MortarThePoint

I might be missing something here. The gas membrane is at DPC level of the outer leaf and I am forming the DPC using the cavity tray that is on the surface of the membrane.

There is a company that makes some insulation for use with cavity trays: https://www.jackon-insulation.co.uk/products/detail/jackodur-cavity-tray-insulation/ I wouldn't be able to get these in time. I wondered about using polystyrene coving and pinning the cavity tray to the face of the inner leaf as then there would only be the curve as it makes a single transition from vertical to horizontal. Then I saw the coving is hollow so wouldn't help much. I'll have to make sure the cavity tray is installed loose enough for the blown beads to push it down onto the face of the inner leaf without applying a tension to the cavity tray and risking dislodging it. With a bit of care, it should be possible to have a radius of curvature of about 25mm or less at both the top and the bottom of the vertical portion of the cavity tray (as shown on the right side of my sketch) and so the missing amount of insulation will be small (about 10% over a height of 25mm). If the DPC material can actually be convinced to fold then it will be a lot less. 450mm is a standard width for DPC but that is a bit tight to make this out of. 102mm outer leaf, 100mm cavity, and 225mm up inner leaf leaves only 23mm stuck in to the inner leaf blockwork bed. Is that enough? If tight and crossing at an angle making maximum cold bridge, it would have about 100mm available for the inner leaf bed. I think I'll get my scissors out and trim down a load of 600mm wide DPC to 500mm.

We have a gas barrier membrane crossing the cavity and as I understand it the rule is that anything bridging the cavity has to have a cavity tray above it. It's a pain, but I think we have to have it.

Cavity trays made using DPC roll are often shown simplistically crossing the cavity at an angle from their top on the inner leaf to bottom on the outer leaf. In reality, even if you try to run the DPC material down the inner leaf before crossing the cavity, you will still create a void behind the cavity tray that will end up uninsulated: Is there a way to insulate this void or is it a matter of trying to keep it as small as possible? Preformed corners and stop ends all seem to adopt the shape on the left and so will create quite a significant cold bridge. I'm using blown bead insulation. There is no route for the beads to get between the gas membrane and cavity tray. Also I wouldn't want the pressure of the bead blowing to dislodge the cavity tray.

I found this LABC guidance: "The bottom of the tray should be taken across the external leaf and preferably just visible at the external mortar joint." That's preferably just visible, possibly not, and definitely not sticking out 10mm. This LABC guidance on the other hand is perhaps clearer: "5. Ensure the external edge of the DPC is visible and not bridged by mortar when completing pointing of the mortar joint."

Yes, but he's saying most of the mortar in the joint is above the DPC, rather than the DPC being smack in the middle.

Thankfully not, though I did consider it back at the time. BCO is fine with / expects the DPC recessed slightly

He does seem pretty young and inexperienced. He may be right 'on paper' though which makes it hard to swing him round or to get a second opinion from his firm / the warranty provider.

It's a mixture of brick and rendered block work with block work inner leaf. Gosh, they're even smaller. They won't have the "Venturi effect outlet tube that reduces water pressure, allowing for optimal drainage" if you believe that. It's quite a small opening for venting through. Do the TW1 work well?

The Invisiweep sticking 10mm proud or the standard ones. Either way, doesn't sound fantastic

Well I couldn't spot them in the photo so they clearly work 🙂

I require them all the way around the house at DPC level as I have a gas membrane crossing the cavity. Down there visibility may not be as much of a concern as pest proofing.

Welcome to the forum! Sounds like a nice project and there will be lots people can help you with here.

What do people think of different weep types? Any recommendations? I like the look of the Invisiweep vent as it makes the hole on the outside smaller than for the standard ones (shown below above the standard ones)

Agreed, it's bonkers. Into the cavity I wouldn't argue with, but sticking out like that it would catch rain and have things growing on it. I need to take a stand on this as, in my mind and pretty much everyone else's, it it is a thoroughly bad idea to have it sticking out like that. I think this is where I'd like to get to, so that the inspector can see it has been installed, but it doesn't act as a rain catcher. My brickie talks of having minimal mortar under the DPC, whereas where I have noticed DPCs they appear to be in the middle of the bed. A complication of this though is that the outer course brick would have to be laid before, or at the same time as, the inner blockwork so that it could be aligned appropriately.