EasyBuilder wrote, in response to the thread re poly beads below timber floors:

''So having read all this I'd like to know what ways there are to well insulate a suspended timber floor without the risk of incurring problems. Historically it seems to be a building element that relied upon draughts and high heat loss to keep it dry and rot free. Is it even compatible with modern energy efficiency standards?''

I wrote:

I don't think there is a way without running the *risk* of problems. I have been involved with hundreds of under-floor insulation jobs, of which probably less than a handful have had any problems (to my knowledge - I acknowledge that I do not visit the ones I was involved with 30 years ago!)

Tips to maximise the chance of problem-free and successful include:

- Always draught-proof the perimeters before insulating.

- Ensure there is adequate *cross*-ventilation.

- Ensure that ventilation entering at joist level is ducted so that it enters the void *below* the final insulation layer.


I have found that quilt between joists with breathable membrane below - all joints and perimeters taped - seems to work well. I have sometimes had issues where I have used Pu to stuff the gap between the last joist and the cold cellar wall - where quilt wold 'suck' - with condensation on the Pu and being transferred to the joist. With regret (as I don't like un-insulated bits) I have sometimes removed the infill at this point.

*I have started a new thread so that this does not hi-jack the poly bead thread*

'Best practice for under-floor insulation?'

Posted By: Nick ParsonsI have sometimes had issues where I have used Pu to stuff the gap between the last joist and the cold cellar wall - where quilt wold 'suck' - with condensation on the Pu and being transferred to the joist. With regret (as I don't like un-insulated bits) I have sometimes removed the infill at this point.

Maybe something like leca or silvapor (if it's obtainable again yet) would fill that gap if you put something underneath to hold it in place?

Posted By: Nick Parsons
I have found that quilt between joists with breathable membrane below - all joints and perimeters taped - seems to work well.

Ed Davies wrote:

VCL above the insulation/joists or not?

Yes when boards have to be lifted. No when insulation from cellar or crawl space.

Posted By: haplessDiyerBut is there an argument for not using a VCL so any moisture can breathe back into the room. This seems to often be advocated for wall insulation but less so for floors.

Because the outside surface of walls can easily get quite warm when the sun shines on them, driving moisture inwards. That doesn't happen for the underside of floors!

Thanks for creating a new thread Nick. I was inspired to raise this more general question not just because of the underfloor poly beads thread, but also one started some months ago that reported excessive condensation on the underside of between joists foil faced insulation.

The accepted way of insulating suspended timber floors seems to be by placing insulation of one type or another between the joists. However, given sufficient insulation and the appropriate atmospheric conditions this places the dew point on or around the bottom face of the insulation, and somewhere inside the joists. Sure there are plenty of people who have done this and so far found no problems. It may be that they have not added enough insulation to create these conditions, or that the damp does not persist for long enough for rot.

Thinking about ventilation under the floor, given the required air bricks and sufficient wind all should be fine. However in winter when it's coldest is often a time of high pressure and no wind. With an uninsulated floor the escaping heat will evaporate condensation and vapor pressure will take it out through the air bricks without needing any wind. But with a sufficiently insulated floor it seems to me that the condensation could just keep building up. Put another way, traditional underfloor ventilation is driven by both wind and by heat loss, and taking away the heat loss will at times inhibit the effective ventilation.

Additional insulation under the joists is harder to install, but pushes the dew point out and down. It would seem to remove most of the problem, but the dew point then may be in the wall plates of the sleeper walls and in the parts of the joists beside the outside walls. Filling the space with poly beads prevents any ventilation at all, and there is probably still a dew point somewhere in a space with timber in it. But with sufficient perimeter wall insulation the dew point should be away from the timber, making this a viable solution.

Here is a method I used once. The brief was to keep the original Edwardian skirting and floor boards. It was very laborious.... expecting to be ridiculed by Viking.... but hey ho... It worked and is risk free. Needs *the right size space* between underside of joists and sub- base

1. Carefully take up and number existing floor boards - hitting nails through boards into joists carefully with a nail punch if necessary (Brads will come out with the boards usually). Cutting some boards on joists will probably be necessary
2. Remove all dividing stub wall and block up any external vents. Ensure however that these are not providing any through ventilation to any other timber floors in the house - If they are this method is a non- starter.
3. Cut out *enough* (full length) joists in the center of the room and keep aside. This to allow installation of insulation. Clean out and remove all timber from where the joists were pocketed into the masonry and brick up holes.
4. Flatten surface beneath joist with stone dust. (awkward and laborious)
5. Lay a DPM on top of the stone dust. Allowing enough to reach up and beyond finish floor level (eventually). Turn up and underneath joists at ends.
6. Install rigid insulation on top of DPM up to 100mm below underside of joists. Layers of 50mm are easier to use and can be overlapped. Foam as you go any gaps.
7. Flood the area between insulation and underside of joists with concrete leaving a small gap - say 5 -10mm. Finish is unimportant.
8. When the concrete is cured, reinstate the previously removed floor joists and pack with plastic window packers and fix (screw) joists to concrete at intervals. This was easy with the floor I did as the joists were the old 4x2's.
9. Cut all joists as far away from the walls they are pocketed into as possible leaving a CLEAR space between the new joist ends and the existing masonry.Clean out and remove all timber from where the joists were pocketed into the masonry and brick up holes.
10. Pull previously folded DPM up and beyond new joist ends.
11. Reinstate floor boards.


With hind sight it may have been easier to completely remove all of the joists and reinstate them after laying a new sub floor - but there were complications in that they were adjacent in places to Edwardian tiled hearths which would probably have become damaged...

Thanks for your comments. Yes, there would have to be a DPM that came up to the DPC with appropriate detailing. But in my case I already have concrete on hardcore oversite below the sleeper walls and the last thing I want to do is add more concrete. The aim is to position the insulation entirely beneath the floor structure so the dew point is nowhere near it, and then have an adaptable floor structure above. There must be alternatives to concrete to spread the floor load onto the insulation. Access flooring varies in price enormously. Not needing computer room properties like static electricity suppression I found one at £4 a panel plus £2 per support.

Toying with the access floor idea a little more here's an even simpler solution. Stand the access floor on the concrete oversite and use the pedestals to support insulation to the underside of the floor panels. Steel pedestals may create an undesirable thermal bridge and be prone to corrosion, but plastic pedestals designed for outdoor decking and paving applications are available. No need for DPM, support over the insulation, or to block the air vents. No materials that will corrode or rot within or below the insulation. Rigid insulation could be supported directly by ridges on the pedestals, or by plastic washers pushed over them.

Sorry Dave, perhaps I wasn't clear. The insulation will no longer be on the oversite but immediately underneath the floor panels where it is supported by the access floor pedestals. So only the pedestals will be in contact with the oversite and so I can't see a need for a DPM anywhere. If the insulation was so thick it extended below the DPC in the walls it would need something there. Will try a diagram later.

Quick Question. Is it worth suspending netting or VPL from from joists abutting walls/sleeper walls and hanging more vapour permeable insulation * below * the joists ?
I determined that: 1) can access from above only; 2) will do parge coat; 3) use 100mm knauf earthwool flexible slab between the joists; 4) VPL beneath the insulation; 5) VCL above the insulation and 6) direct air from airbrick below the joists and 7) continue with rads for heating. Have chosen the 100mm earthwool over expanded board insulation because of fire-resistance and vapour permeability BUT this is not a lot of insulation - I recall seeing 200mm of kingspan under new build underfloor heating! Hence the question at the beginning.

It's reasonable to add more insulation below the joists as you say. Clearly you need to make sure that the airbricks connect flow to the space below the air barrier as you suggested.

Are you planning to take up the whole floor? You will need to in order to push mineral wool batts below and in between the joists. But presumably you are, in order to be able to fit the VCL, sorry for stupid question.

8"= 8 inches

Hi. I'm planning to insulate under our suspended timber floors by filling earthwool between the joists (suspended in netting), laying a vcl above this, making sure no gaps etc, then putting back the floorboards. All being done from the top as we don't have access/ crawl space underneath. We're also planning to lay new engineered wood floors above the existing floorboards and will use an underlay for these above the floorboards. My question is, as many wood floor underlays come with a dam facing on one side, if we were to use one of these, then do we still need the vcl below the floorboards? Seems like if we have both, then the floorboards will be sandwiched between the two with no ventilation options. If we were to have only one of these, then which one would be better to avoid problems with ventilation in the floorboards as well as joists? Thanks.

Thanks Nick. Can you explain why a breathable membrane and not netting? Wouldn't that affect the ventilation around joists? Also concerned re mice chewing through breathable membrane if it is by itself. In which case should we use both? Any suggestions on which ones are best and durable enough? Thanks.

Thanks Ed. Yes, that's the plan. I used to get a lot of people coming back after they had done under-floor insulation, disappointed with the results. Main reasons were failure to seal the perimeters (partic with hollow-fixed skirtings) and thermal by-pass.