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How best to Insulate a Floor?

Updated: Feb 13

Downstairs picture of a 2 bedroom terrace house
Downstairs view of the living and dining room

I curse the day I made the decision not to extend our underfloor heating from our kitchen diner to our hallway. It would have been a lot of hassle to do it, but it would save us the shock of coming through to a hallway that feels like walking over an ice block. So for this project (pic above), even though underfloor heating is way out of budget (there is a perfectly good central heating system in place) I want to make sure the floors aren't the weak link in the thermal cocoon I want to make.

In 2022, the UK building regulations were updated to include revised minimum U values for new builds and renovations. To get into the performance of your insulation, like it or not, you need to get nerdy over the technical data sheets. I've tried to simplify it as much as I can but you need to hit these performance targets if you want your home to lose minimal heat. This means minimising the U-value.

The U-Value is a measure of how much heat passes through the given material expresses as watts per m² so these updates are designed to improve the energy efficiency of buildings and reduce their carbon footprint.

Below is a chart of those new regulations. I've also put in the R-value which is the resistance of a material or materials to the passage of heat (the opposite of U-Value). Most materials are measured with R-values so it's fairly easy to determine. Note the U-value is calculate by dividing 1, by the sum of all the R-values of the various materials. As a minimum I want to achieve the U-values of existing element in existing dwellings, but if possible meet the standards for a new build.

U Values - 2023 Building Regulations




Existing element in existing building




New Elements in Existing Dwellings




New Build




For each option I'm going to calculate the R value of the insulation. This is a measure of how good the material is at resisting the flow of heat through it. The higher the resistance the better. For more information on R-values, take a look at my blog post on insulation HERE

The R-Value is calculated as R = l/λ where l = the thickness of the material in metres and λ (lambda) is the thermal conductivity of the material in W/m.K.

Once I have the R values its easy to calculate the U value.

U-Value = 1/ (Sum of all R-Values) 

R Values - 2023 Building Regulations




Existing element in existing building




New Elements in Existing Dwellings




New Build




Around 15% of your heat is lost through the floor of your home so it makes sense to insulated it as much as possible. I need to insulate the ground floor rooms in a 2 bedroom terrace house. There is no need to do the 1st floor as this is inside the thermal envelope.

It comprises of suspended timber flooring in the dining room and a concrete floor in the kitchen and living room.

Floorplan of the downstairs of a 2 bedroom terrace house
Floorplan of the Downstairs

Kitchen and Living room (Concrete slab) = 25.5m2

Dining room & Entrance (Suspended Timber floor) - 11.8m2

Concrete slab

Concrete floor of terrace house
Living room floor

The sale documents fortunately included the plans of the works that exended what was a 2 up 2 down, into a 3 up 3 down in 1985. The work was done to a decent standard but i'm gutted that they didn't insulate underneath the concrete slab which only really started in the mid 1990s.

1985 extension plans for my 2 bedrom terrace house
1985 extension plans for my 2 bedrom terrace house

For reference, to achieve an R value of 4.54, there needs to be around 100mm of PIR insulation at 0.022 w/mK. (0.1 / 0.022). To achieve new build standards of R 7.69 there needs to be 170mm of PIR insulation (0.17 / 0.022).

To get to the right insulation levels for Options to deal with this are

Option 1 - Dig up at least 200mm of concrete.

If I was going to the trouble of this, I'd dig up enough to get to lay 170mm PIR then screed over the top. Although it would offer unbeatable insulation, this brings me out in a cold sweat thinking about the expense and hassle of this kind of job. It would add weeks to the schedule and thousands of pounds to the budget that I can't afford.

170mm Celotex £21 per m2 = £535

Labour = approx 15 days @ £200 = £3000

concrete for removal. £200 per m3, 0.2 x 25.5 = 5.1m3. = £1020

Concrete pouring at approx £100 per m2 = £2550

Total = £7,105

R Value = 7.72

Option 2 - Overfloor Insulation

Laying insulation board over the top of the floorboards. It's impossible to lay 170mm, or even 100mm of insulation on top of the floor without modifications as the door height is at the absolute limit. It is clear that except for the thinnest insulation of 25mm or less, there will need to be additional work to accomodate the door height and radiators. It's unclear just how easy or hard this would be.

Unless you're willing to raise the whole floor which has implications for moving the pipework, electrics and skirting, you have to stick with much thinner insulation board which isn't as effective. Better than nothing though and if it is draughtproofed, then there will be a noticable difference in temperature.

Measurement of standard door height
Door height

6mm Prowarm XPS Boards - 3 x 20 pack (10m2) = £90

Labour = Approx 1/2 day - £100

Total = £190

R-Value = 0.17

25mm Celotex TB4025 - 2.4 x 1.2m x 9 sheets = £121.50

Labour = Approx 1 day = £200

Total = £321.50

R Value = 1.13

25mm Kingspan Kooltherm K103 - 2.4 x 1.2m x 12 sheet pack = £480

Labour = Approx 1 day = £200

Total = £680

R Value = 1.32

For 50mm I would need to modify the door height and the radiators but will be okay with the electrics. I can't go any higher because of the front door height. The skirting is already coming off.

50mm Prowarm XPS pro Boards - 36 boards x £20.99 = £756

Labour = Approx 3 days - £600

Materials = £100

Total = £1456

R-Value = 1.61

50mm Celotex GA4050 - 2.4 x 1.2m x 9 sheets = £183.60

Labour = Approx 4 day = £800

Materials = £100

Total = £1083.60

R Value = 2.27

50mm Kingspan Kooltherm K103 - 2.4 x 1.2m x 6 sheet pack = £1117

Labour = Approx 3 day = £600

Materials = £100

Total = £1817

R Value = 2.63

My research led to speaking to a representative of Reticel who have Vacuum insulated panels which offer amazing lambda values of 0.006 W/mK! for context PIR board is 0.022 W/mK. This would mean that for 45mm panel you can achieve new build insulation levels in about 26% of the depth or PIR board. Unfortunately they need at least 30mm screed over the top which pushes me over depth. I also didn't have a proper quote because we both realised it wasn't possible but the sales guy told me they would be around £150 per 1200mm x 600mm panel.

Add to this the cost of having to pour screed over the top this pushes it into being much more than digging up the concrete cost wise which is disappointing. I have a few reservations about the panels in that they will lose their insulating properties if punctured or structurally compromised. The panels also cannot be cut to size meaning you will have a strip of poorly performing insulation around the edge. It does however save mess and time of the concrete removal.

Recticel Vacuum Insulated Panels - 45mm - 34 panels @ £150 ea

Labour = 3 days = £600

Materials = £5200

Total = £5800

R-Value = 7.5

Option 3 - Insulated the side of the slab

In researching options, I discovered that much of the heat lost through a floor slab is lost through the sides. Up to 25% according to a paper by Chen (2017). My own surveys have always revealed heat loss through the slab edge so it makes sense to address this.

My thinking is that if digging out the concrete is too expensive and laborious then why not focus on one of the biggest area of heat loss. The concrete itself is an excellent thermal store and is great for low temperature underfloor heating and it can also even out those more extreme temperatures in winter and summer.

However it is still much cooler than room temperature so we need to prevent heat loss. through it by digging out the perimeter edge and insulating.

100mm Celotex £12.64 per m2 = £37

Labour = approx 1 days @ £200 = £200

Total = £237

R Value = N/A

Suspended floor

Option 1 - Underfloor insulation -

Lifting up the floorboards and putting either solid PIR, mineral wool or natural fibres in between the joists. This is pretty invasive so creates a lot of disruption and mess. It is also much more expensive in terms of materials and time but you can typically get a greater depth of insulation under the floor which can save money in the long run.

There is also an option to lay a multifoil within the cavity which is supposed to have the equivalent performance to thick PIR board.

Underfloor PIR Between joist insulation

100mm PIR board insulation 2.4 x 1.2m = £37 per sheet (Insulation superstore and insulation4less)

Joist area

8.28m wide

Approx 8 joists @ 5cm = 40cms

Joist area 3 x 0.4 = 1.2 m2

4 sheets of Insulation board = £148

Wood for battons 16 x 3 = 48m

13 battons (25x50mm 4.2m) @ £4.38 ea = £56.94

Vapour barrier = £150

Materials Cost = £427

Labour - approx 3 days = £600

Total = £954.94

R Value = 4.54

Multifoil underfloor

Superfoil SF60 1.5m x 8m = £270

Labour = approx 2 days = £400

Total = £670

Core R value = 3.46

Claimed R Value - 6.53*

YBS superquilt 1.5m x 10m = £143.40

Labour = approx 2 days = £400

Total = £543.40

Core R Value = 1.35

Claimed R Value = 5.56*

  • Using manufacturers own calculations of a P/A (perimieter/area ratio of 0.3)

Hemp flax Insulation

100mm thick laid inbetween the joists 3 x packs = £208.80

Pro clima solitex membrane 1.5m x 50m = £295.80

Pro clima tescon vana tape 2 x 60mm x 30m = £61.63

Labour - approx 3 days = £600

Total = £1166.23

Core R Value = 2.56

Option 2 - Overfloor insulation

Similar to insulating over concrete section above

Verdict / Conclusion

The concrete slab is the most stubborn part of the house that isn't cost effective to insulate to modern new build standards. I have to make a compromise and think the best option is to overlay 25mm of PIR board before laying 18mm particle board to complete the subfloor.

To improve the insulation I can also cut out a 100mm wide and 100mm deep trench around the slab edges that the wall insulation can slot into that will help reduce heat lost through the slab edge.

Concrete - 25mm PIR board - 18mm Plywood

Choices Choices!!!

The suspended floor is a choice between PIR, multifoil and hemp. I was sceptical about multifoils partly because there have been some extravagant claims in the past that have overstated their performance. They do have BBA approval that gives an independant assessment of the core performance. This core value however is way below the claimed R-values from Superfoil and YBS. The multifoils suggest that it is the air gaps either side of the foil that boost the R-value to the numbers stated. Digging into the tests that BBA have conducted they show that air gaps do indeed increase the R-Value but only by around 0.5. A separate paper by Tenpierik and Haaselaar (2012) also finds that multifoils didn't perform better than an R value of 2.5 even when installed with the required air gap.

Added to this the core performance will also be compromised when compressed over the floor timbers leads me to think this isn't the option to go for. The BBA testing even measures the multifoil in its compressed state and for the YBS superfoil it has an R value of 0.23 which admittedly is over the joists but still isn't great.

The PIR board I will be using also has a reflective surface and their literature doesn't state air gap requirements so I'm assuming there will be an advantage there too as there will be a small gap between the PIR and the floorboards.

The PIR board will be a bit of a hassle to cut and fit, but it offers better performance for the money. The 4.54 of the 100mm added to the 25mm (1.32) and the 18mm plywood (0.17) gives a figure of 6.03 which is above the "New elements in existing buildings" and pretty close to the new build target of 7.69.

However the Department for business, energy and industrial strategy say that best practice for retrofitting suspended floor insulation needs to include a degree of ventilation to prevent excess moisture building up around the joists that will cause them to rot. In dry environments it is acceptable to use PIR board but this is Manchester!

The Solution

So I came across Ecological building solutions who offer sustainable products to insulate your home. The Hemp and jute fibres and made from Carbon negative Hemp, Recycled coffee and cocoa bean sacks and offer an themal conductivity worse than PIR but better than mineral wool (0.039 W/mK vs 0.044 W/mK) and will allow the joists to breathe. It is paired with a breathable "windtight, vapour open" membrane that will improve airtightness.

The 100mm of hemp/jute gives an R value of 2.54 which added to the 25mm of PIR board and 18mm Plywood subfloor gives a total R value of 3.84 which equates to a U value of 0.26. Respectable but not super insulated. The biggest differences will be that it will be fully draughtproofed so special care and attention will be taken to seal up all those gaps.

So there you go, there are always compromises when retrofitting an older property. We can't realistically insulated to the latest standards all the time, but we can get pretty close. The improvements will still be huge and have a significant affect on the comfort and efficiency of the whole house. The next blog post on insulating the walls will be much more simple and will definitely get up to spec so stay posted.

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