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Just to clarify further: Condensation management as part of insulation is half the battle, but frequently overlooked.
The problem comes where cold surfaces meet warm air, and what happens to the moisture in the air at that point.
The spray foam seals the timber in a way that it cannot be accessed from the inside, but generally a membrane in not installed on top of the wood. So warm air can still get through.
If my room is full of lovely 21 degree air, and the outside is zero, then if that air is able to get to a nice cold roof truss, it will be dropping a lot of evaporated water on the truss.
And if that wood can't get sufficient airflow to dry out, it'll get damp. And eventually rot.
Meanwhile, you can't even get to the truss to look at it, because it's covered in foam.
So the mortgaging companies are (very understandably) staying away from that potential hot potato.
You could have a house that looks absolutely fine, until the trusses start collapsing.
The ways we work around it are either ventilation (having the roof itself still vented to the outside), vapour sealing (stopping warm air from getting to the insulation), or using ventilation that breathes (water/vapour can move through it, allowing it to dry out naturally).