter.

 

[Brickie]

A cavity tray isn’t really installed as per the drawing-it would come out across the top of a PIR sheet & then down to the outer leaf. Some people chamfer that sheet below to ensure a fall away from the inner leaf,which isn’t great thermally but probably preferable to any moisture finding itself trapped on a minute rise in the DPC tray. 
All the more reason why I don’t believe PIR is a suitable insulation material for masonry builds. 

[ruggers]

When you say come out across the top of a PIR sheet, do you mean a DPC roll as a Z shape so it's bedded into the inner and outer cavity walls and spans as far as you roll it out?

I agree, trying to split PIR at obscure angles would be terrible. I'm also using T&G PIR and taping joints so thought the plastic Type E or type G might be a way to divert any rain water back out if they have a profile thats the width of a facing brick then the back lip can angle up and rest on the outer face of the PIR.

 

I wasn't sure if trays are ever used at DPC level but doesn't seem as if they are, but how else can water escape out below FGL?

Only other thing I can think of if the trays are a bad idea, is to lay a land drain up against the outside wall for the full house perimeter, set to the same height as the lean infill and direct water away to garden or drains. Maybe drill some holes between the bottom of the perps level with the lean infill too.

[Canski]

36 minutes ago, Brickie said:

All the more reason why I don’t believe PIR is a suitable insulation material for masonry builds. 

I agree even more so if the PIR is tongue and grooved. It would work fine in walls without lintels, cavity trays or openings. 

[ruggers]

I wouldn't like to fit it as a builder daily. On this occasion I'm following on from the brickie to install this so they don't have to worry about the hassle and I can be fussy with cuts, corners and chamfers over lintels.

[Brickie]

Ignore the lintol shown-typical installation is more like this. 
A cavity tray is usually installed around the perimeter at around 150mm above ground level,this is a pretty standard detail now.