UFH or not + LVT onto UFH.

[LHall]

Morning all,

 

I'm after some advice as to whether it would be worth putting in a wet UFH system in our 1950s house that we are renovating, and if so, your thoughts about fitting LVT on top.

 

The house has an old garden room extension at the back which we are opening up to make a 5.7x5.6m kitchen-dining area. There will be a vaulted roof over the old garden room which approximately half the kitchen-dining space will be under. We are planning to put some insulation into the old garden room as we don't think it has any at the moment.

 

The house is built on suspended joists with approximately 600-700mm of void space under the original house and 250mm under the garden room. The joists are only 100mm deep, spaced 400-450mm apart. We were thinking of battening from underneath, adding 75mm insulation and finishing with 25mm UFH and dry screed mix. We ideally don't want to raise floor levels due to the lower ceiling height in the original part of the house.

 

Finally, the house faces west and gets quite a lot of sunlight in the afternoon. We are also having 3 velux windows, bifold doors and a window looking out to the garden. 

 

1) UFH vs two big radiators. Which would you get in this scenario for our project? We want the space to feel warm when we need it to but don't know what would be more effective. Would we lose lots of heat under the house?

 

2) If we were to get UFH then how would you finish the flooring to lay LVT? We were thinking of having 22mm chipboard, then 6mm ply, then LVT. Does that seem about right?

 

3) We'd like a herringbone LVT pattern but were recommended not to go for larger tiles due to the larger gapping that would occur in sunlight. Does anyone have experience of this?

 

Thanks for your advice, I really appreciate it!

[JohnMo]

I would do the insulation in the floor, but personally I don't believe there is enough insulation going below the UFH to make cost effective to run as the downwards heat loss will be large.

 

So would either increase depth of insulation or just stick with radiators.  You can get a couple of nice radiators for same or less cost.

[markc]

Got to agree, also why chip floor then 6mm ply? 

[LHall]

Thanks for the advice @markcSpoke with a flooring company who suggested the ply is needed for a more even surface. The 22mm chipboard was what was suggested from the UFH spec diagrams. Would you do something different? Just ply and no chipboard on top of the joists? Thanks for the advice!

 

Thanks @JohnMo for the advice! So just increase insulation to the full 100mm or would you look to somehow put even more in?

[markc]

22mm chip (tongue and groove) glued and screwed will give a very flat and ideal surface for tiling on.

[PeterW]

34 minutes ago, JohnMo said:

So would either increase depth of insulation or just stick with radiators.  You can get a couple of nice radiators for same or less cost.

Start here - work out how to make the joists deeper for insulation purposes but irrespective of heating choice you need some insulation under that floor.

 

8 minutes ago, LHall said:

Thanks for the advice @markcSpoke with a flooring company who suggested the ply is needed for a more even surface.

This is pretty usual as they are quoting based upon it being old boards or floorboards - they use it to get a stable surface to then feather out any issues and lay the floor.

 

I would look at a 2 stage approach to the insulation but would also question wall insulation too - a 1950’s house may not have the best insulation standard so is UFH going to be capable of heating the room .? Do you have space for the buffer tank as temperature will need to be controlled carefully or the LVT will move.

 

You can sprocket existing joists with 6mm ply and then add more depth to allow for deeper insulation levels but I would question 100mm joists to start with - even at 400mm centres they are the weak point and you’re about to add a lot of static weight with a pug mix UFH system. If you’re about to take up the floor anyway, look to replace with 150mm at 400 centres and insulate as you go. 
 

First stop for me though is a proper heat loss calculation ..!

[LHall]

Thanks @PeterW - very helpful advice! Builder also raised the same issue regarding the weight of the pug mix. I'm think I'm leaning more towards the radiator route and increasing the insulation the floors and walls as much as possible. I'll also look into the heat loss calculation.

[PeterW]

@LHall nothing stopping you pulling up the old floor and then laying netting between the joists and stapling on top and then suspending 200mm of loft type rockwool between the joists. Close it all off with a vapour barrier and then screw the new T&G boards over the top and glue all the joints. Using 22mm T&G chipboard you may get it level enough to only need 3.6mm ply glued to the top but let the floor fitter decide that. 

[Nickfromwales]

2 hours ago, markc said:

22mm chip (tongue and groove) glued and screwed will give a very flat and ideal surface for tiling on.

The only failed tiled floors I’ve replaced have been where a binder such as ply or membrane hasn’t been used  

 

At the absolute minimum you must install a 3.6 ( aka 4mm ) plywood binder, glued down and screwed at 100-120mm centres to give something for the adhesive to get its teeth stuck into. That, or a decoupling membrane such as Ditra.

 

Unless you hugely improve the insulation ( and draught-proofing ) then I would not go with UFH. You’ll be running it nigh-on constantly in the winter, in an ever-losing battle against the significant amount of ventilation and fabric heat losses associated with this type / age of structure. To put UFH over a cold-ventilated subfloor is a difficult task to manage in terms of heat and energy requirements, even in a modern build with block and beam.

 

Rads will give almost immediate heat to the spaces, so you can time the heating-on times to better suit your needs. UFH will likely never turn off in the winter, instead you’ll be toggling between economy heating tempos and then comfort to stave off the long recovery times that the UFH will promote.

 

If you can install some 20mm strips of Compacfoam atop each joist to kill off cold bringing then that will pay big dividends from not creating cold ‘spots’ at each junction, plus that’ll allow you and additional 20mm void to fill with more PIR.