Are these joists too shallow for suspended floor insulation?

[Auric]

Hi all

I live in an 1880s victorian house with a suspended timber floor. The distance from the ground to the bottom of the joist is 20cm, the joists are 9cm deep, and the gap between joists is 32cm.

The plan was to put down a permeable membrane and suspend Rockwool insulation between the joists. We would then lay underfloor heating on aluminum spreader plates, and then finally lay the original floorboards back on top. However, looking at the regulations the joists need to be at least 15cm deep to avoid acting as a cold bridge. So now I'm not sure what to do.

Does anyone have any suggestions?

Many thanks!
Auric

Edit: Attaching the relevant guidelines:
Generally, the following provides an indication of the floor configurations that may be more at risk from cold bridging:
• 8-inch joist (200mm) the U-value will not exceed 0.7 and is therefore highly unlikely to be an issue.
• 6-inch joist (150mm) is also unlikely to create an issue (other than in extreme situations where original hardwood joists and floorboards remain).
• 4-inch joist the U-value is likely to exceed 0.7W/m2k, and therefore Building Regulations will not be satisfied.

 

[Bouy]

If its something you are doing off your own back i.e. not linked to any planning/building submissions there isn't a need to comply with any regs

 

[Auric]

If its something you are doing off your own back i.e. not linked to any planning/building submissions there isn't a need to comply with any regs

Thanks Bouy, I'm more concerned about condensation forming and potentially rotting the joists. If the joists will act as cold bridge then this could happen?

 

[cdbe]

You need to fix strips of celotex under the joists - if you make them a bit wider they'll help to carry the rockwool.