Planning to Lay a New Carpet? Save Energy, Insulate First!

John Adams
Thursday, 22nd December 2011

Buying a new carpet in the sales? If you are changing your carpet, now is the time to insulate under the floor. You can prevent up to 20% of your heat escaping, and pick up a bargain at your local DIY store. PM's John Adams explains how.

In the middle of topping up my loft insulation to current standards, my wife informs me that she has ordered a new carpet for our downstairs guest room. I had realized long ago that insulating under the floors would make a considerable difference to the comfort of the downstairs rooms, if only because it would stop the drafts through the cracks between my Victorian floor boards. But I really couldn't face the idea of emptying rooms, ripping up the floors, suspending insulation and putting it all back together again.

This room would have to emptied and the old carpet removed anyway so the task suddenly looked less daunting. The carpet gone I had a go at getting the floorboards up. A wiring access trap offered an inviting starting point and I discovered that the boards could be lifted with a bit of careful jimmy work and hammering from underneath. Unfortunately a sub-wall in the bay window had been built directly onto the floorboards so that end wasn't going anywhere. The solution was to cut through the boards as close as I could get to the wall. Luckily some were half-length and could then be easily removed. I cut a new piece of timber and nailed it to the face of the joist where the cut was so the relayed boards would have something to sit on. Actually a quicker and easier job than it sounds. This then led to the idea that if I cut the remaining full-length boards in half down the centre of the room I would create two work areas. Half being used for storage, cutting, etc. and the other being insulated. Don't be afraid, if you cut down the edge of a support timber you just have to nail on a bit of timber to provide a support for the loose planks. Do check for cables, pipes, etc before cutting though.

Now to the insulation. The recommended insulation methods for under suspended floors seems to be either 100mm of mineral wool, R=2.25, suspended in netting (I have never seen suitable netting on sale nor found any recommendations) or a layer of thermal foil laid over the joists. The latter doesn't have anything like the insulation, R=1.45 at best but offers better draft proofing. At present major utility companies like British Gas are subsidizing top up loft insulation, so you can buy 150mm, 170mm and 200mm rolls of mineral wool for about £3 a roll from any DIY store or builders merchants.

I already had several rolls of 170mm, insulation, R=3.85, I bought from B&Q to use in the loft, so I thought at £3 a roll, why not use that and go for super insulation. Instead of netting I stapled a semi-permeable membrane over and between the joists, overlapping the joints by about 100mm. I taped the first line of joints but as it didn't stick that well I just left the rest, the slight gaps will let it breath a bit. If you are going to try this yourselves I recommend a powered stapler and 8mm staples it's so much easier than doing it with a hand one. Next I measured the distances between the joists and marked these on a still wrapped roll of insulation. This allowed me to cut through the roll using a carpentry saw and get accurately fitting insulation. This was then laid into the membrane hammocks and allowed to fluff up.


Having started down this super insulation route, I decided to 'give the cat another goldfish' and do the thermal layer on top. I have bought several rolls from different sources and the quality varies enormously. I think B&Q's multi purpose is the best as it is much stronger than the competition, they do a cheaper one too but the R-value is only about 0.46 so you get what you pay for. I stapled this in place, again allowing good overlaps and not taping the joins to allow a tiny trickle of ventilation to the undersides of the floorboards. 

Now to put the floor back. At this stage I realized it would have been a good idea to number the boards as I lifted them but after a bit of fiddling I managed to work out the jigsaw and get them back in place.You have to be very careful not to accidently puncture the thermal film layer. If you do, and you will, repair with the silver tape sold alongside the thermal rolls. Instead of fixing the floorboards down with cut brads again, I opted for screwing them down with decking screws. This proved to be quick and effective and means any board can be easily lifted if required in the future. I now have a draught free super insulated area of floor with an R-value of around of 5.5 (not counting the floorboards, and the underlay and carpet layers yet go on top).

So that's half a room done, now for the other half! It took about a day to work out how to do it, clear the room, cut and raise the boards, lay the membrane, insulate and refloor. I think an average room should be easily do able by one person in a weekend and probably a lot quicker with two of you. Estimated cost £25 for half a room so about £50 in all.

If you are going to re-carpet a room and you have un-insulated suspended floors, it has to make sense to insulate them now. As you will probably be losing 20% of your heat through the floor this just has to add up.

Happy insulating. 


John Adams designs Permaculture magazine and writes the product reviews page where he tests tools, bushcraft gear and practical energy saving products. He has a lifetimes' experience working on DIY building projects, has made his own tipi from scratch and renovated a classic wooden yacht as well as various houses (plus many more artistic and craft projects).

N.B. This is not intended as a practical how to article but as an encouragement to eco-renovation. The methods used may or may not comply with your local building regulations and if you are unsure how to proceed please seek professional advice. 

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peter_r |
Tue, 03/01/2012 - 17:06
Wow, well done, I'm impressed (also by your product reviews and the design of Permaculture - what a hero you are). I've done this myself once, when laying a new floor, using Warmcel suspended in landscaping fabric, which worked fine. Incidentally, a useful tip for lifting the old boards with minimal damage is to use a nail punch to push the old nails down into the joists. This can help a lot if your boards are brittle and are splitting when you try to lever them up. When you re-lay the boards you obviously then have to put the screws to the side of where the old nails were. Another good tip, for getting the first board up without damage, is to use a circular saw to cut through the tongue all down both sides of one board (assuming your boards are tongue and groove - some very very old ones won't be). Anyway, a great article, thanks - I do so LOVE Permaculture mag for the range of topics it covers. Happy 2012.
SuzeH |
Sat, 04/02/2012 - 10:07
Loved the article. I've shown it to my partner and he seems to think it would be a good idea too (we will be renovating the bathroom this year and there are 2 other rooms needing new carpet in the future). I have read elsewhere on internet that there is no need to insulate floors upstairs... but surely insulation between the floors keeps the heat in the room it's intended for! We've also just had a porch put in. The floor is bare concrete at the moment and the roof only felted and tiled, but the heat it's storing even in this cold spell (-7°C last night) is great. I want to insulate the roof and put an insulating layer under the lino we want to put down, I just hope that doesn't make it extra hot in the summer! Only just realised Permaculture website was so active. Love the magazine and i think i'm going to love the site too! Hooray!
lgooding |
Thu, 29/11/2012 - 12:08
Great article, very informative. I've just finished my front room and followed the same method as described. However I used the Insumate plastic support tray for the insulation. No need to use the membrane. So far its worked well and the room seems a lot warmer.
JohnDoe |
Mon, 22/09/2014 - 01:30
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