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Get snug with under-floor insulation

Posted by Tom Bragg on 28 September 2012 at 9:36 am

We have a wood stove in our sitting-room and normally get snug there on a winter’s evening, with the rest of the house often unheated. But the floor has been noticeably cold – it’s a typical old terraced house with suspended wooden floors above about half a metre of space, ventilated by the air-bricks around the house.  So our warm sitting-room has been separated from cold outside air by just the carpet and floorboards.

Anne Cooper of AC Architects recommended this as our next big job to improve our home’s energy efficiency and Alex & Howard Rice inspired us to do it ourselves. 

So we’ve just insulated under our sitting-room floor - a significant DIY project. Even if you prefer to have a professional do it, some of this information should still be useful.


First we took a look under some easy-to-lift floorboards to see into one bay:

A big choice was whether to lift the floorboards and insulate from above or to wriggle into this space and do it from below. We decided to insulate the sitting-room from above, but Anne insulated some our hall from below at the far end of this bay.  Our sitting-room is divided into 4 bays, separated by “sleeper walls”. A local builder makes holes in them to crawl from bay to bay, but this sounded beyond our competence. As our sitting room has a carpet and not very good floorboards boards, we preferred to lift them. Experts should be able to lift floorboards and replace them still looking beautiful.

In favour of lifting the floor:

Poor floorboards  Less worried about appearance- carpet

Tight, obstructed under-floor space
Other under-floor work: electrics, rot

In favour of insulating from underneath:

Good access under floor from cellar, etc Beautiful, bare floorboards  Tongue & groove boards – harder to take up
Willingness to work in tight spaces

English Heritage has a guide discussing this, especially relevant to old buildings. If you need to lift a suspended timber floor for other reasons, then definitely insulate under it.

We wanted a high standard of insulation after doing all this work. For new-build, current building regulations specify a U-value of 0.25W/m2K. By chance our proposed solution, using 200mm of glass fibre matches this, helped by having some insulation under the joists. Heat-loss through the joists is significant, especially when they are close-spaced like ours, occupying 16% of our floor area.

Because of this, 100mm of Celotex between our joists would give a worse U-value for us (0.39W/m2K), even though Celotex is a much better insulator than glass-fibre.  See Thermal Calculations Online.  We could have used Rockwool (mineral wool) or Sheep’s wool (more expensive), instead of fibre-glass. 

Your floor and situation will be different  - we hope this information helps your choices:

1. Taking up the floorboards

We managed without removing any skirting boards. But every floorboard needs cutting above the middle of a joist for a place to begin lifting it (unless it’s been lifted before). If your floorboards are tongue and grooved, like ours, you also need to saw between them to cut the tongue and be able to lift them. We did this with a power jig-saw, tilting the saw to avoid cutting the joists or to start a new cut. Beware of cutting any cables or pipes!

This let us do a pretty neat job, except for a few places where the blade hit a nail.  But it’s worth using new sawblades for this job, because they cut much quicker than old ones!

Before you start lifting boards, label every separate piece of floorboard so you can put them back in the right place. We found that this double-headed lifting bar was a wonderfully satisfying tool for prising up the sections of floorboard, but an old tyre lever and wrecking bar were useful for a few awkward boards in the corners. If the boards are stiff, it helps to loosen the nails to waggle the boards a bit, either waggling up and down with the lifting bar, or by waggling the board side to side (if its stuck under the skirting board for example).  Hammering down on board above the nails can also help. We also borrowed a special nail puller which was excellent for pulling the nails out of the joists once we’d lifted the boards. 

We left the boards that were alongside the skirting board because it would have been difficult to have lifted them without taking off the skirting board, but this didn’t seem to matter- it was quite easy to push the insulation and polythene film underneath.

When removing nails from lifted floorboards, hammer their tip with the board resting on a scrap of wood until the nail head is clear. This reduces splintering of the floorboard’s good side. Save cut nails for re-use, straightening them by hammering against a paving slab.

2. Preparations:  fixing the netting, etc

While your floorboards are up, it’s an ideal time to: • Find and fix any other problems, like rot - we had to! • Add extra electrical outlets • Remove any unused cables or pipes and label those in use • Insulate any hot pipes
• Leave a time capsule for people to discover many decades hence.

Ideally pipes and cables will sit below the insulation, but it can be fitted around pipes. Cables should only pass through insulation for half a metre: more and they can overheat.

If the room has external walls with airbricks to ventilate under the floor, make sure they are clear and have good airways to under the insulation. You probably need to make ducts from the airbricks to below the insulation. We used thin aluminium.

Garden netting is normally used to hold fibrous insulation between the joists, but as we wanted to insulate under the joists too, we chose “Thermacrop”, a tough netting sold as a garden fleece. Compared to open netting it should help keep draughts out of the insulation, while still being porous to water vapour, which is essential. After cutting it into strips the width of the bay, we glued folds to hang off each joist, to prevent gaps between each pocket (see drawing). Normal hot-melt glue melts Thermacrop, but Cool-melt glue, as used in schools, etc works fine: mark it where you need to glue and align the 2 layers over some newspaper. Squirt glue along the line and press it down quickly with another strip of newspaper. This squeezes the glue through the 2 Thermacrop layers, bonding them together. Fix these folds at the right height on each joist with stainless steel staples (avoiding the possibility of rust!)  At the ends of the bays, depending on the position of the last joist, you may need battens screwed to the wall to staple the netting to.

3. Adding insulation - with variations

We used nearly 3 rolls of 200mm thick glass fibre, which was £13 a large roll from B&Q, thanks to the CERT subsidy for DIY use (which will expire in December 2012). Buy it quick!

Saw the glass-fibre rolls, without unwrapping them, into the width of the net pockets. Lay it roughly in place and cut it to length with scissors. Tear a strip off the edge of the glass-fibre where the joist will be and arrange it to fill the pocket, fluffing it up to reach floorboard level.

We also used one small roll of homeECO Insulation, made from recycled PET bottles: unlike glass-fibre, it can be handled without itches and so is good for filling tricky gaps or insulating from below the floorboards. It’s extremely hard to saw widths from the roll, but otherwise is easy to handle and to tear into appropriate strips. 

We took the opportunity to insulate under part of our hall, from below the floorboards. Use a good vacuum cleaner, dust masks and lighting!  A few dust sheets and a camping foam mat to lie on made it nearly comfortable! Anne first stapled pieces of polythene under the boards and down the sides of the adjacent joists – not a perfect seal, but it should cut most draughts. To the same design as the rest of the floor, she then held 200mm of the homeECO Insulation in place by stapling thermacrop netting around it. It’s not a job for the claustrophobic or asthmatic, but by using the non-prickly homeECO insulation it was much less unpleasant than we’d feared.

4. Sealing 

Good insulation needs to be complemented by a really good seal of the floor and its edges. We covered the whole floor with polythene sheet stapled over the joists, sealed at the edges with sealant.

In fact we did the whole job in 2 halves, to always giving us some floor boards to work from. Pro-Clima sealing tape is expensive, but with powerful, long-lasting adhesive, it’s worth it. We used it to join the polythene sheets and to help seal the edges. We cut the polythene to curl around the ends of floorboards under the skirting boards, just short of projecting back into the room. When the boards were nailed down, we applied a bead of clear sealant all round the room between the skirting board and the floor to complete the seal.  In a few places where there were large gaps between the boards near the skirting board, we filled the gaps using brown papers bags mashed up with sealant.

5. Making good

Put the floorboards back and check they all fit. We nailed most of them them back in place, reusing the cut nails, but we screwed down every fifth board so it would be easier to lift them in the future. We labelled the position of these boards under the edge of the carpet.

It cost us about £100 to insulate under our sitting room, roughly 3.6m square, plus some of our hall. It took the 2 of us most of a week, including some other distractions and about a day for fixing the rotten timber we uncovered. It seemed a big job – we were learning the techniques, and was very satisfying.

Energy Saving Trust gives the likely savings from under-floor insulation (for all the ground-floor of a typical house) as £60 per year with a DIY payback time of about 2 years. They estimate CO2 savings at 240kg/yr.  For our sitting room, mainly heated with scavenged firewood, this doesn’t quite apply, but we expect a snugger room, while burning less fuel.  

More information about insulation from YouGen

Insulation information page

Wise up when buying solar, double glazing and insulation

Is fleece or board best for underfloor insulation?

Insulation is king

Find an insulation installer

About the author: Tom Bragg is chair of Cambridge Carbon Footprint, which works with people towards their low-carbon lives. He and his wife, Anne, enjoy steadily improving the energy efficiency of their Edwardian terraced house.

If you have a question about anything in the above blog, please ask it in the comments section below.

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10 comments - read them below or add one

anne miller

anne millerComment left on: 27 April 2017 at 12:48 pm

Hi Sarah. 

As the underfloor half of the "Tom and Anne" team, have a look at the new article here, which describes a new method we worked out, only lifting one board every 1.2m and doing it basically from the top. (we have sleeper walls supporting the joists every 1.2m)

This method is v suitable for doing a bit at a time, spread over several evenings.  Each "bay" in our hall only took a few hours, but our hall is on average only about 1m wide.

The slowest bit was getting the board up without damaging it,  so if you have full access to the underfloor without needing to lift so many boards, that would make it much quicker.

You'll need a nice long relaxing bath afterwards you've been down there tho!

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Tom Bragg

Tom BraggComment left on: 27 April 2017 at 9:48 am

Yes, Sarah, it's less work if you can insulate your floor from underneath.  We've done this in some areas with Celotex, carefully cut to fit between the joists, and wool or fibre insulation (from recycled bottles) in awkward spaces like by the walls and where pipes block the way for Celotex. Both need securing in place, without air gaps.  Glass-fibre or mineral wool are too irritant to work with from underneath!

As you say, where mains electric cables are surrounded by thermal insulation for more than 0.5m, their current rating is halved.  This is discussed on Green Building Forum, with the general advice to secure the cables below the insulation, if possible. If not, fixed to the floor or to the side of the joists at the top, with an air gap around and then insulation below.  It may depend if cables have enough spare length to re-route them. Pipes can go in the insulation.  We found some redundant, disconnected cables that we could remove. An AC voltage detector helped identify these, but be sure they are redundant! Similarly with pipes.

Your pipes and cables may force some compromises with insulation in particular areas, which takes time, but I think a few weekends is about right. It helps to have good illumination, masks, a foam mat to lie on and someone above offering encouragement, support, tools & materials.  As an asthmatic, that's my role!    

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Sarah woodhouse

Sarah woodhouseComment left on: 26 April 2017 at 9:24 pm

We are going to have a try at doing this from our crawl space.  Just a few questions: all our services like gas electricity water pipes are in that space.  Some of the wires drape across the space, and some of them are tacked along the underside of the boards.  The pipes are the same.  Do these need relocating? I saw that there is a risk of overheating if the electric cable travels any distance within the insulation, but if we are going  to try to cover the joists to cut down in thermal bridging not sure where we can put them.  Might the water pipes be better within the insulation?  Also, wondering how long it might take (roughly). We have three large rooms the biggest 6m by 9m. A few weekends? 

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Tom Bragg

Tom BraggComment left on: 20 October 2013 at 6:00 pm

Hi Jon,

I've updated the original article (whihc has pictures), commenting on the underfloor insulation's good performance last winter. 

Putting a vapour barrier on the warm side of the insulation should be no problem - in our case polythene between the floorboards & joists. You probably know not to put a barrier on the cold side, when warm air in the insulation is likely to condense on the cold barrier.

A vapour barrier (or breathable mebrane) has the great advantage of stopping all draughts between the floorboards - its edges need sealing too! There's some discussion on which is best on the Green Building Forum .

All the best,


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JonCageComment left on: 20 October 2013 at 11:18 am

Great article Tom and Anne,

I have a few questions though:

1) You mention a drawing in step 2 but I can't see one anywhere?

2) After a year, have you had any problems or are you still enjoying a nice warm sitting room?

3) Do you think a vapor barrier is worth adding above the insulation?

I've been thinking of doing the same thing to our loune so any advice would be very greatfully received.

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rgreigComment left on: 20 January 2013 at 5:36 pm

I am interested in whether there are any potential future issues with using polythene, which is not breathable and will create a vapour barrier. Will this not potentially lead to rot if condensation forms between the polythene and the floorboards?


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Sims Solar Ltd

Sims Solar LtdComment left on: 7 December 2012 at 2:41 pm

Eternal, while buying materials might sound a cheaper option, you'll be paying with VAT @ 20%. professionall installed VAT will be 5%.

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LgoodingComment left on: 7 December 2012 at 12:05 pm

I think there are also some interesting products coming onto the market to make installing floor insulation even simpler:

I recently found via google: the Radiant Panel from Insulation solutions and the Insumate tray.

May be worth a try to see if they add any air tightness or thermal advantage. 

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EternalComment left on: 5 December 2012 at 12:50 am


I was looking into insulating my living room as well. However I am not very good at DIY nor have the time. Does anyone recommend someone that does underfloor insulation in London that's not expensive?


I am prepared to buy the materials myself...


Any help would be appreciated.


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muymalestadoComment left on: 28 September 2012 at 12:08 pm

Tom and Anne - you'll be quite snug after this effort.  This is very well described, and from similar experience, will be well worth the disruption and expense.

You have emphasised the need to be careful and thorough and to use the best materials.  We didn't, all those years ago, so we can see / feel our lack of thoroughness, which you advise against.

Next you will opening the sitting room door to let heat into the rest of the house.

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