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Posted By: Dominic CooneyMy instinct says exhaust on the right with … the constant fall

That'd be my instinct, too. The exhaust air should often have some water droplets in it which you want trickling outside rather than settling in that U of rectangular duct.

OTOH, that U could be a trap for all sorts of horrors, whether it's used for inlet or exhaust, so maybe you'd want it on the exhaust side so at least you're blowing them outside. Does it have a slope to it - hard to see from the picture but maybe it slopes down to the left - so perhaps it could usefully have a drain at the low point.

Part of my design is that it'll be all downhill from the exchanger to the external vents on both the inlet and exhaust sides.

Posted By: Dominic CooneyAs I understand it the intake and exhaust should be insulated all the way.

Yes, that's right. It was just whether the other ducts also needed insulating, but it seems not.

It slopes downwards as it goes through the wall from inside to outside, so 500mm of pipe sloping downhill to outside with an aluminium louvre/grill on the external wall (fitted with insect mesh). I can't see rain getting uphill that far?

I was just being pessimistic. It depends at least on how steep the fall is, what the design of the external cowl is like and how strong the wind is whether there will be a problem in reality. I'd be inclined to put a small drain on the bottom with a [short] length of clear tube attached before a valve, so you can see if there's any water standing there and drain it if necessary. You can insulate the clear pipe if you wished.

I expect I'm over-concerned. Expect the unexpected and all that.

Nice wall !!!

Posted By: Dominic CooneyWe are currently deliberating how much or little to point it. It’s the only elevation that wasn’t horrifically strap-pointed with cement mortar. All the others have now been redone with lime putty mortar. There are however some big gaps and we are wondering if we can get away with just a bit of sensitive pointing targeted where it needs it most. SWMBO wants to leave it as it is, I would probably veer towards doing the whole lot, because once the cracks and the big gaps are done there’s not a lot left!

If you can leave it as it is, then that's definitely easiest! There are only one or two gaps that leap out from the photo; if you try to do just those, or even the entire wall, then I'd be tempted to colour the lime a very dark brown or black so the wall doesn't change in appearance much.

Having said all that it looks like some bits up top are already pointed with a light-coloured mortar?

+1 on the nice wall comment, BTW.

Interested to know what type of insulation you're planning to add to the ducts, to take it up to something closer to the exterior envelope insulation values?

The intake and exhaust ducts carry air at the temp of external air (a degree or two warmer in the case of exhaust).

Essentially those two ducts are a continuation of your building thermal envelope. So, you should insulate them as you would your walls, roof, floor, etc.

Will the world stop spinning if you don't do that? No.

Will you get heat loss at those ducts, that also reduces the effectiveness of the MVHR? Yes.

Will you get condensation on the surface of the grey insulation? Probably not, though that depends upon the absolute humidity inside the house next to the ducts, and also whether the MVHR reduces intake volumes to prevent exchanger frosting in sub-zero external temps.

I would at least pack some wool insulation around the duct in the floor void, and anywhere else you can sensibly insulate. I assume you went to the trouble of keeping the flat duct against the wall to minimise visibility, but maybe if you box those in, use some laminate plasterboard?