Ensuring a Thermal Bridge Free Footing

[Visti]

We're finally at the point were we've reached one of my most concerning details of our build (so far): the footing between the timber frame and the foundations!

 

As some of you know, we're going for an MBC frame but are unable to make use of their slab due to the clay on our slot mandating a suspended foundation! Hence we're going for a beam and block setup, but have been concerned regarding thermal bridging. This is what our architect has suggested, and I'm curious as to what you all think!

 

You can see that our 300mm wide twin-stud TF (cellulose insulation) will rest at least 150mm above ground level on brick externally and low density block internally. Key component will a single course of foamglass blocks (such as Marmox Thermoblock, Purenit or Compacfoam) to reduce the thermal bridging. We’ll also add in XPS insulation between the structural supports and also a small amount of insulation (undecided what type, rockwool?) overlapping the TF and structural blocks along the inner wall. About 200mm of EPS insulation will go down on-top of the beam-and-block foundation with a 100mm screed floor on top of that.

 

 

I've seen similar alternatives suggested, such as Touchwood Homes method that @bissoejosh suggested and others in the thread below which appear simpler than the above. Are we over-doing it potentially?

 

 

 

Edited December 23, 2017 by Visti

[Jeremy Harris]

Is there no way to just mitigate the clay problem by increasing the depth of hardcore under the slab?  According to the Kore report, this works OK and is something that I'm pretty sure that Hilliard (Tanner Structural Designs, the people MBC use for the passive slab design) have done before.  Our MBC slab is on clay, FWIW, with no problems.  Kore specifically state in their info that the design is "suitable for all ground conditions".  This is an excerpt from the Kore technical report produced by Hilliard for the passive slab system:

 

Quote

For insulated foundations, the standard depth of hardcore under the EPS is 150 – 225mm of 18
– 35mm hardcore, compacted in layers appropriate to the compaction method. The most
common approach adopted where underlying soils are weak is to increase the depth of this
hardcore layer. Since the load spreads as it travels downward, the deeper the hardcore, the
smaller the load exerted on the underlying soil. This works because the hardcore has a
predictably suitable bearing capacity to take the higher intensity bearing pressures nearer the
surface. It is also strongly advised, especially in poorer ground, to lay a geotextile on the
excavated surface before placing the hardcore under the insulated foundation. This layer helps
to spread the loads further and also prevents fines in the underlying soil migrating into the
hardcore, which would reduce the bearing capacity of the hardcore.

For very poor soils, other options may need to be considered, such as piling or other specialist
ground improvement methods like Vibro Stone Columns, as discussed briefly in the previous
section. It is outside the scope of this report to discuss these options further.

 

Edited December 23, 2017 by JSHarris

[Visti]

I believe there is technically, but it's not a course we're willing to pursue further as we've already challenged the authorities (Graven Hill) and the Structural Warranty provider (Premier Gaurentee) several times and gotten nowhere. It got to the point where they'd just reference the plot passport (similar to T&Cs) which explicitly states a requirement for a suspended slab. So better to save our energy for the next encounter! 

[ProDave]

You seem to me making the "mistake" of having the FFL level with the outside ground.
 I would not want a suspended floor like that. For a start how do you ventilate it?

 

Aim for the bottom of the suspended floor structure to be slightly above ground level. Then your frame can start at FFL or even a bit below, not raised up as you have it.

 

I am not even sure that would pass building control. We had to ensure the floor underbuild had to be brought up to at least outside ground level.

[MikeSharp01]

31 minutes ago, Visti said:

some of you know, we're going for an MBC frame but are unable to make use of their slab due to the clay on our slot mandating a suspended floor...

What is wrong with the clay? Our passive slab, AFT, is on clay and nobody has raised any objection. 

[Jeremy Harris]

48 minutes ago, Visti said:

I believe there is technically, but it's not a course we're willing to pursue further as we've already challenged the authorities (Graven Hill) and the Structural Warranty provider (Premier Gaurentee) several times and gotten nowhere. It got to the point where they'd just reference the plot passport (similar to T&Cs) which explicitly states a requirement for a suspended slab. So better to save our energy for the next encounter! 

 

Try showing them this report, that makes it clear that a passive slab will work fine on any soil:  Kore Insulated Foundations Report.pdf

 

Worth pointing out that a suspended slab requires more insulation in practice, as it has to deal with the much lower winter temperatures in the ventilated void, so it makes the house less energy efficient in cold weather and increases the heating system losses if UFH is used on the ground floor.

Edited December 23, 2017 by JSHarris

[Russell griffiths]

I must admit when I first looked at the pic I didn’t like the idea of ffl lower than outside soil level

i have found previously it made for some funky orientation of the dpm to prevent damp penetration to the inside. 

[jamiehamy]

Respectfully, I think @Vistihas made his point - not revisiting! 

[Russell griffiths]

I have been thinking about this detail recently 

has anybody put any thought into a timber frame sitting on top of icf blocks

im thinking two courses of icf up to damp course with the timber frame sat on top with an isolation layer between the two like a bituminous impregnated foam gasket. 

Crazy or genius. 

 

[Jeremy Harris]

50 minutes ago, Russell griffiths said:

I have been thinking about this detail recently 

has anybody put any thought into a timber frame sitting on top of icf blocks

im thinking two courses of icf up to damp course with the timber frame sat on top with an isolation layer between the two like a bituminous impregnated foam gasket. 

Crazy or genius. 

 

 

It might work, but you would need to look at the thermal resistance up through the core of the ICF and compare it with other options, like putting the frame on tom of foamglass, or similar.

 

TBH, any strip type foundation is going to be a fair bit poorer than an insulated slab, the question is really how poor it is and how well it mitigates the sole plate condensation risk under dynamic (not static) temperature and humidity changes. 

 

The main problem is that the standard interstitial condensation models tend to fall over when looking at this detail, IMHO, as they don't take account of the rapid changes in humidity that can occur near ground level in cold weather, the classic case being a very cold night that cools the sole plate down, followed by a bright sunny morning that warms the area around the outside of the base of the wall fairly quickly, creating ideal conditions for driving water vapour inwards, perhaps towards a sole plate that is below the dew point for the local water vapour concentration that penetrates.

[bissoejosh]

As discussed previously we're doing something very similar to your detail. I'd question the benefit of the marmox block on the inner skin, if you carry the cavity insulation down far enough it makes it almost redundant and as I'm sure you know it isn't cheap...

 

Have you got enough depth on the outside face to carry insulation over the sole plate and down the outside edge of blockwork?

 

I don't see a problem with the b&b floor depth as long as it's vented but would be tempted to bring the ffl up to just above ground level if not to allow even more insulation depending on the site levels.

Edited December 23, 2017 by bissoejosh

[Visti]

On 23/12/2017 at 16:05, ProDave said:

You seem to me making the "mistake" of having the FFL level with the outside ground.
 I would not want a suspended floor like that. For a start how do you ventilate it?

 

Aim for the bottom of the suspended floor structure to be slightly above ground level. Then your frame can start at FFL or even a bit below, not raised up as you have it.

 

I am not even sure that would pass building control. We had to ensure the floor underbuild had to be brought up to at least outside ground level.

 

The ventilation is not an aspect I'd considered to date, and I'd not be adverse to raising the FFL either. I'll make a point of addressing this at our next meeting. Thank s for pointing it out!

 

 

On 23/12/2017 at 18:23, JSHarris said:

TBH, any strip type foundation is going to be a fair bit poorer than an insulated slab, the question is really how poor it is and how well it mitigates the sole plate condensation risk under dynamic (not static) temperature and humidity changes. 

 

I am completely with you there and am very aware that the thermal performance will be lower given the typically higher delta T of the air vs the ground. The dynamic nature of the temperature will also be more noticeable internally and more difficult to compensate for in the UFH. Unfortunately our choice at this point is to either to proceed with a B&B or find another build site not associated with Graven Hill! We'll probably be increasing the depth of the EPS from 150mm to 200 or 300mm to combat this!

 

20 hours ago, bissoejosh said:

As discussed previously we're doing something very similar to your detail. I'd question the benefit of the marmox block on the inner skin, if you carry the cavity insulation down far enough it makes it almost redundant and as I'm sure you know it isn't cheap...

 

Have you got enough depth on the outside face to carry insulation over the sole plate and down the outside edge of blockwork?

 

I don't see a problem with the b&b floor depth as long as it's vented but would be tempted to bring the ffl up to just above ground level if not to allow even more insulation depending on the site levels.

 

I like the idea of carrying the cavity insulation down and getting rid of the foamglass... I'm well aware of the price, hence why you can see there's only one course so far! I'm sure we can get the depth needed. 

 

On 23/12/2017 at 17:28, Russell griffiths said:

I have been thinking about this detail recently 

has anybody put any thought into a timber frame sitting on top of icf blocks

im thinking two courses of icf up to damp course with the timber frame sat on top with an isolation layer between the two like a bituminous impregnated foam gasket. 

Crazy or genius. 

 

 

Sounds crazy, but I like the way you're thinking. Some calculations over the holiday may be in order.

[Visti]

An update for you all following your suggestions!

 

• Use of periscope ventilators will maintain a FFL level with the external ground level, 4 in total.
• XPS cavity insulation extended down to the base of the cavity, which will lengthen the thermal path through the inner leaf block as suggested by @bissoejosh
• The foamglass course has been removed, replaced with 50mm of EPS services zone insulation between the inner block leaf and the screed/slab.

The thermal bridge therefore from the underside of the Beam & Block, up the block inner leaf, and though the 50mm services zone insulation has a U value of 0.28 W/m²k. I'll have to ask what the equivalent was with the foamglass!

 

The current U value of the floor is 0.15 W/m²k with 200mm EPS, which we're looking to increase to 250mm (~0.12 W/m²k) or  perhaps even 300mm (~0.10 W/m²k) given @JSHarris points regarding the larger delta T with air vs soil in winter. Given the walls are MBC's 300mm cellulose @ 0.12 W/m²k it'd be foolish in my opinion to leave it at 200mm of EPS.

 

 

[Russell griffiths]

Do you risk water entering the vents in the case of some local flooding, if at ground level ? 

Should they not be a course higher ?  

[Jeremy Harris]

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.  As it's concrete beam and block, does it actually need to be ventilated at all?  The only reason for ventilating a suspended floor is to reduce the risk of condensation on timber joists and floor boards by encouraging airflow.  Is there any reason you can't just lay EPS under the suspended floor to fill the void and do away with the vents altogether?

 

I understand that the beam and block is a plot requirement, but this may be a way to get around that by sticking to the exact letter of the plot "regulations" (which are novel, I've never, ever heard of a plot having such draconian restrictions on build methods before - must be a first for the whole of the UK!) but effectively having a non-load bearing insulated slab.

Edited January 2, 2018 by JSHarris

[Oz07]

It has just occurred to me why do we commonly sit FFL's 150mm above floor level just because DPC is at this level? Is there a reason we don't use have outside dpc higher than inside especially in cavity wall construction? Is this not what we invariably do anyway around the level access opening? Also is this why bottom of timber frames are not level with FFL so no timber in splash zone at level access? Your never going to get the bottom of a suspended floor anywhere near ground level without house sitting 600mm high once beams insulation and screed is accounted for.

 

Excuse me if i'm being stupid it just occurred to me every house seems to be 6" above ground, even when no local risk of flooding.

[Oz07]

4 minutes ago, 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.  As it's concrete beam and block, does it actually need to be ventilated at all?  The only reason for ventilating a suspended floor is to reduce the risk of condensation on timber joists and floor boards by encouraging airflow.  Is there any reason you can't just lay EPS under the suspended floor to fill the void and do away with the vents altogether?

 

I understand that the beam and block is a plot requirement, but this may be a way to get around that by sticking to the exact letter of the plot "regulations" (which are novel, I've never, ever heard of a plot having such draconian restrictions on build methods before - must be a first for the whole of the UK!) but effectively having a non-load bearing insulated slab.

Wouldn't he need a compressible insulation incase of different settlement/heave. Especially if he is not going to dig out between the strip founds and prep as you would for an insulated slab (2/300mm ish hardcore?).

Edited January 2, 2018 by Oz07

[Jeremy Harris]

2 minutes ago, Oz07 said:

Wouldn't he need a compressible insulation incase of different settlement/heave. Especially if he is not going to dig out between the strip founds and prep as you would for an insulated slab (2/300mm ish hardcore?).

 

It only needs a bit of give, though, so perhaps fill the void with bonded EPS beads blown in, or use something like EPS 70, that would have enough give to take up any very slight movement between the ground and the underside of the B&B.

 

I still don't see why vents are needed for a B&B floor, anyway, as there's nothing under there that is going to be bothered by getting a bit damp, is there?  Thinking out of the box, why can't that void just be sealed, perhaps with a small vent to ensure that the pressure is equalised?  An air void would give some insulation value, and I think you could possibly get away with using the method in BS EN ISO 6946 to calculate the lambda value of the sealed air gap.

[Russell griffiths]

Looking at your new drawing is your bottom row of cladding a bit low ?

i would think it would suffer a lot from continually being wet/dry wet/dry, as I believe your design has very little roof overhang. 

[Visti]

Now that isn't a half bad idea folks! There is even compressable EPS insulation that is specifically designed to deal with clay heave. A 100mm of that beneath the slab rather than an extra 50-100mm above ought to fill the air void whilst giving the added U value performance! Ideas ideas!

 

I'll have to check out the fact about the ventilation requirement in both cases. Looks like some bed time reading of the building regs.

 

The roof design has no overhang. There's a hidden gutter behind the cladding, but that's it. Any reason why it's be more of an issue there than terminating further up?

[CC45]

We've done what you suggest but we went up one more row with the vents (TP also sell these telescopic vents).  I would not put them level with the ground floor.

 

Once you've got sign off you could always close them off.

[Russell griffiths]

I was looking at the height of your cladding off the ground level, regarding roof overhang, your bottom row of cladding is below Dpc level and in the splash up zone so it will catch a lot of weather all of the time, where as this could be acceptable if you had a large roof overhang, you do not so that lowest row will be exposed to a fair bit of bad bounce up and could wet your lowest batten as well. 

 

Just a thought, I wondered if sometimes we are putting design before good standard building practices. 

[Visti]

Noted! hadn't thought about that!

[ProDave]

I would be very surprised if building control will allow the sub floor slab to be lower than ground level. up here it WOULD fill with water sooner or later and you would have to make provision to remove that.  This was something BC were very particular about here that the slab level could not be higher than ground level. 

 

This was addressed in our case by creating a trench along the front of the house containing a French Drain that runs around the side of the house to the back. It only works because of the sloping site and ground level at the back is a lot lower than the slab.

[bissoejosh]

Is the outer sole plate likely to be at risk sitting directly on a cold block/brick? I really like @JSHarris idea of sticking with beam and block but working around the regs if possible. I've never understood the need for a concrete beam to have ventilation when a slab is fine without.  If BC insist (ours have) how about keeping a void beneath but fitting additional insulation directly to the underside of the beams? No idea how this could be done though - glue of some sort I'd guess...