Green Building Forum - U values of ground bearing solid floors

U values of ground bearing solid floors

tony — Thu, 27 Sep 2007 18:42:13 +0100

A U value is a measure of the heat loss in steady state. The heat lost from a solid floor should therefore not be the ammount calculated from the structural floor and the insulation alone but should also include any insulating properties that the hardcore, fill and sub soil layers may have.

It may even be that in the majority of cases no under slab insulation is not needed and only edge or wing insulation called for.

The tradition of only insulating walls down to floor level must also be called into question.

Do we need to be wasting so many resources on under slab insulation? It may even be that not insulating under most of the slab is a better way forward!

U values of ground bearing solid floors

chuckey — Thu, 27 Sep 2007 20:22:36 +0100

Under the floor is the earth. Its huge and it keeps a stable temperature (in our latitude) of about 10 deg C. So your floor slabs conduct their surface temperature away to this giant heat sink. Now when you slop around in your slippers, when you stop your feet will get cold, because their heat is now being conducted away into the earth. Conduction into a solid is much better at carrying heat away then conduction into the air. People have experimented and come to a suitable value of "R" which balances the, through the floor heat loss with say, heat loss through the wall, windows, roof etc. The best/cheapest floor is a suspended wooden floor (no downward conduction losses) without any air flow under the floor, the problem is that the floor rots outs.
Frank

U values of ground bearing solid floors

howdytom — Thu, 27 Sep 2007 22:18:58 +0100

So if you were contemplating using an earth floor, would you insulate underneath, if so what with, how thick and how deep(insulation and earth on top)?
Tom

U values of ground bearing solid floors

Jeff Norton (NZ) — Fri, 28 Sep 2007 02:34:04 +0100

The large stable temperatures of the ground is dictated by the avarage temperature of what is above it. The ground outside in the UK will be about 10 deg C. The ground under a house depends on the average temperature of the house, so conduction travels both ways, in summer it has a cooling effect and winter a warming one. With solar design you can manipulate the internal environment for the effect to be a very comfortable as well as a low energy loss one.

Vertical and wing insulation is the way forward on dry ground.

U values of ground bearing solid floors

fostertom — Fri, 28 Sep 2007 12:03:13 +0100

Posted By: Jeff Norton (NZ)Vertical and wing insulation is the way forward on dry ground

To be clear, Jeff means vertical or (even better) wing *instead of*, not additional to, horizontal underslab insulation.

U values of ground bearing solid floors

chuckey — Fri, 28 Sep 2007 20:29:02 +0100

Jeff, in winter there is no warming effect,unless you intend to keep the houses internal temperature below 10 deg C. What you have is a smaller cooling effect then the ambient temperature would give you.
Like wise in summer the "cooling" effect is only evident at ground level, unless you use a fan to lift the cold air off the floor and try to circulate it.
Frank

U values of ground bearing solid floors

Mike George — Fri, 28 Sep 2007 20:59:39 +0100

Posted By: chuckeyJeff, in winter there is no warming effect,unless you intend to keep the houses internal temperature below 10 deg C. Frank

Isn't it possible that the still air adjacent to a ground floor will drop below 10 degrees overnight in Winter? And isn't it also possible that the temperature in the soil below the floor itself may exceed 10degrees as a result of daytime heating and incidental gains?

U values of ground bearing solid floors

Paul in Montreal — Fri, 28 Sep 2007 22:05:21 +0100

Posted By: Mike GeorgeIsn't it possible that the still air adjacent to a ground floor will drop below 10 degrees overnight in Winter?

By what mechanism? There's a roof above the floor surely so radiative losses are out.

Posted By: Mike GeorgeAnd isn't it also possible that the temperature in the soil below the floor itself may exceed 10degrees as a result of daytime heating and incidental gains?

Isn't this whole thread about the fact that the ground under the slab eventually rises to a temperature that's dependent on the interior temperature? It may never equal it (due to losses sideways and downwards) but it should come to an equilibrium above the "outside" ground skin temperature (which is equal to annual average air temperature).

Paul in Montreal.

U values of ground bearing solid floors

Mike George — Fri, 28 Sep 2007 22:23:25 +0100

Okay, so I was playing Devil's advocate a little. I am thinking of houses with a poorly insulated envelope, which are only heated intermittently. Plenty of these in the UK. And we have new legislation to enforce insulation upgrades to floors, in some circumstances this is irrespective of whether the walls and/or roof are insulated.

U values of ground bearing solid floors

Jeff Norton (NZ) — Fri, 28 Sep 2007 22:31:16 +0100

Posted By: Paul in MontrealIsn't this whole thread about the fact that the ground under the slab eventually rises to a temperature that's dependent on the interior temperature?

Yes that is my point, if the Internal average air temperature is 20 degC over the summer (or charging of mass time) period then the ground will become 20 deg C! which reduces the losses to the ground over winter, which in turn removes the need for insulation under the slad (or horizontal insulation).

U values of ground bearing solid floors

tony — Fri, 28 Sep 2007 23:25:50 +0100

Yes exactly -- so what u value can we attribute to a solid floor with no insulation?

Further if the stuff under the house is is warmer than we think the heat loss through the floor will be very much less so less (or no) need to insulate it.

U values of ground bearing solid floors

Mike George — Sat, 29 Sep 2007 08:02:24 +0100

Posted By: tonyYes exactly -- so what u value can we attribute to a solid floor with no insulation?

I dont think u-values are suitable for solid floors. The temperature difference between the air and the ground is too variable for accurate predictions of heat loss.

Posted By: tonyFurther if the stuff under the house is is warmer than we think the heat loss through the floor will be very much less so less (or no) need to insulate it.

Yes, perimeter insulation is the way to go [for new builds]

U values of ground bearing solid floors

tony — Sat, 29 Sep 2007 08:06:47 +0100

Thanks Mike -- now how do we put this to the powers that be? The aim being to stop wasting resources where they aren't doing much good.

U values of ground bearing solid floors

Jeff Norton (NZ) — Sat, 29 Sep 2007 08:19:22 +0100

Each new build can be simulated with dynamic software to prove it's effectiveness (in NZ we call it the modeling method)! I think Mike said IES software is compliant?

U values of ground bearing solid floors

fostertom — Sat, 29 Sep 2007 10:49:22 +0100

Posted By: tonyso what u value can we attribute to a solid floor with no insulation

You'd only be using vertical or wing insulation instead of u/floor insulation if you were sure that the superstructure was set up to make the elevated subsoil trick work. If that's the case, then subsoil is more or less at internal temp, so there's in principle no heat loss through the uninsulated slab, so U-value is irrelevant. If you do have heat flow out through the slab, then you do need to know its notional U-value - but then you'd be using an insulated slab. In summary, the question of U-value of uninsulated slab shouldn't arise - if it does there's something wrong.

U values of ground bearing solid floors

fostertom — Sat, 29 Sep 2007 11:01:08 +0100

Posted By: tonyhow do we put this to the powers that be?

Posted By: Jeff Norton (NZ)Each new build can be simulated with dynamic software to prove it's effectiveness (in NZ we call it the modeling method)! I think Mike said IES software is compliant?

That's right - not having got round to doing fully uninsulated slab yet, I now realise I wouldn't think of going the uninsulated route unless part of a computer simulation/modeling development of the building's solar/thermal/air movement design. In due course we'll gain a 'feel' for it and may be able to design reliably without simulation. Tas is also Building Regs accredited - I wouldn't say 'compliant', as it's anything but compliant with the Approved Document procedure - which is only intended to be one, pre-approved, way of meeting the basic legal Regs requirements.

U values of ground bearing solid floors

Mike George — Sat, 29 Sep 2007 12:21:34 +0100

Tas and IES are currently the only two accredited pieces of dynamic simulation software for use with Part L2A or L2B. Compliance is by the satisfaction of ISBEM type criteria which can be outputed from either software.

It is unclear whether IES or Tas can be used as a means of comliance with Part L1A or L1Bb as the recomended software for use with dwellings is SAP! I do not know about IES, but Tas does not give a SAP rating.

There are also backstop 'u-values' which must be met for floors in all of the Part L Documents. My feeling is that this requirement would need to be relaxed in order for dynamic software to be used.

U values of ground bearing solid floors

fostertom — Sat, 29 Sep 2007 12:51:43 +0100

http://www.construct-it.org.uk/pages/events/members_meetings/November_2006/presentations/Paul%20Carey%20Zero%20Energy%20Design%20-%20CIT%20MM%2015-11-06.pdf pp30-31 implies thast Tas is gd for L2

U values of ground bearing solid floors

Mike George — Sat, 29 Sep 2007 14:10:27 +0100

Yes, it is fine, but there are maximum backstop u-values prescribed in Part L2, meaning that you have to input a floor with that value into Tas. I think it is around 0.6W/m2K from memory.

It is unclear whether you can ignore floor values and input a model with no insulation in the floor! I may be wrong but my interpritation is that even if you hit the notional target, a steady state floor value worse than 0.6 will not comply.

U values of ground bearing solid floors

fostertom — Sat, 29 Sep 2007 14:11:47 +0100

Does the Bldg Reg Relaxations system still exist?

U values of ground bearing solid floors

tony — Sat, 29 Sep 2007 15:22:54 +0100

Seem to be coming back to my question -- what is the U value of the whole lot of floor plus what is under it? Do we have a R value for soil? and multiply that by the median of the path lengths for heat to escape add something for the concrete =?

U values of ground bearing solid floors

Paul in Montreal — Sat, 29 Sep 2007 16:50:22 +0100

Tony,

U value and R value are the same thing! You can't know one without knowing the other.

According to this site - http://en.wikipedia.org/wiki/Earthship - the R-value of soil is "1 per foot". This would be in imperial units, not RSI. The site is interesting since it discusses R-value of soil in the context of thermal mass too. BTW, to convert between R (imperial) and U values, see http://www.sizes.com/units/rvalue.htm

Paul in Montreal.

U values of ground bearing solid floors

tony — Sat, 27 Oct 2007 16:56:48 +0100

Have we finished with this one now?

U values of ground bearing solid floors

biffvernon — Sat, 27 Oct 2007 20:11:45 +0100

I visited a house today, built about 200 years ago where current work has revealed the construction. The floor comprised a foot thick layer of cobbles, roundish stones averaging perhaps four inches diameter, overlain by about two inches of plaster. This was about 25% sand and 75% gypsum. No damp course of course and none needed as demonstrated by the survival of the plaster. The cobble layer may have added significantly to the insulation as there would have been a lot of entrapped air and only limited cold bridging at the stone to stone contacts. Cheap, effective and green.

U values of ground bearing solid floors

tony — Sat, 27 Oct 2007 22:07:42 +0100

nice

Anyone want to estimate its U value?

Or for that matter the U value of an uninsulated "modern" floor slab on dry ground?

U values of ground bearing solid floors

fostertom — Sat, 27 Oct 2007 22:48:10 +0100

Haven't we agreed U-value is meaningless for ground bearing slabs? Are you asking for an 'equivalent' figure that can be used to satisfy Bldg Regs? or what?

U values of ground bearing solid floors

tony — Sat, 27 Oct 2007 23:20:20 +0100

Yes and for calculations. There is still heat loss and U values but presently they are not understood. Consequently the heat losses predicted ary very high.