Posted By: kebl2825100mm deep joists

That doesn't sound deep enough, especially at 450 mm spacing.

I don't know if a thinner-than-structural plywood layer + adhesive + parquet can be considered a structural floor cumulatively if the same thickness of plywood alone wouldn't be considered to be? Does, for example, a 12mm plywood plus rigid adhesive holding a tight-fit 18mm parquet 'act' like the equivalent of flooring with 20mm+ sheets? Or not significantly better than the 12mm plywood on its own?

No, just consider the plywood as the structural part. It would be usual to put the plywood on top of the existing T&G structural floor.

You haven't said much about insulation and haven't said how deep the underfloor void is. Have you considered replacing the timber floor with an insulated solid floor?

edit: personally I would prefer a flexible adhesive but I'm not an expert on the subject.

Just to add. At least some of the floor in the front room and hall is coming up anyway to re-route a soil pipe from our en-suite (which is above the porch+hall). It currently runs across the inside of the front wall just above our bedroom floor, at insufficient fall, smack in the way where I want to add internal wall insulation. New planned route is dropping it down into the ground floor void and across under the house that way.

So that disruption is what motivated me to finally do the underfloor insulation at the same time - the parquet idea is coincidental, leftover from a relative's project and chosen more for its anecdotal value than aesthetic (it is from Manchester Museum of Science and Industry).

Good morning,

For your period of house the 100mm joists sounds about right with intervening sleeper walls to create max 1.8 span of the joists. It's also very common for the joist spacing to be rather random, which doesn't help installation of insulation. You'll probably also find that your joist ends are fitted into the wall. The floor bounce may well be down to the ends of the joists rotting and playing around - but just a guess. I guess you have solid exterior walls and your house was built too early for 1st gen cavity?

To give you some suggestions, I think it's most sensible to start off your floor design with a heat loss calculation to understand first what type of insulation and how thick it needs to be so that your UFH doesn't end up underperforming.

Also keep in mind any insulation fitted under the joists will be a nightmare PITA to fit even with a 600mm void. If it's shallower, you'll most probably want to focus on fill between joists only because you don't want to compromise the underfloor ventilation - so you are, for example, pretty tight with the area of floor where there's only 200mm.

Since you're going to be taking up the existing floor and will likely have to replace joists (probably more than you suspect), refitting new ones into the existing in-wall supports is fiddly and you'll most likely have to get some broken slate roof tiles from your local roof merchnat to use as packers etc. Because timber dimensions have changed you may also find that you need to buy structurally graded carcassing timber in a larger size and have your timber merchant rip them down to the correct dimensions (easier than making large holes in your wall).

However, another option is to remove all your existing joists, fill in the end supports in the wall and then use wall plates to support new, deeper joists. This way you keep all the joist ends out of the wall (better for airtightness and protection against rot), and you can beef up your between joist insulation. If these new deeper joists are installed so they rest on the existing sleeper walls,you'll end up with a nice solid floor. The other advantage with this approach is that when you install your new subfloor of plywood/chipboard/osb, you'll have the joists spaced correctly for the board end (this will also remove the headache of cutting down your insulation for random spacing). If you stick with the existing joists and their spacing, it'll likely be another PITA to line up the new subfloor joinst across the joists.

Otherwise, the simplest buildup would be:

Joists with fullfill insulation between
Subfloor - if using 400mm centres, 18mm TG4 osb or plywood (for osb or plywood you can actually use 15mm boards at 400 centres but the 18mm is the one normally stocked and easy to get hold of) or 18mm chipboard (moisture resistant green I'd say is probably better). Out of these choices, my personal preference would be the OSB as it can double up as the vapour controll layer for the floor and it performs very well. It's cheaper than the plywood. You can nail and glue, or screw and glue these boards.

On top of this subfloor then install whatever the UFH manufacturer recommends and then finished floor again according to the manufacturer.

Regarding the subfloor and its structure, if you're intending to use a screed for the UFH, then you'll have to confirm joist sizing and board thickness accordingly as the ones I've given you are for a typical suspended timber floor buildup without screed.

Because of all these complexities of adding the UFH to your suspended timber floor, I think it would be well worth taking djh's advice and pricing up an insulated solid floor. But if you're doing DIY and your labour is essentially free, then the timber floor is probably the most DIY friendly, even if not entirely straightforwards.

Oh, and don't forget to look at exisitng wiring and plumbig under the existing floors as this will need to be modified accordingly.

HTH.

Posted By: JontiI would start by asking if you need to have heating in the hallway at all? Secondly, have I misunderstood this because are you thinking of installing underfloor heating but no insulation under the floor?

Yes, you've misunderstood. Definitely will be adding insulation as I said near the start of my first post, and to clarify there is none currently.

I don't know if we need do heating in the hallway, how would I go about figure that out? The upper and lower hallway space and stairs are all open to each other and there is another staircase open up to the loft room, so overall it's a large proportion of the total air volume of the house, though on the party wall side of the house (we're end terrace). Currently there's 1 radiator on ground floor, one on first floor, both with their TRV set low. The adjoining ground floor rooms that we spend most time in are heated the most. If the hallways is unheated won't the heating in the heated rooms just need to work harder as there will be more loss to the hall - but maybe that's acceptable, I don't know?

Once insulation is there it will make any heating we have perform better, cost less to heat to same or better comfort. It's only because we're laying parquet that we thought if we ever do want UFH instead of the radiator then now is the time to do it.

Posted By: SimonDGood morning,

For your period of house the 100mm joists sounds about right with intervening sleeper walls to create max 1.8 span of the joists. It's also very common for the joist spacing to be rather random, which doesn't help installation of insulation. You'll probably also find that your joist ends are fitted into the wall. The floor bounce may well be down to the ends of the joists rotting and playing around - but just a guess. I guess you have solid exterior walls and your house was built too early for 1st gen cavity?

To give you some suggestions, I think it's most sensible to start off your floor design with a heat loss calculation to understand first what type of insulation and how thick it needs to be so that your UFH doesn't end up underperforming.

Also keep in mind any insulation fitted under the joists will be a nightmare PITA to fit even with a 600mm void. If it's shallower, you'll most probably want to focus on fill between joists only because you don't want to compromise the underfloor ventilation - so you are, for example, pretty tight with the area of floor where there's only 200mm.

Since you're going to be taking up the existing floor and will likely have to replace joists (probably more than you suspect), refitting new ones into the existing in-wall supports is fiddly and you'll most likely have to get some broken slate roof tiles from your local roof merchnat to use as packers etc. Because timber dimensions have changed you may also find that you need to buy structurally graded carcassing timber in a larger size and have your timber merchant rip them down to the correct dimensions (easier than making large holes in your wall).

However, another option is to remove all your existing joists, fill in the end supports in the wall and then use wall plates to support new, deeper joists. This way you keep all the joist ends out of the wall (better for airtightness and protection against rot), and you can beef up your between joist insulation. If these new deeper joists are installed so they rest on the existing sleeper walls,you'll end up with a nice solid floor. The other advantage with this approach is that when you install your new subfloor of plywood/chipboard/osb, you'll have the joists spaced correctly for the board end (this will also remove the headache of cutting down your insulation for random spacing). If you stick with the existing joists and their spacing, it'll likely be another PITA to line up the new subfloor joinst across the joists.

Otherwise, the simplest buildup would be:

Joists with fullfill insulation between
Subfloor - if using 400mm centres, 18mm TG4 osb or plywood (for osb or plywood you can actually use 15mm boards at 400 centres but the 18mm is the one normally stocked and easy to get hold of) or 18mm chipboard (moisture resistant green I'd say is probably better). Out of these choices, my personal preference would be the OSB as it can double up as the vapour controll layer for the floor and it performs very well. It's cheaper than the plywood. You can nail and glue, or screw and glue these boards.

On top of this subfloor then install whatever the UFH manufacturer recommends and then finished floor again according to the manufacturer.

Regarding the subfloor and its structure, if you're intending to use a screed for the UFH, then you'll have to confirm joist sizing and board thickness accordingly as the ones I've given you are for a typical suspended timber floor buildup without screed.

Because of all these complexities of adding the UFH to your suspended timber floor, I think it would be well worth taking djh's advice and pricing up an insulated solid floor. But if you're doing DIY and your labour is essentially free, then the timber floor is probably the most DIY friendly, even if not entirely straightforwards.

Oh, and don't forget to look at exisitng wiring and plumbig under the existing floors as this will need to be modified accordingly.

HTH.

The only bounce I've noticed is at the dividing wall between hall and the rear ground floor room. Where I have been in the crawlspace there is packing and DPM material between the walls and the joists. I can't easily get to the area with the bounce through the hatches I have but I'm guessing it's just this wasn't fixed securely so there is that one joist covering a longer span.
The front wall is original and has a small 'finger' cavity. The side wall (opposite to party) was rebuilt in more modern brick at some point, don't know when, and don't know the size of that cavity yet.

All the joists I've seen when I've been below are modern (we think from an early 1990s refurb) so hopefully not loads to replace, if any. Wanted to do a single room at once (thinking about moving furniture around) so replacing any joists across multiple rooms will indeed be a pain.

How would I go about doing a heat loss calculation as the recommended first step?
Forgetting the idea of UFH pipes going between joists, then for insulation in the 600mm void areas, I was just going to add wool/batt between joists and EPS boards below as much as feasible. I played around with this u-value calculator https://www.changeplan.co.uk/u_value_calculator.php (with the default interior/exterior surface Rsi/Rse don't know how appropriate they are) to have confidence I was exceeding building regs, and that the dew point formed below the joists. 100mm wool between joists (~85% of the area) and another 100mm below of wool/EPS did that. And I thought EPS vertically on the void walls would be easier to tie to eps under joist than a membrane supporting wool. In my head anyway. From crawling around a bit I know it unpleasant moving around down there to even get yourself in position never mind wielding insulation boards - I lost some motivation to do it all from below after that!

I'm not against the idea of putting in a new solid floor, it just seems an extreme step though, same as all new timber joists throughout. My inclination would be to keep it breathable so limecrete which is presumably more expensive again. Is there a ballpark price/m2? I can't seem to find any online. I better contact a couple places and ask for a speculative quote rather than keep guessing.

The underfloor void is wide open to the neighbour's under the party wall near the porch that would need blocking up if go solid. There is electric wiring, gas line, soon a soil pipe, a (soon if not already) redundant plumbing overflow pipe, and I was planning to put in network cable to the ground floor rooms too while I'm there, back to understairs cupboard and/or porch to run up to the first and second floors. Cabling in the void run through some flexible smooth-bore conduit to allow fishing in future if needed, and to keep it away from contact with the EPS. Opportunity to rewire/derate if needed. I think it is only ground floor socket ring (2 rooms, not much load) loose near joist level, SWA out to the garage along the soil floor, and the earth bondings that run in the void again laid loose over sleeper walls.
I assume something similar is done with electrics running in conduit so they can run through solid floors? If they all have to move out of the void that's more cost and disruption.


Evidence so far seems to point to UFH between joists not being a good option when there are 2 solid floor layers above it.
But now have the new option of a solid floor to look at!

Looking round at some limecrete sites, sent my details in.

Bit surprised to see this
"Wooden floors should be ventilated using either vents or spaces at the perimeter under the skirting."
at
https://www.thelimecentre.co.uk/products/insulating-plaster-render-limcrete-floors/limecrete-floors/

Maybe I've misunderstood. Does a wooden floor finish above a solid floor usually need ventilation at the perimeter? Why?

Leaning towards ditching UFH idea and any thought of replacing with solid floor, unless I find lots wrong in the areas I haven't been able to get to yet. If in future we ever change our mind to go solid the insulation taken out will still have value elsewhere, to someone, and the parquet can be salvaged and relayed if needed too, albeit at great faff.

Thanks for comments so far, has definitely helped.

Thoughts now turn to the insulating job. There are lots of threads about it, so my dumbest questions are probably answered already amongst them.

2nd vote for what MV just said.
We've just done exactly the same here (Torfloor upstairs, pipes in screed downstairs).