Thermal bridging at gable coping upstand?

[SuperPav]

How much am I overthinking this?

 

Cavity wall, 100mm aerated block inner, 125mm stone outer, 125mm average cavity, full fill with icynene.

 

Roof is a room-in-roof with vaulted ceiling, 200mm rafters hung off glulam ridge beam.

 

At the gable ends, there will be stone coping (450mm wide). This needs to rest on both leaves, thus bridging the cavity, but also the upstand where the leaf flashing goes on the inner leaf is exposed to cold air anyway, so either way, the top of the inner leaf gets cold. This obviously just transfers down along the wall.

 

1) How much of a problem is this in reality (the house is far from passive house standards, this is a second storey add-on onto an existing old bungalow)?

2) Do I need to run a single pitched or stepped course of Marmox thermoblock to somehow decouple the upstand from the inner leaf below?

 

I'm not as fussed about the actual amount of heat loss as much as I am about potential interstital condensation on the inner blockwork at the top of the gables alongside the pitched rafters which are insulated.... Same with the kneeler blocks at the bottom of the gable, which would normally bridge the cavity, something I'm trying to avoid.

 

I'm ready to pay £500 in thermoblocks for the 3 gables if it would help and I can figure out how to actually get it laid, but don't want to do it if it's marginal as the budget is quite tight anyway....

 

 

[SuperPav]

So this is really bothering me now and I can't seem to find any decent details/sections of how housebuilders do coping on cavity wall construction without resulting in a cold bridge via the inner leaf!

 

Instead of building all the way up to the coping in blockwork, would something like this work? i.e. block only up to the sheathing, then put a thermal break PIR strip sandwiched by 2 or 3 bits of 4x2 to get the required upstand, and then just sit the copings onto the stone and timber? Could even screw retainer brackets into the 4x2s...

 

(just dumping brain ideas/thoughts onto here for my own sanity...)

Gable wall detail.pdf

[Radian]

This looks like a 'cold roof' How are the rafters ventilated?

Of more immediate concern to me is your cavity closer. Your copings will have mortar joints so the underlying surface will see a fair bit of water. You have 75mm flashing over the inner leaf only. You need a waterproof layer capping off the entire span.

[tonyshouse]

On my house I stopped the inner skin to the gable wall at ceiling height? no thermal bridge then!

[SuperPav]

Thanks both, sorry for not clarifying.

 

It's a room in roof, build up as follows> Plasterboard, 25mm PIR under rafters, 200mm wool insulation between rafters, OSB TG4 over rafers, counter battens, battens, reconstituted stone slate tiles. 

 

The cavity closer can go across the whole thing, can be anything really I suppose? cement board/6mm marmox, as long as the edge of it can be hidden by a 10-15mm mortar join on the stone wall.

 

I'm just really struggling to find a detail for this, as the only ones I can come across are solid stone walls, where it seems these things just get plonked on top and nobody cares about any water infiltration through the mortar joints, and certainly nobody cares about any thermal bridging...

 

 

[Radian]

We built our house in 1997 and the Architect left the details of setting the copings to the masons. The very first snag we ran in to was water appearing around window reveals in the gable wall. This was because the coping stones were simply set to span across the cavity on slates with no overlap. Evidently they thought water wouldn't make it to the joints. Huh. The whole lot was stripped off and re-laid on a continuous roll of lead. This has to be turned down over the edges I'm afraid (no problem on the roof side) but water will seep into your external stonework if you hide it back in. Up to you. I don't know how it could safely be made much prettier.

 

  

 

As for thermal bridging, we have a cold loft space so there's no issue.

[Russell griffiths]

Marmox block built into the inner leaf instead of that pir?????

just thinking out loud. 

[SuperPav]

That was my first thought, the problem is Marmox is 65mm thick, which itself isn't enough for the upstand. Sticking a diagonal course of masonry on top of the marmox on a 42-43 deg slip plane doesn't seem right, so would I just keep the marmox block down with timber and just screw all the way through it to the masonry below. In which case I might as well use the PIR as much cheaper than a marmox board,  given there is hardly any loading on this.

 

I'm also now interested where the hell originally the idea of coping gables came from historically? Surely running the roof tiles to the end would have been more robust, cheaper and easier for all involved. It's not like this saves on any materials (e.g. lead etc.) that would've been hard to come by in the past. Was it purely a design thing?? 

[Radian]

4 minutes ago, SuperPav said:

I'm also now interested where the hell originally the idea of coping gables came from historically? Surely running the roof tiles to the end would have been more robust, cheaper and easier for all involved. It's not like this saves on any materials (e.g. lead etc.) that would've been hard to come by in the past. Was it purely a design thing?? 

There are numerous reasons. I think Parapet roofs were mainly used as a way of preventing the spread of fire between buildings but they can also reduce wind loads on the roof. Ours was the whim of an Architect and we liked the look of it. Too bad the trades that put it up were out of their depth.