Hello, just joined up here. Searched but didn't find similar so wonder what folk think about retro-fit insulation under a timber/joist floor, ground floor over bare earth/rubble? We have an old (1897) house built on brick foundations and piers directly in heavy clay - London, UK. Basically beneath the ground floor boards is a void of 0.5m and then clay rubble. It's well ventilated with air bricks and subsequently very drafty in winter - cold air can be felt coming up through skirting and if we remove carpets as planned it's going to be ridiculous! I need a cheap, ECO and DIY solution and have come up with this. Knauf EkoRoll 100mm batts beneath the floorboards and between joists, fixed by stapled straps or some kind of fabric roll (similar to garden weed supressant) or board out beneath joists with something like HARO fibreboard underlay. I can do this at a cost of approx £5 per m2. I'd appreciate any feedback on this idea, especially the use of boards and mineral wool .. is it overkill? I believe its a good idea to continue to allow the house to breath and this seems like a good compromise to using insulated board and foam which also isn't as green.

Air sealing the sub floor is at least as important as insulating it, but both are necessary.

Fitting 100mm batts perfectly without gaps between antique joists of random spacing can be difficult, but it may be possible. It needs to be tight to the underside of the subfloor, and have an air barrier (fiberboard is good) in contact with the under side of the insulation.

For air-sealing the plank sub-floor you may need to staple a wide polymer material rather than caulking between planks. Something like Tyvek or similar vapor permeable but air-tight housewrap would work, folded & caulked where it meets the joists. The batts would then want to have a fiberboard air-barrier supporting it (sealing the seams of the fiberboard with tape/caulk/mastic). Without pictures or a better description of the geometry it's a bit hard to advise on exactly how the fiberboard would be installed. You may have to install supporting strapping timbers to the joists first, then slip in the fiberboard & insulation. It needs to be a slight compression fit, with no air-gaps between the fiberboard & batts, or between the batts and the subflooring above, or any air leakage into the gaps can substantially reduce the thermal performance of the assembly.

Rather than overkill, if you're heating the place to 18C even 200mm of fiber insulation would usually be economic in your climate, but if you're only holding the line at 10C indoors 100mm is probably enough. It partly depends on your fuel type/cost, but also where the money for that "extra" 100mm of rock wool might be better spent on improving the thermal performance of an antique building like this. But 100mm is just a start. In the US code minimum requirements for new houses in climates with comparable heating degree-days to London would be R30 (US units) = R(S) 5.3, which is would be about 200mm of high-density rock wool. The 100mm Knauf EkoRoll product is rated R(SI) 2.25, so even 200mm would still be only R(SI) 4.5, still more than 10% below what would be required in the US.

Hi, thanks Dana1, very helpful reply.

The joists are surprisingly straight though the spacing is a bit random! My U calcs show that even 100mm of EkoRoll and fibreboard should give me a 5x insulation over what's currently there i.e. just floorboard and carpet!

Cheers
Mike

As Dana says, plan on some type of air barrier on both sides of the insulation. When it comes to air (not vapor) flow, you don't want a house to breath through the partitions.

Consider taped rigid foam instead of fiberboard.

thanks jonr, note my requirement for solution to be eco ... fibreboard and ekoroll are composed of mostly recycled materials and have low carbon footprint, foams although better insulators are worse environmentally and generally more expensive per m2. I've ruled out using rigid foam on that basis, also on basis that they need to be sealed well to be effective. It's a tricky balance, cost and environmental impact but I think I'm heading towards a solution based on your comments so far. Many thanks.

While you are working on air sealing and insulating the floor, it might also be a good idea to confirm that the crawlspace ventilation is adequate. If it is bare earth, you might also consider placing a heavy vapor barrier down over that. If it were me doing it, I would place a plastic vapor barrier(at least 6 mil) down first. It will ultimately help control moisture, but for the job it will make moving/sliding the insulation around to install it a lot easier. It will also help to keep it(and you) clean at the same time and overall make the insulation job not quite as nasty a job in a half meter crawlspace

If you are looking for something for the crawlspace, see the best vapor barrier at http://www.barrett-inc.com/thebarrier.php
The Barrier is a 100% vapor, water, moisture barrier as well as a thermal break of conductive heat transfer and radon resistor all in one product. The Barrier uses an extruded EPS with laminated heavy-duty polyethylene film on both sides: it stops 100% of any and all forms of moisture transfer, try that with rigid foam sheets. The Barrier's multi-layer product make up is an effective radon retarder and when proper sealing and gas infiltration procedures are used, the Barrier blocks the radon transmission. Radon gas is the 2nd leading cause of Lung Cancer and builders should be aware that this product is a RADON Protector for their customers. Termites also are deterred by the naturally occurring boron in the Barrier.

Hi, thanks for those comments. The crawlspace floor is currently a jumbled mess of dried clay clods, scrap metal, broken bottles and building debris from over 100 years! I'll definitely do something to it before I venture in though no need for radon barrier. Less than 1% radon potential where I live in UK :-)

Posted By walthamstowmike on 03 Mar 2016 05:28 AM
Hi, thanks Dana1, very helpful reply.

The joists are surprisingly straight though the spacing is a bit random! My U calcs show that even 100mm of EkoRoll and fibreboard should give me a 5x insulation over what's currently there i.e. just floorboard and carpet!

Cheers
Mike

Right!

And what's reasonably economic is 10-15x what you currently have, (more, in high energy price areas). 5x practically nil is just getting started.

You neither need nor want a vapor barrier anywhere in the stackup in your climate. Were you to build out the walls and completely enclose the space under the floor from the outdoors you would need ground vapor barriers and insulated walls to that enclosed space, but that is probably a much bigger project based on the description.