Non-vaulted roof, condensation risk

[LinearPancakes]

Hey folks. We're nearly done with all of the air tightness detailing in our renovation. One last hurdle is the porch.

 

It's not vaulted, and has recessed spotlights. For this, we're looking at applying air tightness membrane along the underside of the rafters, to have space for two recessed spotlights and avoid them needing to penetrate the membrane.

 

The detail here will be a pain, as we'll need to ensure the gable walls on both sides are also made air tight and that this is tied back nicely to the edge of the air tight layer on the house. You can see the flapping membrane in the picture.. A pain, but do-able I think.

 

What's bothering me is that there was no insulation put above the membrane. So it's just roof, 125mm void, membrane, and then we'll be having mineral wool above the plasterboard. It seems the problems with this are:

• We shouldn't be insulating all around the spotlights, and if this is the only insulation, we'll have un-insulated areas.
• That's not going to be a lot of insulation generally. Though it's only a small area.
• Will it be like a tent? The warm moist air rising up through the gaps, past the insulation to the cold membrane surface and condensing?

 

Seems like it would be better to have say, 75mm PIR insulation between the rafters, leaving 50mm air gap, then the membrane. That would reduce the chance for any temperature differential and mean the spotlights aren't completely uninsulated.

 

Any thoughts?

 

[LinearPancakes]

OK, looking at Ubakus, as I feared, we'll get significant condensation on the membrane, if I entered it correctly:

 

Looks like something like this would be better, and I have a good amount of 70mm Kingspan PIR lying around:

[Dave Jones]

does it matter as its a cold roof ?

 

these cups are good for sealing downlighters.  https://www.thermahooddirect.com/product/thermahood-round-airtight-downlight-cover/