Insulation thickness and what wattage for UFH?

[ozbird]

Hi and thanks in advance for all advice.

we are making 2 bathrooms in the top floor of an old barn, (south west France) .each is approximately 15m2. The ceiling height varies from 3m to 5m. One bathroom has 2 external walls, the other 1 external wall. There is an empty space underneath them of 1,2m and underneath that is our kitchen. . The ceiling of the kitchen has insulation (20cm hemp loosefill) and the ceilings of the bathrooms - which follow the slope of the roof - are insulated as well. that thin reflective stuff.

We have screwed down the old tongue and groove flooring into the joists, and reinforced the joists to minimise the movement in the floor.
Now we are putting 18mm external grade plywood over the old flooring, and levelling the 'gaps' underneath using hemp fibres. There is not much to fill

We want to put underfloor heating as the main heat source for the bathrooms, which will be tiled. Our house is NOT centrally heated, we use 2 wood heaters to heat the house. These 2 bathrooms are coming off 2 new rooms which are also in the barn space and they do not currently have heating. We had thought of putting electric radiators since one is a bedroom and one a living room, but that's probably for another forum, just to give you an idea of the background heating.

SO the question is: since we don't have a space(height) restriction we thought of putting 60mm insulation. Is it better to have the extruded polystyrene with or without the cement layer either side? and do we need to prime or seal the plywood before putting it down (flexible adhesive and screws)

And since we have a big volume, should we be going for 200w system or 150w system?
I've read some opinions that say the 200w will heat up faster but then cost more to run and others that say that since it is on a thermostat it will require less heating time so won't be more expensive...... so i am confused.

I am glad to have found this forum now but wish i had found it 3 years ago when we did our kitchen floor. I waited over 3 months for a quote from the local electrician, who was then insisting that i put 50mm insulation over a cement slab before the UFH (and that room has a lower ceiling) and couldn't provide me with any details, so in the end we didn't put it and i regret it (almost to the point where i would consider ripping the floor out and redoing it! with what i now know)

 

[Ajax123]

Wattage will depend on the heat loss which will need to be calculated for each room space first. Heat loss can be minimised by insulation so it follows that the more insulation you can get in the better? The running cost depends on the heat used not the output capability of the system but the more energy you can pump in the faster It will all warm up?

PIR (Kingston and cellotex type insulation) has a better insulation performance that extruded polystyrene which typically as an r value of 0.29w/mk ainst te PIR which is 0.023. The better the insulation the less you need.

You woud be far better to insulate between the joists than create a floating floor if you intend tiling.

 

[ozbird]

thanks for your information, i haven't heard of PIR before but will look into it.
We can't insulate between the joists, they sit over a void. and we needed to put down the plywood because the surface of the floorboards was all over the place (they were in an old grain store, never house quality). So we have to base ourselves now on the plywood.

 

[ozbird]

anyone got advice on whether 60mm insulation is too much? can you ever have too much insulation? is there enough difference between say 20mm thick and 60mm thick to warrant the price difference and possible return on saved electricity?

btw the plywood is screwed down to the joists and the boards underneath,

ANyone?

 

[Uheat - Keith]

INSULATION.
As with Ajax above, we always say Construction insulation boards over plywood etc.
Elektra MD160 or MD200 Underfloor Heating Mat for installing on Construction insulation boards, or conrete etc, ( or extruded polystyrene insulation boards if those boards are on concrete etc )
Can not put 160w/m2 - 200w/m2 outputs on plywood etc.

200w/m2 will not cost anymore, as above, all down to the heat loss of the area.

Warm regards,