Here's a conundrum..

[Tim Lamb]

Insulation between joists below under floor heating.

Having promised no wall hung radiators I am now stuck with getting the job done! Ground floor no real problems: grooved insulated sheet to take the pipes covered with standard chipboard or screed for the new bit.

Upstairs life gets expensive and difficult. All the suppliers call for insulation below and in contact with the heated floor. In a chalet bungalow, there is no such thing as a standard floor joist spacing. The dormer cheeks are double or triple timbered and the intervening space filled with ladder work on joist hangers. Elsewhere walls have double supporting timbers.

I have been considering cross battens but this adds a huge cost and the suggested sizes (30mm x 70mm) are not found off the shelf and particularly not untreated.

Pre-grooved chipboard is an alternative and cheaper solution. Both systems call for insulation between the joists and in contact with the underside of the floor. PIR foam is not cheap, needs careful cutting/fitting together with supporting battens. Rockwool is cheaper and easy to cut and fit but... how do you stop it ending up resting on the ceiling plaster board, light fittings and electrical wiring?

Chicken wire netting? Some sort of cross batten or?? My floor joists are

9" deep and Rockwool 6". Perhaps lengths of plain galvanised fencing wire stapled between the joists? Any impact on wi-fi transmission?

[Andrew Mawson]

When we did up the house and holiday cottages, I bought rolls of garden type green nylon netting off ebay, and stapled it to the joists, laying insulation batts suspended by it.

Andrew

[newshound]

I was going to suggest string rather than wire, but I like that idea even more.

[Brian Gaff]

Yes I've seen that orange netting often used around peoples new driveways for stopping public walking on wet cement. Tat stuff is used by construction companies by the roll and then tossed away cos its cheap, Used indoors it should last for ages. Brian

[harry]

Rigid foam bats can be bought far more cheaply as "seconds". (Minor damage) Just Google "insulation boards seconds" to find some local toyou.

[Tim Lamb]

OK chaps. Several ideas to investigate there.

The green nylon garden netting with wide spacing for Pea and Bean supports is not UV stable but I guess that is not a problem between floors.

Staples with soft wire zig zag? Easy to do in modest lengths.

Seconds PIR I had not considered but it would need to be hugely cheaper as I estimate 150mm Rockwool is around a quarter the cost of 75mm foam. And there is still the cutting plus support battens.

I suppose one has to consider fire risks and any impact on through ceiling mounted light fittings.

Architect site meeting tomorrow. I'll run this by him.

Thanks

[Peter Parry]

The upstairs under floor heating here has the heating pipes clipped in thin aluminium sheets to act as heat spreaders. The heat spreaders were cut to fit odd shapes. Insulation is fibreglass laid below them. The flooring chipboard is fixed on top.

Upstairs under floor heating is generally much less critical than downstairs as the downstairs heat rises and (assuming a well insulated house) keeps the upper floor at a constant and slightly lower temperature than downstairs even with no upstairs heating.

[tabbypurr]

I'm struggling to see the point of much underfloor insulation upstairs. Upstairs UFH will add a limited amount of heat to downstairs, and downstairs UFH compensates accordingly.

NT

[Harry Bloomfield]

Tim Lamb explained :

It will block, or severally reduce the range.

[Peter Parry]

I agree, I suspect ours was added because the instructions said so (it was 25 years ago).

[Tim Lamb]

In message , Peter Parry writes

Could be:-)

There is also the issue of de-rating electrical wiring run through insulation to say nothing of elevated temperatures in the void. Perhaps a supplementary query for the electricians.

I have been following the LED thread but not learned much yet:-)

Looking at the thermal finning on some recessed LED fittings, I rather wonder if surface mount might be better.

[Tim Lamb]

In message , Harry Bloomfield writes

There is always Devolo as I don't know of any radio hams within 400m.

[rick]

I did similar ... just put the aluminium reflector foil under the pipes (industrial bacofoil) then 150mm rockwool pressed in place .. and used a lattice of nylon cord zig zagging from joist to joist ... stapling in place on the joists.

By having rockwool slightly oversize it self supported itself ... cord just stops sag over time.

[Tim Lamb]

In message , Phil L writes

You are right of course but I think this is more to do with heat management in local zones. Not much point in having individual roomstats if the whole house is thermally linked.

[David]

How about fibreglass inside a plastic sleeve - as used for insulation everywhere?

The sleeve holds it together so it doesn't need very much support.

Not much good in your case, but if insulating a ceiling above plaster board it is very easy to staple the plastic to the joists which holds it in place until you get the plasterboard up.

String or netting stapled below the insulation should work well. Doesn't have to be very robust with squishy fibreglass rolls.

Cheers

Dave R

[Tim Lamb]

I'm going with the garden netting stapled to the joists.

Interesting conversation with the architect concerning fire protection. Apparently 12mm plasterboard has only a 20 minute rating. I asked if the glass fibre would stretch this to the required 30 minutes and avoid the need to add 9mm board over the existing.

He thought it might be worth running by building control but reckoned that the 9" joists might be good for the extra time anyway.