Underfloor Insulation

WageSlave_3

Hello all.

I am in the process of renovating my property and with the soaring price of heating oil (+100% in a year), I am focusing on reducing my energy use as far as possible. I've had a log burner installed and am having cavity wall and loft insulation done fairly soon.

I recently had to lift some carpets to repair a broken floor board and I noticed cool air enters from under the floor. I have fairly ancient underlay and carpets (replacing them is someway down the road) but I wonder, more in curiosity than anything else, if anyone has had insulation installed below the floor? I have a decent enough crawl space so access is no real problem. The floor beams are of a standard spacing.

I'm mainly interested in knowing if it is possible to do, if it is of any great benefit and whether the cost is worthwhile? Heat obviously rises, so I'm not entirely convinced it would be worth it, but as they say, every little helps!

WS

Fred56

Almost certainly be worth it. Suspended timber floors are amajor source of heatloss. You should aim to get the U value of the floor down to at least 0.25. In common insulating materials the following would be the approx thickness

Glass or mineral wool (rockwool) 200mm
Rigid expanded polystyrene 135mm
Rigid polyurethane (kingspan or Xtratherm) 95mm

Aim a bit higher because the wood joists conduct much more heat away than the insulation. For example about three times the rate of conduction of mineral wool. If your joists are spaced at 400mm centres they will constitute 11.75% of the area so have a really significant effect.

I don't know how deep your joists are, they tend not to be in old houses and often have supporting wall at mid-span. If they are not deep, say 100mm, you could extend them downwards to give more depth.

The ideal way of doing this job is to lift the boards, install the insulation, then a vapour barrier and then put the boards back directly on the vapour barrier. The vapour barrier keeps the warm, vapour-laden air in the warm side of the construction. This is important if you want to avoid condensation forming in the structure (interstitial) and causing rot. However, that may make the job bigger than you like and you do have the crawl space to exploit. I'd suggest that, if you leave the floor insitu, you use mineral wool and close across the bottom of it with a breathable membrane of some sort.

Make sure you get all the gaps filled with insulation. There no point insulating if the cold air can just go round it. One of the biggest heat losses in houses is due to air infiltration. Try to close as many sources of draught as you can.

You may want to lead a vent up somewhere your wood burner - it needs a appropriate airflow which depends on it capacity. Check the datasheet.

Watch out for the area of wall below the damp course. I know they say rockwool does not wick moisture but isolating the wool from the wet wall won’t hurt

Point to note.
Ensure you maintain a clear airflow in the void below, this is very important as you don't want you joists rotting or nasty growths on the substructure. Make sure the airbricks are clear and that the air can get from one side to the other. Wang in more airbricks if you like but use pukka cavity sleeves and make sure all the airbricks are vermin proof because mice love insulation.

You can always throw yourself on the mercy of the Building Control Officer. These people are engineers and are usually happy to help, at least they are where I come from.

thundercats

Hi

Just saw your post , i insulated under my suspended timber floor a fortnight ago

In my case it has made a quite a difference to my 20's semi. we had underlay, carpet and boards, and laminate in the hall, took about 1/2 day a room (my rooms are large), its killed the draughts , room now holds its temp much longer, and floor temp is higher too, checked with on of those infra red laser thermometers, a useful toy to find cold spots!

It actually took quite long to do as as i only had 25 inches clearance to the bottom of the joints, from the ground, i could do a room in about 5 hours, its fiddly having no space and the brick piers in the centre of the room don;t help. so if you have standing room its should be much quicker. either way i found its easier than doing loft insulation on the hottest day of the year, and having to haul rolls of insulation into the lost by your self


here is what i used

200mm spaceblanket from knauf ( its encapsulated) currently on BOGOF at B&Q
put silverside to mother earth side, an orange to floor boards think it was £5.99/roll

theres usually lots of this stuuf in b&q and often some are outside of the plastic cover, ask B&q for a big discount, if you see this- maybe a couple of minor tears, just fix with tank tape

1 Staple gun ( heavy duty) and long staples - i used a stanley -B&Q
1 50m x 1m wide role of orange plastic safety fencing ( to hold the insulation up) £16.95 -ordered off the net - its the stuuf you often see at car boot sales, or building wworks

1 worklight
1 roll of tank tape - seal up ends of eachroll - to keep dust /dirt down while installing!

in my case the insulation went up relatively easily, moving in a confined space is a real problem,so did a quarter of a room at a time due to the brick piers in the middle. and i cut the safety netting into usable lengths at first, until i got the hang of what i was doing.

* Ensure all leccy cables are ground side and not hidden by insulation*

Basically ,staple one end of netting to rafter, push up insulation , pull across netting, staple up other side move onto next rafter, repeat until complete, then go to other side of pier and repeat, then remove excess of each roll or add if necessary to be later used to block up between rafters and wall, bottom of door frames etc etc

i wouldn't even contemplate using the traditional loft insulation rolls under a floor, if there is no headroom. Those rolls are 1m wide , thats quite a big hole u need sawing into your floor, if ur not planning to take boards up, never mind the hassle trying to cut into the right width! space blanket rules on this job!

i did consider celotex boards , but from my experience - old house . lots of electric cables, ch pipes etc it would have been far more difficult to do and its far more expensive using celotex than using space blanket when its on BOGOF. i bought a single celotex board for where my "trapdoors" were



hope this helps

only the kitchen to do - i ran into a bees nest under my cooker so just waiting for the cold to kill em of before i finish off the whole ground floor

Dave

ps i bought my loft insulation off ebay £90 for 10 rolls of 170mm knauf mineral wool including delivered - (B&Q at the time was over £20 /roll )- just happened the seller was local to me...I gave up trying to get a deal through the energy companies as their deals actually arent that cheap if u can diy

sebastianj

Very useful info I must say, I did not even know under floor insulation is done, so you only staple the net and not the insulation? sounds like a big job lifting floor boards etc, hope I can sleep tonight.
seb

thundercats

yep

you staple the net -that takes all the weight of the insulation

superdon

Is this the stuff you used - Knauf Space Blanket Loft Insulation (T)200mm?

Its £5.99 at B&Q

Does it go between the joists? What if the gaps between joists are wider or narrower than the roll of space blanket?

thundercats

yep thats the stuff i used

mine was a close fit give or take an inch it sits quite well as its resting on the netting

WageSlave_3

Thanks, Fred and Thundercats. I had a feeling (especially after researching further) that this was a big, big job and considering we're towards the end of summer (did we actually even have one), perhaps this is one for next year. I've got myself into a situation where I have started a lot of jobs but not really finished any of them, so starting yet another job will only add to the problem and prevent me getting to the others.

I suppose renovating a house is one very long project and especially so when work demands are very high on my time at the moment.

WS

silversnoops

calling thundercats....
What to do in my old house where very few of the gaps conform to 37mm width Knauf rolls?? I have lots at 41mm and even one at 54mm.
Please don't ask me to cut up the metre wide stuff....

latecomer

Thats the trouble with old houses, things are not always uniformly spaced out. I did this recently under the floors but I used the cheaper Knauff loft insulation and cut it to suit. I used a respirator that I bought for when I was spraying and didn't find the job that bad. Ok it wasn't pleasant but it was worth it to me for the cost saving. The standard width (abuot 370mm) was close enough for most what I did.

rgvkid

hi all,
i am just about to insulate under the floor in my front room (about 4 ft. crawl space),got the B+Q reduced insulation,sadly couldnt go for the foil stuff as the joists are too wide.
has anyone used some sort of membrane sheet once the insulation is in place? or just put insulation in,netting to hold it and that's it ??
also has anyone seen the spring wire clips which can be used to hold it in place? looks a lot easier than stapling netting,but would assume they are quite dear in comparison.
many thanks,
col