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dinosharks and roof insulation

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ian:
So, my office now has no ceiling. The office is an extension on the side of the house with a sloping roof. The roof is contiguous with the house roof (so the office roof is the house roof, if I'm making no sense). There's fibreglass insulation between the, erm, eaves or whatever they are called. Now it's pretty black in places, particularly near the apex. I'm scared of mould. Really scared of mould. More so than sharks and dinosaurs. More so than even dinosharks. Or sharkosaurs.

Since the discolouration doesn't appear to affect the wood etc. would it be safe to assume that it's thirty years of dust filtered out by the fibres (the previous ceiling was tongue and groove wood and thus probably quite porous).

Secondly, if it is dust, I assume there's been lots of airflow into the main roof cavity (which was reinsulated back in 2010, who knows what was there before). I'll ask the contractors, but I don't to be honest think this is their area of expertise, but should it be replaced before they plasterboard and seal, and does something else need to happen? We've also had some soffit vents put in because the main roof space wasn't properly vented when the roof was replaced (sometime in the 2000s). Googling talks about vapour barriers and some such. It's all foreign to me.

Canardly:
Assuming this is a traditional pitched roof, rather than be taking out insulation you will probably  need to put some more in, certainly if less than say 150mm thick. Blackness around the eaves is probably dirt and ditto around the apex..  Older types of sarking felt have a habit of becoming brittle and perishing along the eaves but mould in a roof space IME is rare and yes roof space needs to be vented. You should be able to detect air movement up there. Its a bit unclear from your description what kind of insulation you have i.e. fibreglass mat/batts between joists/rafters. Given your comment ref apex would suggest between rafters. Up to 250mm of quilt laid to ceilings is not uncommon nowadays.

ian:
Rafters, that might be the word I'm looking for. But yes, it's batts of fibreglass insulation between them, pretty much to the depth of the beams which is probably about 250 mm (ish). Thinking about it, it probably went in when the roof did (sometime in the last decade). There's no actual roof space, the previous ceiling was nailed to the rafters to seal the insulation in. I'm not really clear where the air was going before the vents went in, the roof was pretty much sealed. You may guess that I'm not especially confident about someone that put on a roof but didn't put a vents in. That said, it's quite possible the soffits went in at a later date, they're uPVC. May have been done when the windows were.

Given that there's nothing unsavoury growing on the wood or roof lining (at least not that's obvious, I can't touch it, if it is mould I will die), I'm thinking it's dirt and dust. Of course, if there are problems or it does need to be replaced, the time is now, as a new ceiling will go in next week and there won't be access without pulling it off again.

Canardly:
Ideally there should be some air space above the insulation to allow air movement. There should also be a permeable vapour barrier above the insulation which may already be built into the 'batts' if they are purpose made panels. This is to prevent moisture ingress/condensate being trapped in the structure. English heritage doc below contains some good illustrations and narrative.

https://www.english-heritage.org.uk/publications/eehb-insulating-pitched-roofs-rafter-level-warm-roofs/eehb-warm-roofs.pdf

Asterix, the former Gaul.:
Ventilation is your friend. However well insulated a space may be it is nevertheless possible to get condensation and hence mould if the air movement is inadequate and there is any moisture in the air (i.e. anywhere not like the Atacama desert)

 

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