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You mean the 70mm little round ones, or the long vent strips? Soffits are sheltered, so I'm not worried about those so much, these vents are for the side walls to vent the cavity behind the vertical T&G cladding. Normally that's vented behind the cladding along the top edge and bottom edge, not on the actual wall face which is pretty exposed to the prevailing wind, so just trying to think creatively here.

Thanks, I will try this. Do you think these vents are OK to place towards the top and bottom of the cladding? I'm worried they may let in too much rain. Are there more suitable vents around?

Thank you both. Yes, it should have been counter-battened, however this hasn't been done. Partly due to the fact that the wall thickness would increase even more and my 150mm sills barely cover the cladding I'm doing now, etc. So what I was proposing was creating air channels in the horizontal battens to allow air to pass through. The only problem is that it might not be enough and I'm not sure how many or wide that would need to be. I would also leave the back open (where it leans against the brick wall) to allow air to pass through. Finally a couple or louvre vents in the cladding. I'm not sure what else I can do, or that will be enough. Board on board cladding is a good call, but I've already got the T&G stuff here so I need to stick with it to manage the budget. I'm even a bit worried about installing a couple of louvre vents in the cladding too, since there's not much protection against rain coming in through them. But maybe these are better than no airflow at all.

Anyone here built one of these, or something similar. I'm halfway through a small Garden Room which is a lean-to structure against the corner of my garden. This is how it stands now and I'll be fitting vertical T&G larchwood cladding: I was starting to think about ventilation and just realised I've probably got the wrong end of the stick so need to get back to basics and re-learn what I need to do here. There are 3 main structural cavities in the building: 1 - The one between the outer timber cladding and the exterior OSB face. (I'm just using 19mm battens, not counter-battens, before fitting vertical T&G larchwood cladding) 2 - The one between the interior OSB face and the PIR insulation. About 15 - 20mm in my case 3 - The room itself The only ventilation I was considering was just the interior of the room (3) with trickle vents on the doors and one 229x154mm 'passive' hit and miss vent. I'm now wondering if the additional vent is really necessary at all? Cavity 3 (the actual room) doesn't really need additional ventilation I believe for the room - it's more for the benefit of the inhabitants rather than the building itself, so trickle vents are sufficient and in the summer the doors can just be opened up. Is that right? I mean with an internal volume of about 15.5m3 how much airflow is required? If in the summer the doors could be opened out and overnight there will be nobody sleeping in it. Is it dangerous to have no other ventilation? Cavity 2 functions in tandem with the insulation and should be air sealed, so no need to ventilate this. It's Cavity 1 that I've probably got wrong. This should have been counter battened to enable unimpeded airflow from bottom to top. I'm just fitting horizontal battens however, onto which my vertical cladding will be secured. Ergo, no airflow. To try to improve this I propose cutting out several 2 inch gaps into the horizontal battens, just less than the width of a cladding strip, and also secure stainless steel rodent mesh at the bottom batten gap and at top one too. At the rear of the structure I won't fit cladding right up the rear back wall of the lean-to. Instead I'll leave a 1 inch gaps here and block it off with another strip of mesh. I'll also install a louvre vent in the cladding in each of the two side walls to vent cavity 1. None of this is quite as effective as counter-battens, but is it likely to be effective enough? Any better tricks I can employ instead? I don't want to do counter battens at this stage because of the additional wall thickness and the door/window sills are already going to be too short. Thanks for any advice.