Situation: Basement floor, 175mm reinforced concrete over hardcore on
clay but ~2m underground. I want to put travertine tiles (12mm) over
electric underfloor heating for comfort in the winter and primary
heating in the summer when the rest of the house is warm anyway.
The U-value calculator say's given the area and perimeter that U-value
excluding floor coverings is 0.67. Manufacturers of insulated tile
backing board (marmox etc.) say that even with 6mm of insulation under
the heating elements I will feel the benefit of quicker warm-up and
lower running costs. The U-value calculator says that with 6mm of
insulation the overall U-value is ~0.63 i.e. not very different.
Since the heat-loss from the room is largely unchanged I can't believe
the argument about reduced running costs is true? Certainly a gain of
0.04 in U-value doesn't justify the £400 cost of the insulation. Is
there something I'm missing?
On warm-up time surely 12mm thick tiles will still heat up faster than
the 175mm concrete so even in the warm-up phase the effect of the 6mm
insulation will be pretty much nothing. Again am I missing something,
apart from the desire to get the cost down?
I accept that if I had 50mm of insulation it would make a difference,
but this is a basement with finished headroom of 6'6" so I don't have
space to play with.
Should I spend less money (since it's cheaper) on a bit more wall
insulation to keep the overall heat loss down?
Personally, I'm not sure you really have space to install it as is. You
definitely need the 6mm, and you really need the 50mm. Would it be possible
to dig out the floor and reinstall something more suitable with DPM (you
don't mention one) and 50mm of kingspan? You could even install wet
underfloor, then, which will be much more efficient.
Remember that because the heating is buried in the floor, you get direct
conduction of that expensive and environmentally unfriendly electric heat
right into the ground, so the u-Value is much more significant than a
standard floor, which is unlikely to be excessively problematic even
We are definitely singing from the same hymn sheet here, Christian.
I've got 50mm polystyrene, and if I did it again I'd use 70mm celotex.
My problem is a suspended concrete floor with an icy draught
underneath... I may block up the vents once the BCO has buggered off -
well some of them anyway.;-)
Sopudns about rioght. You don';t want 6mm of exep-snice crap, you want
60mm of expensive insulation. Polystyrene, or best of all Celotex or
The rugs that you pit down on the tiles for one..the sofa that you sit
on for another..I've got UFH and UNDER THE SOFA its about 10 degrees
warmer than UNDER THE RUG which is 5 degrees warmer than OVER THE
LAMINATE which is 5 degrees cooler than the tiled parts...
The amount of energy that goies down3wards relative to the amount that
gies uopwards and is useful, is entirely down to the relative U values
in each direction. A big sofa on a rug is a pretty good insulator. To
get te heat to sone up you have to stop it going down.
Then get digging. For my money, putting in UFH with any less is a waste
of your time and your money.
With building regs forcing the walls to be about 0.1-0.3 U valuse, 0.63
is going to soak away about 70% of the heat before it even gets into the
And with electricity at 13p a unit or whatever, thats a damned expensive
way of making the earthworms cosy.
No. Either do the job properly and excavate and fit 50mm+ insulation,
or scrap the idea of UFH and tiles, and fit a THICK carpet and some
I am the biggest fan of UFH there is, but the underfloor insulation on a
ground touching floor is MANDATORY - its not an option, its part of the
Digging down further is NOT an option. Not because I'm lazy (I already
dug it down 500mm over a 25m2 area. It's an old house and the cellar
walls don't reach floor level. The structural engineer proposed a
reinforced bath-tub as earth retaining but didn't want me to dig down
more than absolutely necessary due to the undermining effect on the
house (which didn't fall down so that was good!).
Anyway I don't expect there's many volunteers to come and dig up 175mm
concrete with A142 mesh in it.
I'm still not convinced by the figures. Google it and you'll see that
large concrete floors don't require under slab insulation as perimeter
effects dominate heat loss. I accept mine's not big enough for that to
be the case but if you do the calculations adding 50mm of underslab
heating reduces U value to 0.3, which given the loss through the walls
isn't significant. I'm not subect to building regs part L a) it's
existing and b) it's listed and they accept that part L standards can't
always be achieved in a manner sympathetic to the aesthetics of the
building and can cause real damage to the fabric by messing around with
moisture equilibrium levels.
To be honest compared to the rest of the house it's the best insulated
room because the ground has an effective insulation value particularly
in cold weather as it's always at ~12C once you get more than a meter
below. The u-value calculator I used had special settings for basement
floors and walls including data on average ground temperatures across
the UK. I accept that warm-up is based on relative u-value above and
below the heating, but that's why I don't think 6mm insulation makes
any difference since it doesn't significantly shift the u-value below
the heating elements
I also stick with the argument that when the room is at a steady
temperature the heat-loss calculations are valid for underfloor heating
as for rads, if not why not?
If we take the finances. If you believe (and feel free to argue, but I
want to know your theory not just an insulation is good mantra!) that
heat loss is the same, and it's just warm-up time, let's work it out:
Additional cost for insulating (excluding the social effect of reduced
headroom) is £400 if I use 6mm Marmox. At the quoted 13p/kWh
(incidentally I'm paying 10p) this represents 3076kWh. Total heat loss
from the room is 1700W (assuming no additional wall insulation) and
output from UFH is 2000W. If it takes an extra hour to warm-up then
this is 1500 days running, which is a minimum 5 yr payback time based
on expected usage. However since heatloss is unaffected and the floor
acts as a better storage heater without the insulation I should be able
to turn the heating off 0.5 hrs earlier increasing the payback time to
10 yrs MINIMUM.
By the way I have radiators in the room anyway as I said in the first
post as primary heating in the winter, the UFH is just to stop the
little ones from getting cold toes since it's their playroom. For the
radiators I'm absolutely convinced that the only consideration is the
total U-value of the surfaces, floor and walls.
What I really want you all to explain is how the extra heat gets out
given that the U-value doesn't change with the addition of the 6mm
Come on take up the challenge show (not tell) me how I'm wrong because
I don't see it in the posts so far.
For those interested in facts not hearsay take a look at
This states clearly in the intro that insulation is NOT required for
all ground contact floors. For my basement at 2m depth 0.8
perimeter/area ratio, floor value from the table is 0.56 and walls are
0.58 (R=0.5) for no insulation. This is pretty close to what the
u-value calculator said.
The problem is not with having the floor insulated for general u-Value
reasons. As you have found out, floor insulation is rarely that critical.
The problem is burying the underfloor heating in it. Because the floor is
now hot, rather than the 21C to about 10C temperature difference, it is
coping with something like 30 or 40C to 10C, so the heat loss could be three
times as much. Coupled with the fact that it is electric, I would say that
underfloor heating is not appropriate. Have you considered a different
What you haven't calculated, and for which there are no tables that you can
use is that cold strikes upwards from the floor, and being in a cellar, the
walls too, you seem to have talked yourself into having no insulation
despite everyone here telling you that you'll need it....no one is going to
agree with you because I think everyone here knows that come December, this
basement is going to be uninhabitable at night time, and certainly most of
the daytime too if we get a cold spell but it's up to you in the long run
Which cold is this that STRIKES from the floor. Average temperature
1.7m below ground in the UK varies between 10C and 16C. So in the
coldest month which is January not December the situation is that you
have a room where the equivalent outside temperature is 10C and there
are no windchill factors (since it's not very windy underground), so in
fact there's very little difference in the temperature in the basement
all year round. Which has the great advantage that it stays cool in the
hottest days of summer and is NOT uninhabitable in the coldest days of
Anyway I'm not talking about no insulation, I'm talking about doing it
where practical so I will be putting some insulation behind the
dry-lining but not so much that I generate lots of condensation.
Reconsider the comparison to the rest of the house. Uninsulated
u-values for the basement are ~0.6 all round. Elsewhere given solid
stone walls the average wall u-value is >2 so it's there where I'm
pouring heat out of the house to try and warm the town (and of course
the planet). If you're right about the basement being uninhabitable in
winter (which of course you aren't) then I should probably abandon the
whole house to the rats since you clearly think it's not a viable
This group is really good on practical issues but seems to fall over at
least a little (don't want to be too rude) when it comes to theoretical
analysis. Maybe I should try alt.climatechange or something like that,
maybe theres even an alt.thermodynamics group!
Insulation helps prevent condensation. It doesn't cause it.
If you propose an ELECTRIC underfloor heating system with no insulation in
alt.climatechange, if such a group exists, you are likely to be firebombed
by environmental activists.
Remember in your calculations that your basement floor is (a) at a much
elevated temperature and (b) loses two air<>solid interfaces. It is solid
throughout, enabling straight conduction all the way through without having
to swap from solid to air, which provides considerable insulation in its own
right. Typically in an uninsulated floor situation, the floor surface
temperature itself is significantly lower than the room temperature, further
reducing actual heat loss.
You're probably right about alt.climatechange! So with your sensible
advice Christian can I make a summary of the sensible points.
1. For efficient heating of the space overall I should stick with
radiators since these are running on gas anyway, therefore I'm
producing less carbon if I run uninsulated on rads rather than pretty
well insulated on the electric UFH.
---This is OK and what I plan to do anyway
2. If I stick with putting in the UFH for comfort heating then I am
wasting my money with the 6mm insulation, since most heat comes from
the rads so payback on the insulation is somewhere well over the
horizon, probably beyond the lifetime of the installation.
---This is OK since I don't have the money or the headroom.
3. For control of the UFH I should use the floor temperature sensor and
set it as low as possible (say 20C) to keep the duy cycle to minimum so
it just does the job of keeping the floor at a comfortable temperature.
Are there any other points I'm missing.
One more area of interest.
The issue with insulating the walls is that it moves the point where
the condensation is generated. Basically uninsulated any condensation
forms on the inside walls of the basement. If I insulate it then forms
on the inside surface of the wall membrane. This is why I didn't follow
the installation advice for the cavity membrane installation. The floor
membrane laps up the walls inside the wall membrane to ensure that
water from the walls (and there is some) drains behind the floor
membrane and ends up in my sub-floor drainage which is pumped away.
They recommend sealing the floor wall joint with waterproof tape. I
couldn't see the point since the top of the membrane is unsealed and so
water vapour can escape at the top anyway (dealt with by heat-recovery
ventilation and dehumidifier if necessary). My thinking was that if I
don't seal the joint then any condensation which forms on the inside of
the wall membrane should (on the whole) run down the gap between the
wall and floor membrane and so end up in the drainage system rather
than on the floor or at the base of dry-lining.
Does this seem sensible. If I'm going to seal the joint now would be
the time as I've put up the stud framework (inside the membrane) but
haven't boarded it yet.
It's been a fun thread!
However its normal to put a vapour barrier on the inside of the
insulation in this sort of situation.
Ventilation then gets the moist air out before it can condense on that.
On outside walls you generally have plasterboard with foil backing, then
insulation, then stone..
The trick is to have the vapour barrier where its too warm for condensation.
And allow the permeable outer walls to breathe outwards.
Controlling vapour is crucial when insulatiing.
What is the intended finished floor surface? If carpet or wood based, I
wouldn't use UFH at all.
Certainly. Personally, if I really wanted UFH heating in there and there
really was only a choice between 6mm and 0mm insulation, I would still go
for the 6mm. Note that 10mm boards are the same price and the additional 4mm
What you haven't considered is that if the insulation is adequete, then the
wall temperature will be much higher. Provided the ventilation is up to
spec, which seems to be very much the case with heat recovery ventilation
mentioned, then the relative humidity at the vapour barrier will be much
less than 100% and no condensation at all will occur. As the temperature
drops through the insulation, what little air (with closed cell insulation)
that gets through will be very low in absolute humidity due to the vapour
barrier, so no interstitial condensation will occur there either.
Thanks again, I'm signing it off now. I would put the 6mm in if it
weren't so damn expensive but 10mm is really pushing the headroom issue
(partly because the guys who poured the floor were worried about not
having enough concrete so ordered the extra 0.5m3 which meant the slab
ended up about 10mm thicker than planned so I had less to play with
than I thought. BCO is very understanding about stair headroom and
stuff like that but I don't want to push him too hard. The problem with
the condensation is that the ventilation is inside the room (for the
occupants) and doesn't ventilate as well the space behind the
insulation and in front of the membrane. Essentially there is likely to
be a humidity gradient through the wall.
Anyway the arrangement of the membrane should ensure that condensation
on that interface isn't an issue so I will be insulating between the
studs for the dry-lining. Before anyone comments on bridging by the
studs and using over stud insulation, I know, I know but I don't want
to keep bringing the walls in. Anyway U-value for the wall construction
even accounting for the bridging is pretty good (I'm going for ~0.3
although heat-loss calculation was based on 0.6). I'm really not going
to notice this room on the heating bill.
Thankyou to everyone.
The vapour barrier and insulation will prevent condensation coming from the
U-values are not the only reason to avoid cold bridging. One of the main
problems is that you will get localised cold lines, which will lead to
condensation where the studs are. The condensation might not be excessive,
but can often lead to obvious staining from moulds. Can you do the final
"plasterboarding" with something like Kingspan K17/K18? This is most
typically done using a 37.5m board (25mm insulation+ 12.5mm plasterboard).
However, thinner boards are available and will make a significant difference
to the cold bridging situation.
I know all that..been there dome that etc. However for that to be
effective - using soil as an insulator effectively - you need a LONG
time to build up temps.
Because the UFH is not *at* room temp., Its a lot higher..say 30Deg C.
so more heat gets lost before the room even gets warm. Your heating
element is always ABOVE room temp and the colder walls are always below.
You are placing the heating element directly against the cold wall..with
no air between. Now if transfer from floor to room is good, thats not
such a big issue, but if the floor is actually masked from the room by
insulation - carpets/sofas even a dog lying on the floor - its like
having a heating element halfway outside the house. The temp locally
rises, so heatloss to te outdie imncreases..but the heat transfer into
the room falls.
IN terms of heat output inside the house, to heat output inside the
house, teh ratio is simply the localised U values. Any obstructions on
the floor make UFH worse. whereas in a conventional system they make it
better. UFH under a carpet is like hanging curtains over your radiators.
They get hot and most of the heat goes outwards.
Look, in the end its your choice, but don't be misled by a simplistic
interpreation of the maths.
I certainly agree that 6mm insulation is a total waste of money..What
concerns me is that the whole UFH without the underfloor insulation -
down to 0.1- 0.2 U value - that is what I think is ideal, may also be a
waste of money.
Which will vary depending on the wind, the draughts, the ventilation,
the curtains, the carpets and the furniture.
We have single glazing. With a room temp of about 22C the curtains -
doubled lined with thermal lining - allow frost to form inside teh
windows at -5C outside..the air space is about 7-8C estimated behind the
curtains...so U value of window from building regs book about 3-4..u
value of curtains with airspace behind, about .8...?
Drawing the curtains probably halves the heat loss from the rooms with
the french windows in.
However, if all you want to do is shove in a bit of warm to stop a tiled
floor feeling icy, then none of this is relevant. Just don't expect the
heatloss calcs to actually be the complete picture.
We have an aga running in a slated floor kitchen. Its enough - because
we have 50mm insulation under - without the UFH being on - to make the
I never said that. I said that less than 50mm was useless, and that UFH
as a primary heat source without insulation of at least that underneath
was hugely uneconomic.
Why does the FLOOR feel cold right NOW with the room being warm?
Because its at a lower temp then the room?
If now its at a higher temp than the room, is it not concievable that
MORE heat will be lost through it than is currently the case?
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