Insulation for suspended flooring

Fri 30 Jul 2021, 22:22

Not to above, underneath! It's OK to put a damp proof membrane on top to prevent spills soaking in.

Co-operators renovating, with ridiculously low money budgets, many old street houses, had a lot of experience in the self-build short-life housing coop movement in the 80's. Rescuing and trying to insulate old houses (often in dreadful condition, some not quite drums, but getting close to it). We were a creative bunch, and not "schooled" in the architectural conventions, came up with wild and woolly solutions to major structural problems or changes that turned to Housing Associations' architects and engineers hair gray, usually using scrap materials gathered from heavens-knows-where to prop up very strange changes in the buildings to meet our needs.

40 years later those buildings stand and prosper. The conventional renovations of the houses the local authorities and HAs undertook the "right way" aren't...

So. 

Using solid board, despite its 4:1 advantage in insulation thickness efficiency, led to Very Bad Outcomes.

Its this practical experience which leads me to avoid sealed insulation to create a still-air space, and instead ensure lots of airbricks and cross ventilation underneath, with additional airbricks or gratimg if required, 7" of mineral wool, open underneath and sealed on top , worked out best, even if that wool was so thick.

Granted that many of those buildings had dry rot infestations and had yards of joists cut off and bolted partial replacements, hyphae can reach up to 30 feet in search of water - straight through masonry as well - and spores can persist for centuries; it wasn't surprising that any, even modest, increase in water content in the old, remaining wood, or in new wood attached to it, could cause the infestation to revive. It's startlingly virulent, a revived old rot, than a new rot starting afresh.

Very Bad Outcomes.

Mayhap this caution doesn't pertain in a structure that has been maintained and kept dry enough, so my gut-reaction to panelling is over-phobic. 

What comes to mind is a vague memory that dry rot needs a minimum of 18% (??) water content to work, otherwise remains dormant.

I was an unnofficial "consultant" to a Buddhist monastry in an ornate and badly run-down and large mansion in the early 80s. They had a big problem in extensive parts of ornate rooves - lots of oddly arranged tree trusses holding things up, complex flashings, lots of gullies etc. Dry rot was winning.

By happenstance I knew someone in the maintenance department of a palace, and had heard about their conundrum with the expenses of replacing the woodwork holding up onion domes etc. The restoration budget was impossible. Allowing a listed structure to collapse was impossible.

Some bright spark pondering the palaces problem had the idea - if rot won't be active if the wood is too dry, it would be cheaper - even over the very long term - to heat and dehumidify the timbers sufficiently to drive the fungus into dormancy, rather than cut them out (provided the wood's structural strength wasn't too badly compromised, and it's surprising how much strength a dead tree trunk maintains, even though to the eye its got a lot of rot in it).

So I appraised the Abbot of this circumstance and that it might help them out of a hole. 40 years on, both the palace, and the monastery, are still there. Or at least no Buddhist abbot has pursued me with a suit in court! ;-]