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Posted By: jwdWhen insulating a suspended timber joisted floor (with vented space below)if I just put insulation btween the joists will there not be significant cold bridging issues? Surely the timber wont insulate as well as extruded polystyrene.
Posted By: tonyIf it was mine I would do 150 between ans 50mm batts under the floor joists and nothing above the joists but use a thin sheet of poly there as a combined vapour/air sealing barrier.
Posted By: evanWhen you say 50mm batts under the joists (I assume cross-ways) what type of product do you mean exactly? I was thinking of something fairly rigid, polystyrene board of some sort, which can be nailed up to support the material beneath the boards. Is that right?
Posted By: tonyIt is nice to mitigate the thermal bridging through the joists by fitting insulation under them#I prefer not to mix insulation and although sheet insulation is tempting I would use a rigid or semi rigid form of the same insulation you have between the joists.
Posted By: mjansenI had been thinking about the same and though Knauf Space Board might do for underneaththe joists, perhaps fixed with insulation retaining washers/fixings into bottom of the joists.
Posted By: davidfreeboroughPolystyrene is OK on top of or between the joists, but I wouldn't use it under the joists because it's not vapour open
Posted By: jwdA breathable alternative might be something like pavatherm or Steico wood fibre board insulation under the joists. It insulates well - lamda values around 0.04 depending on the exact product. I guess the real question is what is the least amount of insulation required to stop the cold bridging but i dont have the knowledge to work that out.
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