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Author Topic: block and beam floor... insulation  (Read 2642 times)
chasfromnorfolk
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« on: October 24, 2013, 04:57:16 PM »

I've happened upon a job while up here in Scotland which puzzles/torments me: a block and beam floor under construction, due to have a wet u/f system installed over it. There'll be a 150mm void beneath and down into the subsoil sufficient to bring ffl back up to about 150mm above ground.

Now, in a conventional solid floor with u/f you might start with type1/consolidated hardcore, then Jablite then slab then 50mm Cellotex then 60/75mm screed enclosing the pipework.

Why then has Building Control insisted that the void below the block and beam  is not only maintained, but it is ventilated - thus ensuring a nice  cooling effect below the floor?

Anyhow, the real question is: what percentage heat 'leaks' through 50mm Cellotex and the obvious follow-up: if any of the heat leaks through 50mm, what thickness do you need to ensure all heat goes up and not down?

Chas
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desperate
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« Reply #1 on: October 24, 2013, 07:06:43 PM »

Any void outside the heated envelope like that needs ventilating I would say, it would be almost impossible to stop any moisture getting in, so it would tend to build up and get really rank leading to all kinds of bacterial and fungal nastiness.

It would be quite easy to work out the amount of heat lost if you know the temperature difference across the insulation and the U values of the components, the screed above the pipes is a pretty good conductor of heat though, so as a proportion I would guess at less than 10% wastage. No matter how thick the insulation some heat will always escape.

Desp
« Last Edit: October 24, 2013, 07:58:55 PM by desperate » Logged

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JohnS
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« Reply #2 on: October 24, 2013, 07:48:45 PM »

Radon?
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Countrypaul
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« Reply #3 on: October 24, 2013, 08:31:18 PM »

Sounds almost exactly like the block and beam floor we have having put in at the moment. 65mm screed, 100mm kingspan, membrane, beam and block, 150mm (min) void which must be ventilated, then ground. Reason given to me for the ventilation was to prevent condensation and damp developing.

Paul
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clockmanFR
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« Reply #4 on: October 24, 2013, 08:34:17 PM »

Its the New Hypercourse underfloor heating, from the Roman times.
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A.L.
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« Reply #5 on: October 24, 2013, 09:22:16 PM »

hello,

Anyhow, the real question is: what percentage heat 'leaks' through 50mm Cellotex and the obvious follow-up: if any of the heat leaks through 50mm, what thickness do you need to ensure all heat goes up and not down?

-the U-value of a 10m x10m suspended concrete slab with 50mm of lambda 0.025 insulation is about 0.3 W/m2C (100mm 0.19/200mm 0.11)

- bare concrete slab can lose about 11W/m2C to the room above it, the ratio of heatloss to outside air/heatloss to room will depend on the slab to outside air dT/the slab to room dT/the slab, air and room temps

- if we assume outside air=0C, room=20C, slab=25C then ratio is (11*5)/(20*0.3) or 9.17 or 10.9%

(n.b. editing slightly to maintain consistency - then ratio is (20*0.3)/(11*5) or  0.109 or 10.9%)


- of course there will be a floor covering which will reduce transfer to room and possibly require an increase in slab temperature, both factors which would increase % loss to outside air
« Last Edit: October 25, 2013, 11:44:45 AM by A.L. » Logged
Tiff
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« Reply #6 on: October 25, 2013, 08:18:59 AM »

Chas - our house has block and beam floor (ventilated) exactly as you describe and its bloody cold in the winter. When we had the extension built some of the existing floor was broken up revealing the insulation, a very disapointing thin layer of expanded polystreyene.

I too would be interested to know if it really is essential for the void to be ventilated. The only reason I can think of is that if it gets too damp and the concrete beams saturated then the steel bars could corrode. However, my thinking is that the block and beam would be a warm surface and very unlikely to suffer from condensation.

I would be interested to learn more on this subject.
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chasfromnorfolk
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« Reply #7 on: October 25, 2013, 11:08:21 AM »

My suggestion was (coming from a point of dangerous part-information) that he should look into the possibility of having the void filled to overflow with cavity-wall insulating foam - if Building Control's worry was condensation. But then is that type of foam impervious itself? I'd guess that the type of foam used by the industry as insulation under unfelted roofs - the stuff that would stick tiles/slates together and lock the whole thing together would do the job... but that's only a guess.

Which leads me to A.L. Thanks for the calcs, it demonstrates why I shouldn't be let loose as a consultant without access to consultation.  Could I nudge you the bit further and ask the second part again: if I understand you that 11% can be 'lost' downwards through 50mm , what depth of insulation reduces the loss to zero or a more tolerable %?

Cheers,
Chas
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A.L.
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« Reply #8 on: October 25, 2013, 11:55:36 AM »

hello again,

Could I nudge you the bit further and ask the second part again: if I understand you that 11% can be 'lost' downwards through 50mm , what depth of insulation reduces the loss to zero or a more tolerable %?

O.K. - it would take an infinite amount of insulation to reduce the loss to zero  Smiley

however on the same basis as previous post 100mm would give 6.9%, 200mm 4%, 300mm 2.6%
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chasfromnorfolk
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« Reply #9 on: October 25, 2013, 04:21:04 PM »

hello again,

Could I nudge you the bit further and ask the second part again: if I understand you that 11% can be 'lost' downwards through 50mm , what depth of insulation reduces the loss to zero or a more tolerable %?

O.K. - it would take an infinite amount of insulation to reduce the loss to zero  Smiley

however on the same basis as previous post 100mm would give 6.9%, 200mm 4%, 300mm 2.6%

Good grief... to get down to what I'd consider bearable you'd need to teeter around on a foot of Cellotex?

This is on top of the slab and ffl is critical, so assuming he keeps to 50mm between slab and screed for his pipe layout it's got to be better to add as near to 250mm as poss below the slab and hope that escaped 10% is not getting any further... assuming he can get over the ventilated void problem.

I can only assume 'the Industry' quietly accepts the 10% loss - it's not something I've ever seen as a warning against the process - speed, convenience and dryness being the promoted plusses.

Chas
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car-mark
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« Reply #10 on: October 25, 2013, 04:26:29 PM »

Hi there

There was a mammoth post on the green building forum about condensation in sheepwool floor insulation.

Although it may seem unconnected with your question it goes into a lot of detail as to why an air gap may be useful under your concrete floor

yours mark
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desperate
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« Reply #11 on: October 25, 2013, 04:51:06 PM »

10% loss may be a bit on the high side, but that is assuming an outside temp of 0C according to A.Ls calcs, which is probably only a small amount of time each year. But on the other hand I have seen quite a few serious cases of rot in peoples houses caused by lack of ventilation and cold/damp roof spaces and floor voids, a small loss of heat is well worth while to protect the building fabric. Any small saving on the energy bill is dwarfed by the cost of a re-roof or floor replacement.

Just a thought

Desp
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offthegridandy
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« Reply #12 on: October 26, 2013, 12:03:13 PM »

Chas,

on a technical note, after the blocks have been laid on the beams a weak slurry of sand and cement should be brushed across and into the whole surface.  This will creep down between the blocks helping to seal any gaps and locking the blocks tight together.  We have beam and block with a 600 mm plus void beneath. The walls are 600mm thick stone and I installed cranked ventilators through the walls to the void.  Our floor is b and b with slurry, membrane,75mm celotex, wet UFH then 75mm screed.  Floor heat up time is good.

NB tape the joins in the celotex on the floor and connect/tape the celotex to the wall/perimeter insulation.  I'm sure if laid correctly the losses are minimal.

Edit,  the walls or piers that support the beams should have a DPC under the beams to stop rising damp into the b and b floor.

Andy

« Last Edit: October 26, 2013, 12:06:29 PM by offthegridandy » Logged

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chasfromnorfolk
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« Reply #13 on: January 29, 2014, 11:22:57 AM »

I've risked 'pinging' this as an unexpected email dropped into the box this very day...too late for my mate in Scotland, but maybe worth a bookmark for my own dotage-house build: insulative blocks (claims 5x better than normal lightweight ones) requiring only 60mm screed over for uniform domestic load - I've yet to establish what happens under partitions.

http://www.cemex.co.uk/readytherm.aspx

I have a feeling I'd be happier with some Cellotex as well as a thicker screed over them, but maybe less concerned about underfloor void cold with these in place between and under the beams...

Chas
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