Hello,

I've been reading the various threads about insulating suspended timber floors, and thanks to everyone who's contributed - we've learnt a lot. Our situation is slightly different to the other threads and we'd appreciate some thoughts on the best options for us:

We're insulating the full ground floor (approx 50sqm) of a Victorian house, with suspended timber floors that we can easily access from below - there's a cellar running underneath the full house. For this reason (and because we're living in the house and couldn't manage the disruption or cost of taking up the floors) we'll be insulating from below.

The joists are 180mm deep, and we've already had a re-wire completed, with most electric cabling running underneath the joists, but some notched through the joists. We've also got some heating pipes underneath the joists.

We're improving the ventilation to the cellar as part of this work, but have a bit of damp down one section of the external wall. The joists run parallel to the wall, and the closest joist is about 50mm away from the wall

Finally, our finished floor surface above the floor will be stripped wooden boards. We've got some fairly gappy boards, and may look to seal these from below or from above, if it's worth the effort.

From what we've gathered from the other threads, our approach will be to use an inorganic quilt insulation between the joists, and to staple a breathable membrane below the joists to help with wind wash of the insulation and to ensure any moisture can escape. Our questions:

- the insulation material we're planning to use is rockwool, or Earthwool (which is slightly cheaper) - any reason to pick one or the other? We plan to use 200mm which compresses slightly to fill the depth of the joists

- for the breathable membrane there are lots of different ideas across the threads and elsewhere online, from a weed membrane, through to a Pro Clima membrane, and so a lot of price difference. What do we need to look for, here? Balancing out cost and performance, would Tyvek Housewrap work? The Pro Clima Solitex Plus comes in at circa £4/sqm, whereas the Tyvek is £1.63/sqm, and the weed membranes might be cheaper than this?

- I've seen in one thread on here the idea of using masking tape from below to seal the gaps between boards - is this worth doing, if you're insulating and putting a breather membrane below? We'll likely have some minor liquid spillage onto the floor above (we have a toddler!), so if it's worth it to prevent water interacting with the insulation then we might do this

- for the section of damp wall, what do you recommend? As detailed above, there's a joist running parallel to this wall at about 50mm from the wall, so we could foam this gap with an air-tight foam, but I don't know if that's a good idea? If we were to do this, would you recommend running the breather membrane below this, and therefore taking it to the external wall, or would we be better taking this to the closest joist to the wall, but no further?

- how important is taping the joins of the breather membrane and how important is taping it to the external walls? We're having a tradesperson do this work for us, as we have hectic jobs and the toddler, so keeping the work as simple as possible is best for making sure it gets done well

- we'll chat to our electrician about the cabling notched into the joists, and whether this needs to be de-rated

- finally, do we need to consider whether we're making the floor too airtight? We don't have mechanical ventilation, so will be relying on opening doors and windows to ventilate the house above. We don't have a woodburner or anything that would further compromise our air quality.

Thanks in advance for your ideas

I would seal the gaps between the boards, strips of wood glued on from underneath, pushing stuff in the cracks, sealing with mastic etc.

Any quilt is fine, I wouldn’t bother with breather membrane, strawberry net for me but I would add 50mm eps across under the joists to mitigate their thermal bridging, between the last joist and wall eps too but inspect it by taking out periodically

Hi KmcK,

I'd say you've pretty well summarised the best approach, and along with PiH's & DJH's comments, I would add the following, having spec'd and installed this for clients...

- I always use Knauf earthwool, as it's so much less itchy than any others I've used (check the lambda/heatloss values, so comparing like with like. Ideally the 32 version, 0.032 W/mK)
- use a membrane, but just get the cheapest you can find. I usually try to get the the widest roll, last time it was 4m wide. Staple it and then add large head felt nails for a bit of security, if you can swing the hammer.
- would be good to get a layer under the joists too, but I realise headroom and those existing cables needing derating could be an issue. It would help with the exposed pipework, being wrapped inside the wool.
- I wouldn't put much/any effort into trying to seal the floor. It's been acting as vapour removal from your house all these years, and improving the ventilation in the cellar will ensure than vapour is better removed. The earthwool is very vapour open.
- I wouldn't bother trying to tape the edges of the membrane, as it won't last. Take your time to get decent overlap that doesn't show big saggy gaps. Using the 4m wide roll will generate fewer joints.
- floor perimeter is really important, so rigid insulation board pushed into the 50mm gap you mentioned, and foam nozzle jammed up the interface insul to wall/joist will close gaps and glue it in place...but understand the damp first.
- I sometimes run a narrow strip of rigid insul board along the wall edge where the joists butt. Fix it into the underside of the joist, like a sort of counter batten, and then foam around the mating surfaces. This closes off the edge where the membrane/wool butt against the underbuild/wall, which can be a slightly weak spot for gaps in the wool.

You're really lucky to have good access from below, so you can take your time, and do a nice job of it. Good luck.