Ensuring a Thermal Bridge Free Footing

[Jeremy Harris]

I think that the ventilation under beam and block is just a lack of thought about the real situation, and sticking to the old dogma that a suspended floor must always be ventilated underneath.  It's not logical when there are no materials involved that are going to be affected by damp, but the building industry in general seems a bit conservative (with a small "c") and reluctant to change things they have been doing for years.

 

If it were me, then I'd probably just pump the void full of bonded EPS beads afterwards and then partially seal up behind all the vents with some permeable fabric, to allow the pressure to equalise but resist the wind from blowing through.

[Visti]

The principle concern with ventilating the void beneath the beam and block appears to be the prevention of gasses accumulating rather than damp. Even though the likelyhood of gasses accumulating is remote, it's not something I think we can rule out 100%. Filling the void with EPS of some sort would solve this, but in the shrinkage scenario a void would be created anyway... not quite sure how to address that particular issue! 

 

I'll be raising it with out warranty provider to get an early steer, but it's the BC that ultimately makes the call.

[Jeremy Harris]

The gases problem is easily fixed, lay a radon barrier on the ground.  If it's good enough to keep radon out of the void it will be good enough to keep any other gas out.  Don't run any gas pipes through that void either. 

 

In terms of movement/shrinkage, the movement is small enough for soft EPS to take up.  The quick way to fill the void would be to blow in bonded EPS beads, which would be slightly permeable so would still allow pressure to equalise and the under croft to be vapour permeable.

[TerryE]

  On 02/01/2018 at 18:27, JSHarris said:

It's a pity that you're forced to have a void under the floor like this, as it does mean increasing the floor heat loss in very cold weather

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I know this is an off-the-shelf thought, but if you have to have a block and beam floor then why not just put a poly membrane and 100mm whacked MOT as an under base and then fill the void with blown EPS?  

 

Bugger, JSH beat me to it.

Edited January 4, 2018 by TerryE
Cross posting codicil

[bissoejosh]

Looking at this some more what is the outer leaf of brickwork for? As far as I'm aware the Larson truss used by MBC doesn't place any vertical load down the outer stud - or have I go that wrong MBC customers? Assuming not could the whole outer leaf be removed, replaced with EPS and then have brick slips stuck to the outside face as external finish?

Edited January 10, 2018 by bissoejosh

[Jeremy Harris]

  On 10/01/2018 at 09:25, bissoejosh said:

Looking at this some more what is the outer leaf of brickwork for? As far as I'm aware the Larson truss used by MBC doesn't place any vertical load down the outer stud - or have I go that wrong MBC customers? Assuming not could the whole outer leaf be removed, replaced with EPS and then have brick slips stuck to the outside face as external finish?

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Yes, you're right, the outer member of the twin stud doesn't take any significant vertical loads and yes you can fit counter battens to it, then cement board to those and use brick slips.  Some here have done just that.  You then have a thermal bridge free outer skin that rests on the EPS upstand and looks like a brick wall.

[Luckylad]

  On 03/01/2018 at 13:59, JSHarris said:

The quick way to fill the void would be to blow in bonded EPS beads

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Is it possible to diy  this?

 

Edited January 10, 2018 by Luckylad
Bad grammar

[Jeremy Harris]

  On 10/01/2018 at 10:26, Luckylad said:

Is it possible to diy  this?

 

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Possibly, if you can get hold of the kit.  The bit of kit is pretty simple and is the same as used for blowing in bonded EPS bead wall insulation.  It's a big hopper full of loose EPS beands (bean bag size) that has a blower that squirts them down a pipe with about a 20mm bore (that's a guess from looking at one).  There's also a tank of diluted PVA, and a pump that squirts that via a smaller pipe to the metal nozzle at the end.  The PVA coats the beads as they are blown in, so a few hours later, when the PVA has dried, the whole lots becomes a solid mass of EPS.

 

EPS beads are cheap, as is blown-in bonded bead wall insulation, so I reckon the best way to tackle it would be to have some injection point holes drilled (these seem to be around 16 to 20mm diameter, AFAICS) spaced over the floor, then just get the beads injected to fill the cavity.  Bonded EPS beads are vapour permeable, so there shouldn't be any problem with water build up if there are still some vapour permeable external vents (perhaps a bit of vapour permeable membrane behind normal under-floor vents).

[bissoejosh]

  On 10/01/2018 at 09:33, JSHarris said:

 

 

Yes, you're right, the outer member of the twin stud doesn't take any significant vertical loads and yes you can fit counter battens to it, then cement board to those and use brick slips.  Some here have done just that.  You then have a thermal bridge free outer skin that rests on the EPS upstand and looks like a brick wall.

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Excellent, it would seem to address a lot of the strip foundation / beam and block issues people encounter if a passive slab isn't viable.

[Visti]

I did not know that about a Larson Truss... I suspect it's because we just weren't aware of the loading (yet to engage on engineering, but we're kicking off promptly). 

 

More insulation is always a win, but the beads + cement board + brick slips appears to be a much more complicated setup (both in time and cost). I'll have to do some costings! Using solid EPS may make it more viable as a DIY method thinking about it. 

 

As for the ventilation of the void, I've done some digging in the Bregs and it's clear that it needs to be there. Reg c 4.19. The EPS beads may still be an option however, though I'm still awaiting our warranty provider for confirmation of acceptance of such a solution.

[bissoejosh]

  On 10/01/2018 at 21:02, Visti said:

I did not know that about a Larson Truss... I suspect it's because we just weren't aware of the loading (yet to engage on engineering, but we're kicking off promptly). 

 

More insulation is always a win, but the beads + cement board + brick slips appears to be a much more complicated setup (both in time and cost). I'll have to do some costings! Using solid EPS may make it more viable as a DIY method thinking about it. 

 

As for the ventilation of the void, I've done some digging in the Bregs and it's clear that it needs to be there. Reg c 4.19. The EPS beads may still be an option however, though I'm still awaiting our warranty provider for confirmation of acceptance of such a solution.

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I'm sure I've read somewhere of someone who simply stuck brick slips to solid EPS. This is what I'm hoping to do although I'm yet to research it.

[Jeremy Harris]

  On 11/01/2018 at 13:22, bissoejosh said:

I'm sure I've read somewhere of someone who simply stuck brick slips to solid EPS. This is what I'm hoping to do although I'm yet to research it.

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Not easy to do, I think.  You need to allow for differential thermal expansion/contraction, which is why it's more usual to put brick slips on to a solid substrate (a cement board of some type) that has the same sort of coefficient of thermal expansion as the slips.  This way there can be slight movement between the cement board carrying the slips and the EPS, without causing cracks to appear.

 

It might be possible to render on to a mesh mechanically fixed to the EPS, then fix the slips to that.  Whether that's more cost effective or not I'm not sure.

Edited January 11, 2018 by JSHarris

[bissoejosh]

  On 11/01/2018 at 13:36, JSHarris said:

 

 

Not easy to do, I think.  You need to allow for differential thermal expansion/contraction, which is why it's more usual to put brick slips on to a solid substrate (a cement board of some type) that has the same sort of coefficient of thermal expansion as the slips.  This way there can be slight movement between the cement board carrying the slips and the EPS, without causing cracks to appear.

 

It might be possible to render on to a mesh mechanically fixed to the EPS, then fix the slips to that.  Whether that's more cost effective or not I'm not sure.

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Ah that makes sense, certainly something I'll look at nearer the time, long way off yet!

 

[PeterW]

There is a method of brick slip that uses a preformed and skinned EPS backing board that is about 25mm thick. It has tracks built into it that space the slips vertically, and then allow the slip placement in the grooves. 

 

I spoke to the supplier who said it was used for retrofit solutions but could be used on ICF. I’ll need to dig the details out. 

[Moira Niedzwiecka]

Hello @Visti

Have you had any response from BC or your warranty supplier about using EPS beads to fill the void under the beam & block?

I would consider doing the same if it seems possible.