Floor Insulation

Home Retrofit
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Agreed, mostly. Ideally the tape should be to the brickwork rather than the plasterboard. This will involve parging or priming the bricks/blocks first. Dot & dab plasterboard is notorious for air circulation (behind the board) and permeability, which we don’t want to link to the underfloor space. If the property is wet plastered (unlikely in this case) then the membrane can be taped to the clean plaster.

The OP said that he suspects that there is a large gap between the floor and the bottom of the plaster. If so he could wet plaster the gap, taping to whichever surface seems appropriate at the time.

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Hi, here is something that you might be able to get hold of in the future- for now it is in North America- to the best of my knowledge and specific for floors.
Earthwool underfloor roll with a wind wash barrier is an exciting new product for the thermal upgrade of existing floors. The wind wash barrier improves the thermal performance of the insulation by providing a protective barrier against air movement under the floor. Installing the insulation is also quicker as the insulation can be directly stapled in place:

High thermal performance- R-1.7
Wind wash barrier – protection from air movement under the floor
Super-soft Earthwool insulation – made using recycled glass and with ECOSE® Technology -Can be stapled in place

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For comparison with other European (including British) products I think R=1.7 is approximately U=0.59W/m2K. If that is the case a thicker version is needed here. I notice in their illustration that it is less than joist depth.

Being a US site the units used might be
in2 and °F, in which case the U value above is total rubbish!

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Closer to home. There are some good eggs here:

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Bottom of the page. I like these solutions. Maybe even with the wire mesh supporting underneath. There should be enough spring back to deal with ageing of the floor and insulation.

Interestingly, gutex website does not list their thermoflex product, linked in the article, as a solution for a floor system.

One problem with natural fibres are that insects might like them. Mineral wool products have no such issues. So it could well be better value and a superior product, considering both gutex and jute are imports from Germany. Bloody Germans.

I personally think that worrying about condensation and the need for hydric redistribution/ hygroscopic etc materials, in this context, is more marketing than anything. As long as it’s open enough, so the water isn’t trapped behind a barrier, you’ll be fine because the vapour pressure will always be into the crawl space in our climate zone.

I’m asking a friend, who’s moving into a new place with a similar suspended floor system, what he’s going to do to flatten out the floor. (He’s a carpenter for the last 15 years.)

I’d also be interested to know if it’s worth insulating up the stem wall to reduce thermal bridging from the ground.

I’ve also seen people put poly down on the earth and tape it to deal with soil gases/damp. This would be taped to your DPC. It would reduce damage to your mortar from efflorescence, as there will be no evaporation from the brick, so the salt doesn’t get left behind to crystallize in the first place.

To think building are relatively simple in the grand scheme of things.

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In this context it is worth checking whether you live in a high radon area, although radon is more of a problem with solid floors.

https://www.ukradon.org/information/radonsearches

I have heard that if you have a radon survey the answer is never less than “you should consider a radon barrier” as the surveyor cannot be sued for advising a barrier but can be if they say no barrier is required and it is found that the house fills with radon.

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Hi all, here is an image of under floor detail I recorded from a visit to a super home. The door was accessible from the basement and wood fibre was used.

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My situation is a little different and mineral wool is < £4/m2, Jute £6/m2 and Gutex is in short supply. As I am not going to be looking at it mineral wool wins as long as it is effective and readily available. I will try to document progress to share learnings.

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29

Nice example!

It would be interesting to find out what the modelling suggest is best for the thickness of the insulation and how far down the wall it should cover.

Anyone got access to static 2D thermal bridging software? Or know of a guide to how much insulation is worthwhile considering the height of the stem wall etc? The chances are it’s just whatever is pragmatic to install, but it’s nice to know there’s some theory backing it up.

I don’t like the tape from the roof membrane to the stem wall. Firstly, it should be a vapour barrier, integrating into the DPC and the air control layer of the floor above, as there’s no need for the stem wall to dry through the insulation into the cavity. I’m sure a vapour open roof paper is wrong here (which most roof paper is). Secondly, these tapes do not adhere well or firmly to old, uneven and dusty masonry.

I just don’t like taping in these situations, adhesion to different materials, no easy flat surfaces to compress the tape, difficult access - all looks very fiddly.

I would prefer a mechanical method, such as a batten, fastened to the stem wall, holding the vapour barrier up against the masonry with a continuous bead of bituminous sealant behind it.

@lloydham 's example, the cavity is only ~75mm. I would guess you could just fold the stem wall vapour barrier round onto the ground and use the ground as a surface to friction fit the insulation flush against the wall and floor system. A 25mm batten clamping the vapour barrier against the stem wall, bituminous behind it, with 25mm rigid insulation strip above it, with a second 25mm layered over the top of both. I doubt it’d be worth going to more than that.

*edit: Come to think of it, a liquid applied vapour barrier would probably be much easier. Use a water-resistant insulation system in contact with the ground and do away with the batten. I’d consider using a floor system ledger board (replacing the pocketed joist in the wall to avoid end grain rot) to clamp the air control of the floor to the DPC and stem wall liquid applied vapour barrier.

This is where spray foam starts to make sense - if it wasn’t such an awful product. It adheres to almost everything, fills complex cavities consistently, insulates, air seals and acts as a vapour barrier, all in one application.

Again, I don’t know how much we should worry about taping around the underside of the insulation and the joists - I respect the wind washing theory, but the relatively tiny draft in a suspended floor ventilation cavity does not concern me anywhere near as much as the same phenomenon in an exterior wall.

Just make sure you have insulation batts can elastically deform and spring back nicely into imperfect cavities. Cut the batts ~10-15mm too large, depending on the insulation, and slightly compress them in the cavity. Arch them upwards, against gravity, where possible. If you don’t have access to the crawlspace to fasten the insulation up against the subfloor, then chicken wire with a strip of insulation offcuts used to fill the slack in the mesh to hold the insulation firmly up that way.

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@Frank_Reif , what you say about the difficulty of taping to various substrates is very true. That is why I often suggest parging or priming first. Your suggested solution of holding in place with battening is fine as long as the join is airtight, which can be a real problem if you are looking for Passivhaus or EnerPHit certification but fine for just making improvements to the existing fabric.

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I’ll draw up a 3D model of all of this next weekend.

I’d like to know if it’s possible to add a final detail: a thermally broken ledger board continuous with the perimeter insulation of the under floor heating system.

There will be products for exterior applications for porches, decks and larger overhangs and the likes.

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A nice introduction to the science of crawlspaces is at New Light in Crawlspaces

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I have seen this article before, from a link elsewhere in this forum if I remember rightly.

The advice given would work in the UK but the option to completely open the underfloor space would probably hit building regs and planning issues over here. It could be done relatively easily for certain timber frame constructions. The house in the opening illustrations is timber frame, as are most New England homes.

Remember that the east coast of the USA is VERY warm and humid in summer

34

Improving Air Tightness of suspended floor.
We have a suspended floor which I have insulated with rockwool from below.

When Marianne came round, pre-covid, to do our Whole House Assessment she suggested using OSB board on top of our existing floor boards to improve the air tightness.

Has anyone tried this. If so did you use OSB or plywood. I am wondering if I can get away with a thinner plywood so as not to have to plane all the doors to fit.

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OSB is the go to product for airtightness. Potentially you could use heavy grade plastic sheeting with a thinner ply on top. remember to tape all joins, overlapping and taping the plastic sheeting if that’s the way you go.
By tape I mean appropriate airtightness tape that will stay airtight for years.

Making the floor airtight will improve your comfort and enable the insulation under it to do a better job.

36

Thanks Tim I’ll stick with OSB

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Interesting read folks… however, is there not nowadays a hefty, very-thick floor covering that can be used to substantially reduce heat loss thru suspended-timber floor? - albeit this would potentially put such a floor on ‘the wrong side’ of the insulation.

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Are any U values quoted for these floor coverings?

Ideally you want a U value <=0.15 W/(m²K). Remember too that the airtightness layer needs to be on the warm side of the insulation, which seems unlikely in the scenario you described.

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Excuse my syntax…! I meant, should there not be such a thing, nowadays - but I’ve not come across it, other than a thick rug or carpet. As you say, there could be (if it existed) a wrong-side-of-insulation problem.

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As far as the timbers are concerned it doesn’t matter if they are on the cold side of the insulation. It is the humidity (partially as a result of that) which is critical.

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If you are rich enough I suppose you could use aerogel as an underlay. I believe that a vapour impermeable version is available.