Replacing timber floor with insulation and screed

[Del-inquent]

Current ground floor is slab, brick dwarf walls, joists, 16mm T&G floorboards. Floorboards are sat 255mm-ish above slab.

We have issues with a fair number of joists, replacement floorboards (a fair few would be needed) would have to be machined to spec as there is nothing remotely close available. We would like underfloor heating. Several people both on here and other forums have suggested the same thing:

Rip it all out down to slab, level with a bit of sand, DPM on top, 170ish mm of insulation (some said 20mm of sand probably to level slab, so 100mm eps plus 50mm eps on top) pin UFH to it, 85mm-ish screed to bring it back up to same level.

I was sold on the idea, but now I've had people telling me this will most likely introduce serious damp into the place, and to just stick PIR between joists (after making good) and re-floor. I get that the suspended floors are draughty as anything and a good amount of wind passes through, but is a screed job likely to cause that serious of an issue? I quite liked the idea of not having to wear snow socks and ski boots when I go for a pee in the middle of the night to stop my feet freezing to the floor.

[joe90]

24 minutes ago, Del-inquent said:

I've had people telling me this will most likely introduce serious damp into the place,

How? That’s how my new build was done (but with 200mm PIR)

[JohnMo]

36 minutes ago, Del-inquent said:

level with a bit of sand, DPM on top, 170ish mm of insulation (some said 20mm of sand probably to level slab, so 100mm eps plus 50mm eps on top) pin UFH to it, 85mm-ish screed to bring it back up to same level.

Nothing that wrong that except 

 

Bring DPM up the walls

Add a perimeter insulation to top of screed.

Replace EPS with PIR, 150mm EPS isn't good enough for UFH. PIR compared to EPS will result in 50% better U value. 

 

The floor as is, will be drafty so is part of the original ventilation strategy. So you need to think about house ventilation also.

[Del-inquent]

2 hours ago, joe90 said:

How? That’s how my new build was done (but with 200mm PIR)

I'm guessing because at the moment the house leaks like a barn with the doors open and this will cure one of those areas of ventilation 😅 Seriously now the carpets are down we could use the place as a vertical wind tunnel. Not so bad in the summer but winter was... testing.

 

[Del-inquent]

1 hour ago, JohnMo said:

Nothing that wrong that except 

 

Bring DPM up the walls

Add a perimeter insulation to top of screed.

Replace EPS with PIR, 150mm EPS isn't good enough for UFH. PIR compared to EPS will result in 50% better U value. 

 

The floor as is, will be drafty so is part of the original ventilation strategy. So you need to think about house ventilation also.

Noted on PIR, ta.

I'm guessing those who have said about the damp (a S.E, a friend that's a surveyor, a friend that likes pissing on any ideas he didn't come up with) also are thinking of ventilation strategy, but I'm not sure what I'll need. It's having new windows at some point but I doubt the trickle vents are going to pass as much ventilation as the single glazed, glass falling out, rotten twisted framed wooden ones in there at the moment.

 

[JohnMo]

10 hours ago, Del-inquent said:

ventilation strategy

Your simple option is trickle vents, but only in dry rooms, but you don't want the standard open or shut ones, you need to upgrade to self modulating or humidity sensitive. Then you need dMEV fans in every wet room.

 

Next level of complexity as you need to add ducts, is a central extract unit (MEV) with humidity control.

[joe90]

I agree with trickle vents and fans in wet rooms but by the sound of it (and @Del-inquent will correct me if wrong) the house is not exactly “air tight” like a new build so extensive ventilation is not required.

 

i once argued (and won) with a BCO that night opening windows ( a window that could be locked slightly open) negated having trickle vents.

[Del-inquent]

1 hour ago, joe90 said:

I agree with trickle vents and fans in wet rooms but by the sound of it (and @Del-inquent will correct me if wrong) the house is not exactly “air tight” like a new build so extensive ventilation is not required.

 

i once argued (and won) with a BCO that night opening windows ( a window that could be locked slightly open) negated having trickle vents.

About as air tight as colander at the moment, though in all fairness a lot of that is being rectified in the renovations. Things like the inch gap around the door frame, holes through the walls... 

[kandgmitchell]

As long as that dpm comes up to dpc level as @JohnMo says then I can't see where the damp would be a problem. In a very old house without "proper" dpc arrangements, putting an impervious floor covering down can drive damp further up walls as it can no longer evaporate from across the floor surface but I assume we aren't talking historic here?

[Del-inquent]

50 minutes ago, kandgmitchell said:

As long as that dpm comes up to dpc level as @JohnMo says then I can't see where the damp would be a problem. In a very old house without "proper" dpc arrangements, putting an impervious floor covering down can drive damp further up walls as it can no longer evaporate from across the floor surface but I assume we aren't talking historic here?

 

Not that historic, 60's ish. Definitely has a full DPM all round it, both on external and internal structural walls.

I've just had a quote from one screeding company (I'm getting pushed for time and considering getting the screed itself done by a pro rather than do it a room at a time myself) but they reckon they only need 50mm deep if I go for their liquid screed. Bit pricey though!