Squash insulation between floors or leave gap for aeration?

SwordofLight

HI All,

Any thoughts on this one?

I want to improve the sound proofing and heat retention in a bedroom in my old house. Lime rubble exterior walls, internals are studwork with rockwool I think. Floors have some fibreglass insulation down.

First issue is whether to squish more earthwool or similar on top of the existing fibreglass, which wouldn't leave much of a gap for airflow. For more sound reduction, I was going to put a layer of 9.5mm plasterboard under the floor boards, then underlay and carpet. Probably on the crazier side of ideas but thinking about mass.

Second issue is the airflow! I took some plasterboard off at the window which was very musty smelling, and found a load of smelly rubble behind. I cleared it out and put on the foil backed insulation (in the picture). But then I came back to it later to adjust a board, and noticed it was really warm in the space behind it (between the wall and insulation), and also feeling moist, which can't be good. I was thinking to drill holes at a downward angle out through the brickwork to allow airflow into that space, which would also serve the space under the floor between the joists.

Any ideas?

Lumen

That seems like a bit of a mess tbh.

Suggest you do some reading on recommended approaches to insulating old walls, e.g.

https://fmgarchitects.ie/conservation/research/how-to-insulate-old-walls/

and linked from that...

https://www.heritagecouncil.ie/content/files/Deep_Energy_Renovation_of_Traditional_Buildings.pdf

Mellor

The scenario is a bit chaotic tbh. Rockwool, earthwool, fibreglass, foil backed. A lot of different types there, doing different things at times.

First issue is whether to squish more earthwool or similar on top of the existing fibreglass, which wouldn't leave much of a gap for airflow. For more sound reduction, I was going to put a layer of 9.5mm plasterboard under the floor boards, then underlay and carpet. Probably on the crazier side of ideas but thinking about mass.

First of all, squashing in more insulation will increase density, which will improve sound insulation, but reduce thermal insulation. Which do you need in this situation? It's a floor, so unlikely to be both.

Plasterboard under the floorboards will be crushed and crumble. Don't do that. Is this a first floor?

Second issue is the airflow! I took some plasterboard off at the window which was very musty smelling, and found a load of smelly rubble behind. I cleared it out and put on the foil backed insulation (in the picture). But then I came back to it later to adjust a board, and noticed it was really warm in the space behind it (between the wall and insulation), and also feeling moist, which can't be good. I was thinking to drill holes at a downward angle out through the brickwork to allow airflow into that space, which would also serve the space under the floor between the joists.

Foil back insulation is not breathable. It will contribute to musty/sweaty issues in the cavity. A rock/glasswool product as per the existing walls would be better.

Realistically, it's an old house and probably a bit under insulated. The best thing to do is to strip it all and re-insulate. Might seem daunting, but anything else is just plugging gaps badly.