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Discuss Pug/bisuit Mix For Ufh, Timber Joists, Wet Or Dry in the Tiling on Underfloor Heating area at TilersForums. USA and UK Tiling Forum

J

John McBretney

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We are doing wet UFH on suspended floors with very strong timber joists (9 x 3in at approx. 400mm centres, max span 4.8m). We are setting the pipes between the joists on a 45mm thick PIR base and pug mix (8:1 sharp sand: cement) between and around the pipes, as commonly used. I understand the pug mix is usually mixed and laid dry but my builder would like to lay it wet, in fact he has already started and it looks ok.

Is there any disadvantage or problem laying the pug mix wet?
 
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J

J Sid

It's a bad idea....
If you over board with ply and then Ditra uncoupling membrane over the top you will seal in any moisture and this will happen a year or so down the road
WP_20130418_002.jpg
WP_20130418_007.jpg
This was kiln dried clay balls but even this had to much moisture in it, so any moisture trapped does alot of damage.
The ply rotted and turned to dust......
 
OP
J

John McBretney

It's a bad idea....
If you over board with ply and then Ditra uncoupling membrane over the top you will seal in any moisture and this will happen a year or so down the road
View attachment 80389
View attachment 80390
This was kiln dried clay balls but even this had to much moisture in it, so any moisture trapped does alot of damage.
The ply rotted and turned to dust......

Wow, what a mess! Looks as though the joists are ok though.
We aren't planning to have a membrane above the pug mix, but more important we aren't going to use plywood or chipboard for the structural floor. We are using 18mm T&G cement impregnated particle board (1200x600 sheets) which are very resistant to moisture.
Even so, I guess we need to allow the pug mix to fully dry/cure, or at least acclimatise to the normal room moisture levels, before we overlay it with the CIPBs.
I wonder if it is ok to turn on the UFH to speed up the process. I guess the pug mix will crack but since it's not structural it should be ok? What do you think?
 
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J

J Sid

Yes joists were ok, it was only a small amount of moisture but did alot of damage.
Your board will act to hold in any moisture the same as Ditra did I would of thought!
I would have thought you need to almost treat yours as a heated screed, allow to dry then turn on heat and then test moisture level before covering, would hate to see the joists to suffer and damage thought water or moisture.
What tiles are being fitted?
 
OP
S

Spud

The pug mix acts as a sound and heat insulation when laid dry mix ,it takes low frequency noise , laying it wet stops it being a sound barrier and reduces the thermal insulation , I would say it is a bad idea , top post from Julian there
 
OP
J

John McBretney

The pug mix acts as a sound and heat insulation when laid dry mix ,it takes low frequency noise , laying it wet stops it being a sound barrier and reduces the thermal insulation , I would say it is a bad idea , top post from Julian there

Yes, one reason for using pug mix instead of alternatives, e.g. spreader plates, is that it gives some acoustic insulation from the floor below for airborne noise. We are laying it damp now, not wet like mortar. But why would laying it wet reduce its sound barrier properties -- I thought the barrier to airborne noise depended mainly on the mass of the barrier, so there would be no difference, wet or dry, once cured?

Also, why would laying it wet "reduce the thermal insulation"? Anyway we don't mind the thermal insulation being reduced -- the less the thermal insulation, the more easily the heat will come through.

In the bathrooms we may have ceramic floor tiles, maybe on 6mm thick cement backer board to make up the levels to the bedrooms. The combination will have good thermal conductivity, particularly important in the bathrooms

In the bedrooms we intend to use 14mm thick engineered timber flooring. Between the engineered timber and the structural CIPB floor we intend to lay 6mm thick iso sonic rubber matting to reduce impact noise.

Grateful for any more advice from anyone
 

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