Garden Room - Structural Design Advice

Questions questions! This time regarding vapour barrier. As you might tell from the pic above. The OSB on outside of the framing has expanding foam around the edges, which I guess is semi-permiable. On the inside I'm installing foil-faced Kingspan PIR board insulation which I'll attempt to fully tape up with aluminium foil tape. Will this consitute an adequate vapour barrier, or should I also use a dedicated sheet of polythene or something before the plasterboard? Are the sealed edges between the frame and the OSB a problem or should it have been left with the gaps to expel damp air to the outside?

I was also going to install a 229x75mm vent in the wall through to the interior, but now I'm thinking maybe there should be a couple of these vents in the wall and only airing the cavity, not the actual room. Any advice on this?

Thanks!
 
I use 75mm tape to go across a 45mm edge of stud if insulation flush with stud, if set back Id go 100mm.

if fully taped you dont need VPL

if you mean the cavity behind the external cladding -that should be vented at top and bottom with insect mesh.
 
Thanks for the reply. That clears up a lot, but I've clearly got the wrong end of the stick in terms of ventilation so need to get back to basics and re-learn what I need to do here. There are 3 cavities:

1 - The one between the outer timber cladding and the exterior OSB face. (I'm just using 19mm battens, not counter-battens, and 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 this is really necessary at all.
Cavity 1 I assumed would be naturally ventilated due to all the gaps and the open edges of the cladding. I will look into fitting insect mesh where needed. Do you have a link to anything suitable? Is it stainless steel stuff or fibreglass mesh? Is it only when doing counter battens?
Cavity 2 was the one I was concerned about, but seems difficult to ventilate because it's not interconnected. This is the void in which condensation may occur if warm air meets cold, right? Obviously the one in the roof is ventilated at the soffit, but what about the walls?
 
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OK, I think I've got my head around it better now.


Cavity 3 (the room!) doesn't really need additional ventilation I believe - 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?

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 design in instead?
 
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Hi there - wasn't sure where to ask this question so will try here! I'm framing the front wall of my garden build to fit a 2.4m wide sliding door (PVC probably). How do I ensure the timber header above the door is okay to take the weight of the roof given that it spans 2.4m? Thanks!
 
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