My question is about in-slab hydronic heating generally - not specifically solar hydronic heating. I hope that's not heresy on this site! We are about to build a new house and are looking at putting hydronic heating in the concret slab. The slab will be covered with timber floorboards glued onto baseboards. We are told this works fine with hydronic heating but we are concerned about whether the insulating properties of the timber floor will make the hydronic heating inefficient. Any advice from experts? Would we be better off putting in wall radiators?
In slab hydronic heating with timber flooring on top
(24 posts) (11 voices)-
Benny
MemberIf you put in radiators - all the heat they give out will be used to heat the air inside the house. Can we call that 100% efficient ? If you put heat in to the slab "some" will inevitably be lost to the ground - just how much depends on how well insulated the slab is - I would think that if you are going to use in-slab heating then it should be mandatory to insulate the slab very well. Perhaps you could look at a compromise solution - "in-screed" installation (just in the top layer of the slab) or even on top of the concrete directly under the floorboards.
From what I can find, timber has an Rvalue of about 0.1 - 0.2 for a 15mm board, so it wouldn't be very insulating anyway.
There are some very interesting comments on another forum here ...http://forum.homeone.com.au/viewtopic.php?f=20&t=17657&start=0
Posted Tuesday 30 Jun 2009 @ 12:59:48 am from IP # -
ghostgum
MemberThe R values of the wood flooring is well known. Benny's figure R 0.1 to 0.2 sounds right. My timber stud/floor joists/ceiling joists are about R1.5 to 2. See if you can find the R value for heat transfer from the slab to the ground, and make sure it is much higher than the R value for the overlay flooring. I read that the depth of the hydronic pipes in the slab affects efficiency. You want the pipes as near to the top of the slab as possible, hence the screen method for putting them in the top 30-50mm.
Posted Tuesday 30 Jun 2009 @ 4:01:23 am from IP # -
Benny
MemberIn screed just means in the top layer Jon. Instead of burying the pipes half way down in the slab - probably installing among the reinforcement and then pouring the concrete - the pipes are just pushed(?) in to the very top layer. Screeding is the finishing process for the concrete slab when they flatten /smooth it out.
Posted Wednesday 1 Jul 2009 @ 2:21:37 am from IP # -
f2design
MemberSlab hydronic system described in other forum topic is in a slab, which has a mixture of timber and tile covering. System is working well but there is a difference in warmth under foot. Tile areas do feel a little warmer.
Detail of timber selection and installation is important. Dense, stable, recycled old forest growth timber is preferable to plantation grown timber especially softwood. Cell structure of new plantation grown timber makes it more susceptible to dimensional change due to moisture content and with a heated slab the likelihood of the timber drying out is enhanced. We used northern NSW forest reds timber, which had been used as industrial purlin members remilled to 15mm T&G. Flooring glued to sealed slab using HB Fuller adhesives system.
Slab probably should have a surface sealer applied before gluing timber even if slab moisture is at correct level, to give peace of mind that any possible future moisture ingress does not rise into boards. Floating timber systems should not be used as air gap certainly will act as insulator.
Standard procedure for installing heating coils, whether water of electric is to wire tie them to the top of the reinforcement mesh increasing the slab thickness by the diameter of tube to ensure minimum reo cover is not reduced, usually 25mm.
There are differences of opinion on insulating under slabs. Insulation isolates the slab from the thermal mass of the ground below it and will accordingly reduce summer cooling capacity. It is probably better to insulate around the perimeter of the thickened edge beams to stop leakage of heat to surrounding ground. Any heat going down into ground will be stored in the ground. At the start of the winter heating season the
Posted Wednesday 8 Jul 2009 @ 1:40:21 am from IP # -
tintac
MemberAs I'm researching this at the moment, I might revive this thread.
While I understand the comments by f2design, and they make sense, would it not be true that the vapour barrier of the slab would act to prevent the thermal properties in summer as described above?Also, if you had passive solar design as well as hydronic heating, would it be correct to say that the timber floorboards would act as a thermal barrier to prevent the concrete acting as a heat sink in winter?
Thanks for your comments.Posted Saturday 29 May 2010 @ 11:10:33 am from IP # -
Eco
MemberVapour barrier should have negligible effect (though damp concrete would have different thermal properties to dry concrete).
My opinion is that timber flooring is detrimental to the operation of a thermal mass slab in passive solar design. It has been stated above that the R value of 15 mm of timber is "only" 0.1 to 0.2. This seems very small. But compare it to the R rating of 15 mm of dense concrete which would be about 0.01. For timber what this means is that a greater proportion of the solar energy initially absorbed by the top layer of the timber would be convected off to the room air rather than conducted to the slab mass, compared to uncovered concrete. (Tiles, having similar poor insulative effect as concrete, would not be detrimental.)
Timber would not be as detrimental for the operations of hydronic heating.
Posted Saturday 29 May 2010 @ 11:41:38 pm from IP # -
dymonite69
MemberBut timber has thermal mass, unlike soft floorings. It can absorb heat fast enough to also allow it to also conduct it into the slab. Carpet rapidly heats up without much of it going downwards. Thats is the main reason why it shouldn't be used, it has no mass.
Posted Monday 31 May 2010 @ 3:10:10 pm from IP # -
Interesting point d69. Wikipedia says wood has a specific heat of 1.7 J/(g.K) compared to that of concrete of only 0.88. The thing is that concrete is 3 to 10 times more dense so that the specific heat per volume is still marginally to strongly in favour of concrete depending on which type of wood.
Posted Tuesday 1 Jun 2010 @ 1:31:38 am from IP # -
dymonite69
MemberI am not contesting that concrete does not have a higher volumetric heat capacity (i.e. heat capacity per volume) than wood. However, heat transfer will be coupled from wood to concrete via conduction where it is finally stored.
Since timber has some ability to buffer a temperature rise, it gives more time for heat to pass downwards. Carpet can't do that.
Of course, the wooden floor surface will probably reach a higher peak temperature than a bare concrete surface. This may incur more heat losses. But there is still a limit at which that even a concrete surface can absorb direct radiant heat. It won't remain cool in strong sunlight.
But in comparison, carpet will rapidly heat up to uncomfortable temperature when exposed to sun with a lot less heat passing down into the slab (much of it being reflected, re-emitted or convected back into the room).
Posted Tuesday 1 Jun 2010 @ 1:46:23 am from IP # -
Anonymous
UnregisteredI think hydronic heating uses hot water as a heat transfer medium. A boiler or heat pump transfers its heat to water which is then circulated to heaters throughout the home to provide the most comfortable form of heating possible. These heaters can be radiator panels, towel rails, fan coil units or through in-floor heating. This gentle, silent, radiant heat does not scatter dust, mites. Is it true? Waiting for your reply. Thanks
Posted Tuesday 1 Jun 2010 @ 4:09:54 am from IP # -
Eco
MemberI don't think I have ever read a book on passive solar design where the author advocates covering the thermal mass floor with timber - not that I have read huge numbers. Your Home Technical Manual doesn't recommend timber over thermal mass – and in fact recommends against it - http://www.yourhome.gov.au/technical/fs49.html#typical.
Dymonite69 says that the surface of a timber floor over concrete is likely to to get warmer than straight concrete (until the concrete catches up - which I don't think it will). If the floor is warmer it will lose more energy to the air quicker. This means the air will get hotter and the house will therefore loose the heat through conduction and ventilation, more than if the heat goes straight into the uncovered slab.
Posted Tuesday 1 Jun 2010 @ 10:43:44 am from IP # -
dymonite69
MemberIn fact there is very little direct evidence in the passive solar literature about the impact of timber overlying a concrete slab. Maximal absorption will be gained from having the slab exposed. However, the question is how much more effective this is compared to the alternative.
I know of a passive house with a polished slab that has about the same thermal performance as our with our bamboo flooring. However, what we do find is that our carpeted rooms are much less effective at storing heat.
Most calculations for thermal mass are based on computer models or simple experiments involving a single wall. There is very little empirical data on real world performance of different configurations such as what is being described.
Posted Tuesday 1 Jun 2010 @ 11:19:01 am from IP # -
dave lawrence
Memberthe great debate about timber vs. concrete slabs rarely takes into account the fact that concrete slabs are terrible to walk on , and shocking to live on for your back. insulated sprung floors on the other hand are wonderful for the back . Thermal mass is great , IF it receives winter sun when you want it to. when designing homes (which I do ) I always try to incorporate thermal mass into INTERNAL walls . No body has to walk on them. Well designed Clerestory windows can help to heat them.
And I would never recommend gluing timber directly onto slabs , always batten off the slabs and fix to the battens ... 2 advantages are the timber still breathes both and secondly you get the advantages of a semi sprung floor, as the battens have some cushioning effect.Posted Sunday 13 Jun 2010 @ 9:10:05 pm from IP # -
gav3500
MemberHi,
I'm new here. I'm planning an extension which is roughly an 11mx6.5m box facing NNE (couldn't do any better). I am advocating tiles on concrete whereas my wife is in favour of wooden floor. I have an idea for both where the first quarter is tiles and the rest is a floating floor with the battens aligned along the long dimension. The idea is to take air from close to the ceiling and pump it down along the length of the slab under the floating floor and popping it out through a vent at the end (creating a closed loop). This should help transfer heat to/from the slab, thereby utilising the thermal mass under the floating floor.Another idea is to continue this air under the wooden floor of the old house and pop it up in the rooms through vents.
The air won't be particularly hot (25C at best) and there are concerns about noise, pumping costs, air leakage, air quality and effectiveness.
What do you think?
Posted Tuesday 17 Aug 2010 @ 3:57:19 am from IP # -
dave lawrence
MemberI did exactly that on our house in WA 20 years ago .it is a great idea . you can even limit the tiles to exactly in front of north facing glazing . you wont regret the timber floor .. but strictly speaking it wont be a floating floor.. the battens are fixed to the slab and t/g fixed to the battens (70 x35 hwd as a minimum).
Posted Tuesday 17 Aug 2010 @ 5:43:41 am from IP # -
gav3500
MemberThanks Dave. Yes not not strictly floating floor. Do you have any tips on fan size? The slab would be sealed with acrylic and I suppose it might be worth putting some foil between the battens and the hwd floor to seal the "ducts" from dust percolating through the wood joints. Finally it might allow the running of cabling under the floor coming up in the skirting boards. Brick walls aren't too good for cabling.
Posted Tuesday 17 Aug 2010 @ 6:01:08 am from IP # -
dave lawrence
Memberimportant thing with slab is that it drys out properly ,that takes months . I would not seal the top if it immediately, i would give it time to dry out ,otherwise the water vapour that eventually comes out could damage the battens and flooring.
also make sure you don't seal the timber flooring with impervious coating . it needs to breathe. Re : fan size: not my area of expertise ,but as others have said don't try to under do it. what about running hot water pipes between battens ,but make sure you can divert in summer . that was my plan in WA , but the budget beat me on that one .Posted Tuesday 17 Aug 2010 @ 9:25:24 am from IP # -
gav3500
MemberMonths to dry might be ok if the flooring is done last. Water pipe doesn't gain you much. A solar HWS will be typically underdone in winter so not to be overdone in summer. So there really is no excess heat in the hot water. On the other hand there will be some excess air heat if there is a large window to floor ratio and there is less concrete floor in direct light. The real questions are whether the fan consumes more power than is warranted and can we use that underfloor thermal mass in summer.
Posted Tuesday 17 Aug 2010 @ 11:25:48 pm from IP # -
dave lawrence
Membergood point re :minimal winter heat gain in winter , I just remembered we had a slow combustion stove with wetback connected to overhead solar HWS ...and an attached greenhouse which i had ideas of thermally linking via additional copper pipes looping between underfloor /slow combustion stove and green house . as i said it never actually got done , nor did i do the maths on the viability of it all.
One of the main issues if you proceed with exposed thermal mass on floor is that north side eaves prevents summer heat gain . But it sounds like you are on to that.Posted Tuesday 17 Aug 2010 @ 11:45:59 pm from IP # -
gav3500
MemberSigh! Yes summer heat gain. I've done the calculations for eaves that let in full sun/provide full shade for 6weeks either side of the solstices (at midday). That's all good, but those are only for true north. My extension will be NNE so I'll need exterior blinds or some other shading device for the morning summer sun. Might start a separate thread on my extension.
Posted Wednesday 18 Aug 2010 @ 5:16:44 am from IP #
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