Insulating a suspended timber floor

In a departure from my normal blogging, this post is going to be about how I’ve retrofitted insulation to some of the flooring in my house and improved its airtightness. This has resulted in a noticeable increase in room temperature during the cold months.

Setting the scene

The kitchen floor in my house is a suspended timber floor, built over a 0.9m tall sealed cavity (concrete skim floor, brick walls on four sides, air bricks). This design is due to the fact the kitchen is an extension to the original house, and it’s built on the down-slope of a hill.

The extension was built around 1984, shortly before the UK building regulations changed to (basically) require insulation. This meant that the floor was literally some thin laminate flooring, a 5mm underlay sheet for that, 22mm of chipboard, and then a ventilated air cavity at outside temperature (which, in winter, is about 4°C).

In addition to that, there were 10mm gaps around the edge of the chipboard, connecting the outside air directly with the air in the kitchen. The kitchen is 3×5m, so that gives an air gap of around 0.16m². That’s equivalent to leaving a window open all year round. The room has been this way for about 36 years! The UK needs a better solution for ongoing improvement and retrofit of buildings.

I established all this initial information fairly easily by taking the kickboards off the kitchen units and looking into the corners of the room; and by drilling a 10mm hole through the floor and threading a small camera (borescope) into the cavity beneath.

Making a plan

The fact that the cavity was 0.9m high and in good structural shape meant that adding insulation from beneath was a fairly straightforward choice. Another option (which would have been the only option if the cavity was shallower) would have been to remove the kitchen units, take up all the floorboards, and insulate from above. That would have been a lot more disruptive and labour intensive. Interestingly, the previous owners of the house had a whole new kitchen put in, and didn’t bother (or weren’t advised) to add insulation at the same time. A very wasted opportunity.

I cut an access hatch in one of the floorboards, spanning between two joists, and scuttled into the cavity to measure things more accurately and check the state of things.

Under-floor cavity before work began (but after a bit of cleaning)

The joists are 145×45mm, which gives an obvious 145mm depth of insulation which can be added. Is that enough? Time for some calculations.

I chose several potential insulation materials, then calculated the embodied carbon cost of insulating the floor with them, the embodied financial cost of them, and the net carbon and financial costs of heating the house with them in place (over 25 years). I made a number of assumptions, documented in the workings spreadsheet, largely due to the lack of EPDs for different components. Here are the results:

Heating scenarioInsulation assemblyU-value of floor assembly (W/m2K)Energy loss to floor (W)Net cost over 25 years (£)Net carbon cost over 25 years (kgCO2e)
Current gas tariff
(3.68p/kWh, 0.22kgCO2e/kWh)
Current floor2.60382308017980
Thermojute 160mm0.22327301700
Thermoflex 160mm0.21308601450
Thermojute 300mm0.121810201190
Thermoflex 240mm0.1117910820
Mineral wool 160mm0.24355401680
ASHP estimate
(13.60p/kWh, 0.01kgCO2e/kWh)
Current floor(as above)(as above)113701140
Thermojute 160mm1420290
Thermoflex 160mm1520110
Thermojute 300mm1410410
Thermoflex 240mm128080
Mineral wool 160mm1290150
Average future estimate (hydrogen grid)
(8.40p/kWh, 0.30kgCO2e/kWh)
Current floor702025090
Thermojute 160mm10602290
Thermoflex 160mm11702010
Thermojute 300mm12001520
Thermoflex 240mm10901140
Mineral wool 160mm8902320
Costings for different floor assemblies; see the spreadsheet for full details

In retrospect, I should also have considered multi-layer insulation options, such as a 20mm layer of closed-cell foam beneath the chipboard, and a 140mm layer of vapour-open insulation below that. More on that below.

In the end, I went with 160mm of Thermojute, packed between the joists and held in place with a windproof membrane stapled to the underside of the joists. This has a theoretical U-value of 0.22W/m2K and hence an energy loss of 32W over the floor area. Over 25 years, with a new air source heat pump (which I don’t have, but it’s a likelihood soon), the net carbon cost of this floor (embodied carbon + heating loss through the floor) should be at most 290kgCO2e, of which around 190kgCO2e is the embodied cost of the insulation. Without changing the heating system it would be around 1700kgCO2e.

The embodied cost of the insulation is an upper bound: I couldn’t find an embodied carbon cost for Thermojute, but its Naturplus certification puts an upper bound on what’s allowed. It’s likely that the actual embodied cost is lower, as the jute is recycled in a fairly simple process.

Three things swung the decision: the availability of Thermojute over Thermoflex, the joist loading limiting the depth of insulation I could install, and the ease of not having to support insulation installed below the depth of the joists.

This means that the theoretical performance of the floor is not Passivhaus standard (around 0.10–0.15W/m2K), although this is partially mitigated by the fact that the kitchen is not a core part of the house, and is separated from it by a cavity wall and some tight doors, which means it should not be a significant heat sink for the rest of the house when insulated. It’s also regularly heated by me cooking things.

Hopefully the attention to detail when installing the insulation, and the careful tracing of airtightness and windtightness barriers through the design should keep the practical performance of the floor high. The windtightness barrier is to prevent wind-washing of the insulation from below. The airtightness barrier is to prevent warm, moisture-laden air from the kitchen escaping into the insulation and building structure (particularly, joists), condensing there (as they’re now colder due to the increased insulation) and causing damp problems. An airtightness barrier also prevents convective cooling around the floor area, and reduces air movement which, even if warm, increases our perception of cooling.

I did not consider thermal bridging through the joists. Perhaps I should have done?

Insulation installation

Installation was done over a number of days and evenings, sped up by the fact the UK was in lockdown at the time and there was little else to do.

Cross sections of the insulation details

The first step in installation was to check the blockwork around each joist end and seal that off to reduce draughts from the wall cavity into the insulation. Thankfully, the blockwork was in good condition so no work was necessary.

The next step was to add an airtightness seal around all pipe penetrations through the chipboard, as the chipboard was to form the airtightness barrier for the kitchen. This was done with Extoseal Magov tape.

Sealing pipe penetrations through the chipboard floor using Extoseal Magov.

The next step in installation was to tape the windproof membrane to the underside edge of the chipboard, to separate the end of the insulation from the wall. This ended up being surprisingly quick once I’d made a cutting template.

The next step was to wedge the insulation batts in the gap between each pair of joists. This was done in several layers with offset overlaps. Each batt was slightly wider than the gap between joists, so could easily be held in place with friction. This arrangement shouldn’t be prone to gaps forming in the insulation as the joists expand and contract slightly over time.

One of the positives of using jute-based insulation is that it smells of coffee and sugar (which is what the bags which the jute fibres came from were originally used to transport). One of the downsides is that the batts need to be cut with a saw and the fibres get everywhere.

Some of the batts needed to be carefully packed around (insulated) pipework, and I needed to form a box section of windproof membrane around the house’s main drainage stack in one corner of the space, since it wasn’t possible to fit insulation or the membrane behind it. I later added closed-cell plastic bubblewrap insulation around the rest of the drainage stack to reduce the chance of it freezing in winter, since the under-floor cavity should now be significantly colder.

As more of the insulation was installed, I could start to staple the windproof membrane to the underside of the joists, and seal the insulation batts in place. The room needed three runs of membrane, with 100mm taped overlaps between them.

With the insulation and membrane in place and taped, the finishing touches in the under-floor cavity were to reinstall the pipework insulation and seal it to the windproof membrane to prevent any (really minor) wind washing of the insulation from draughts through the pipe holes; to label everything; insulate the drainage stack; re-clip the mains wiring; and tie the membrane into the access hatch.

Airtightness work in the kitchen

With the insulation layer complete under the chipboard floor, the next stage in the job was to ensure a continuous airtightness layer between the kitchen walls (which are plasterboard, and hence airtight apart from penetrations for sockets which I wasn’t worried about at the time) and the chipboard floor. Each floor board is itself airtight, but the joints between each of them and between them and the walls are not.

The solution to this was to add a lot of tape: cheaper paper-based Uni tape for joining the floor boards, and Contega Solido SL for joining the boards to the walls (Uni tape is not suitable as the walls are not smooth and flat, and there are some complex corners where the flexibility of a fabric tape is really useful).

Tediously, this involved removing all the skirting board and the radiator. Thankfully, though, none of the kitchen units needed to be moved, so this was actually a fairly quick job.

Finally, with some of the leftover insulation and windproof membrane, I built an insulation plug for the access hatch. This is attached to the underside of the hatch, and has a tight friction fit with the underfloor insulation, so should be windtight. The hatch itself is screwed closed onto a silicone bead, which should be airtight and replaceable if the hatch is ever opened.

The final step was to reinstall the kitchen floor, which was fairly straightforward as it’s interlocking laminate strips. And, importantly, to print out the plans, cross-sections, data sheets, a big warning about the floor being an air tightness barrier, and a sign to point towards the access hatch, and put them in a wallet under the kitchen units for someone to find in future.

Retrospective

This was a fun job to do, and has noticeably improved the comfort of my kitchen.

I can’t give figures for how much of an improvement it’s made, or whether its actual performance matches the U-value calculations I made in planning, as I don’t have reliable measured energy loss figures from the kitchen from before installing the insulation. Perhaps I’d try and measure things more in advance of a project like this next time, although that does require an extra level of planning and preparation which can be hard to achieve for a job done in my spare time.

I’m happy with the choice of materials and installation method. Everything was easy to work with and the job progressed without any unexpected problems.

If I were to do the planning again, I might put more thought into how to achieve a better U-value while being limited by the joist height. Extending the joists to accommodate more depth of insulation was something I explored in some detail, but it hit too many problems: the air bricks would need to be ducted (as otherwise they’d be covered up), the joist loading limits might be hit, and the method for extending the joists would have to be careful not to introduce thermal bridges. The whole assembly might have bridged the damp proof course in the walls.

It might, instead, have worked to consider a multi-layer insulation approach, where a thin layer of high performance insulation was used next to the chipboard, with the rest of the joist depth taken up with the thermojute. I can’t easily change to that now, though, so any future improvements to this floor will either have to add insulation above the chipboard (and likely another airtightness layer above that), or extend below the joists and be careful about it.

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