Foundation aerated blocks, marmox, beads below DPC

[SBMS]

We currently have the following 'standard' detail for foundation arrangement:

 

 

We are using EPS beads for cavity insulation.

 

Have seen a number of different schemes to reduce thermal bridging. 

 

Enhancement one: the 'denby dale detail' where the bottom four courses of inner block are switched out to aerated concrete.

 

Enhancement two: putting a course of marmox thermablock adjacent to the screed to minimise bridging (about £1400 additional cost for that).

 

Enhancement three:  instead of the concrete cavity fill before ground, we could bring the beads right down below ground level.

 

Which of the above would recommend incorporating into the above detail with regards to a cost vs benefit analysis?

[JohnMo]

10 minutes ago, SBMS said:

Enhancement one: the 'denby dale detail' where the bottom four courses of inner block are switched out to aerated concrete.

 

Enhancement two: putting a course of marmox thermablock adjacent to the screed to minimise bridging (about £1400 additional cost for that).

 

Enhancement three:  instead of the concrete cavity fill before ground, we could bring the beads right down below ground level.

Start by looking at continuity insulation.

 

First place its missing on the edge of the screed. So enhancement is required there, add insulation width equal to being flush with the inner edge of the skirting

Second your inner wall of blocks is directly connected to ground, so you need a thermal break there. Two rows of aerated blocks will be fine. Move dpm so the aerated blocks are inside and stay dry.

Enhancement two adds nothing to the party already being done by aerated blocks

Enhancement three brings nothing additional to the party there is no thermal bridge there.

[SBMS]

52 minutes ago, JohnMo said:

Start by looking at continuity insulation.

 

First place its missing on the edge of the screed. So enhancement is required there, add insulation width equal to being flush with the inner edge of the skirting

Second your inner wall of blocks is directly connected to ground, so you need a thermal break there. Two rows of aerated blocks will be fine. Move dpm so the aerated blocks are inside and stay dry.

Enhancement two adds nothing to the party already being done by aerated blocks

Enhancement three brings nothing additional to the party there is no thermal bridge there.

Thanks. Architect has specified PIR upstand around screed (not on engineer drawings above). Is that what you mean?

 

Just to clarify - your two courses of aerated blocks - are they the ones directly below FFL (adjacent to the slab, insulation and screed?)

 

Where do you mean regarding the DPC movement?

[JohnMo]

8 minutes ago, SBMS said:

Architect has specified PIR upstand around screed (not on engineer drawings above). Is that what you mean

Yes, but make a thick as practical especially if you have UFH.

 

9 minutes ago, SBMS said:

your two courses of aerated blocks - are they the ones directly below FFL (adjacent to the slab, insulation and screed?

Start at top of screed and down 2 levels.

 

DPC to keep aerated blocks dry

 

 

[SBMS]

25 minutes ago, JohnMo said:

Yes, but make a thick as practical especially if you have UFH.

 

Start at top of screed and down 2 levels.

 

DPC to keep aerated blocks dry

 

 

Thanks. Out of interest if this is as effective as marmox thermablock - why would anyone use them instead of the above detailing?

[JohnMo]

31 minutes ago, SBMS said:

Thanks. Out of interest if this is as effective as marmox thermablock - why would anyone use them instead of the above detailing?

Marketing?

[SBMS]

1 hour ago, JohnMo said:

Yes, but make a thick as practical especially if you have UFH.

 

Start at top of screed and down 2 levels.

 

DPC to keep aerated blocks dry

 

 

With that DPC - how would we get the beads below the DPC?

[JohnMo]

Sorry slight redraw, shown in blue

[Alan Ambrose]

The way I look at this is that concrete, bricks, screed & standard blocks are very good heat conductors compared with any kind of insulation. See the yellow arrow below. From memory, by a factor of x50.

 

You can model it all in therm, but that's the long and short of it. The task is to disrupt that heat flow by putting insulation across the good themal conductors. Insulation (1) at the edge of the screed helps, but you still have the path up the blocks and into the room. Insulation (2) blocks the path completely.

 

You can estimate the U-value / psi-value by just taking the thickness of the insulation and ignoring everything else. Ideally, Marmox as thick as the insulation under the screed?

 

[Alan Ambrose]

p.s. here are the the values for various kinds of aerated blocks compared to PIR i.e. you need between 3 and 6x the thickness of block as insulation vs. PIR / EPS etc. Obviously this is a tradeoff vs. strength.

 

 

 

 

 

 

 

Edited August 29 by Alan Ambrose

[SBMS]

4 minutes ago, Alan Ambrose said:

p.s. here are the the values for various kinds of aerated blocks compared to PIR i.e. you need between 3 and 6x the thickness of block as insulation vs. PIR / EPS etc. Obviously this is a tradeoff vs. strength.

 

 

 

 

 

So, unlike @JohnMo’s suggestion you’d advocate some marmox instead of aerated blocks?

 

I wonder what the payback would be based on additional cost of marmox.. we’ve got 50m perimeter. 

[Dunc]

The way I looked at it very crudely [hope I don't embarrass myself here!]:

 

Thermal conductivity (lambda, k) of a Marmox block is 0.05 W/mK (https://www.marmox.co.uk/products/thermoblock).

Maximum thickness of marmox is 0.1m (100mm) and you can't stack them. So thermal resistance (R) = 0.1/0.05 = 2 m²K/W. The U value (1/R) is 0.5 W/m²K.

 

Thermal conductivity of an aircrete 7 or similar is about 0.18 W/mK (https://www.forterra.co.uk/product/thermalite-aircrete-hi-strength-7/).

Assume you've got a single 300mm block R = 0.3/0.18 = 1.67 m²K/W. The U value (1/R) is 0.6 W/m²K.

 

So not a lot in it as long as the aircrete block is kept dry by appropriate routing of the DPC?

I've gone with a standard thermal block because round my way, no groundworker was likely to bother reading up how to install the Marmox blocks correctly.

Edited August 29 by Dunc

[JohnMo]

2 minutes ago, Dunc said:

The way I looked at it very crudely [hope I don't embarrass myself here!]:

 

Thermal conductivity (lambda, k) of a Marmox block is 0.05 W/mK (https://www.marmox.co.uk/products/thermoblock).

Maximum thickness of marmox is 0.1m (100mm) and you can't stack them. So thermal resistance (R) = 0.1/0.05 = 2 m²K/W. The U value (1/R) is 0.5 W/m²K.

 

Thermal conductivity of an aircrete 7 or similar is about 0.18 W/mK (https://www.forterra.co.uk/product/thermalite-aircrete-hi-strength-7/).

Assume you've got a single 300mm block R = 0.3/0.18 = 1.67 m²K/W. The U value (1/R) is 0.6 W/m²K.

 

So not a lot in it as long as the aircrete block is kept dry by appropriate routing of the DPC?

I've gone with a standard thermal block because round my way, no groundworker was likely to bother reading up how to install the Marmox blocks correctly.

Same calculation I did when I did mine.

[SBMS]

9 minutes ago, Dunc said:

The way I looked at it very crudely [hope I don't embarrass myself here!]:

 

Thermal conductivity (lambda, k) of a Marmox block is 0.05 W/mK (https://www.marmox.co.uk/products/thermoblock).

Maximum thickness of marmox is 0.1m (100mm) and you can't stack them. So thermal resistance (R) = 0.1/0.05 = 2 m²K/W. The U value (1/R) is 0.5 W/m²K.

 

Thermal conductivity of an aircrete 7 or similar is about 0.18 W/mK (https://www.forterra.co.uk/product/thermalite-aircrete-hi-strength-7/).

Assume you've got a single 300mm block R = 0.3/0.18 = 1.67 m²K/W. The U value (1/R) is 0.6 W/m²K.

 

So not a lot in it as long as the aircrete block is kept dry by appropriate routing of the DPC?

I've gone with a standard thermal block because round my way, no groundworker was likely to bother reading up how to install the Marmox blocks correctly.

@Dunc that’s really useful thank you

[torre]

@Dunc +1 we did the same calculation - similar conductivity, but the high strength aircrete are much cheaper and easier to install

[SBMS]

3 hours ago, Dunc said:

The way I looked at it very crudely [hope I don't embarrass myself here!]:

 

Thermal conductivity (lambda, k) of a Marmox block is 0.05 W/mK (https://www.marmox.co.uk/products/thermoblock).

Maximum thickness of marmox is 0.1m (100mm) and you can't stack them. So thermal resistance (R) = 0.1/0.05 = 2 m²K/W. The U value (1/R) is 0.5 W/m²K.

 

Thermal conductivity of an aircrete 7 or similar is about 0.18 W/mK (https://www.forterra.co.uk/product/thermalite-aircrete-hi-strength-7/).

Assume you've got a single 300mm block R = 0.3/0.18 = 1.67 m²K/W. The U value (1/R) is 0.6 W/m²K.

 

So not a lot in it as long as the aircrete block is kept dry by appropriate routing of the DPC?

I've gone with a standard thermal block because round my way, no groundworker was likely to bother reading up how to install the Marmox blocks correctly.

Did you do similar detailing re DPC as suggested? Were you on PIR for cavity insulation or beads?

[SBMS]

Is there any reason why aircrete blocks would be needed if we brought the cavity insulation (EPS beads) right down to the bottom of the cavity?

 

Would that negate any thermal bridging? I don’t know whether a Cavity tray at DPC level would still be needed. 

[JohnMo]

9 minutes ago, SBMS said:

Would that negate any thermal bridging?

No! This thermal bridge would remain, your aercrete blocks get rid of this.

 

8 hours ago, JohnMo said:

Second your inner wall of blocks is directly connected to ground

 

Edited August 29 by JohnMo

[SBMS]

14 minutes ago, JohnMo said:

No! This thermal bridge would remain, your aercrete blocks get rid of this.

 

 

At the risk of sounding daft (I don’t mind!).. if the cavity below ground is filled with insulation, how is this thermal bridging any different to above ground - ie the same makeup with a block wall and then 200mm of insulation? Is it because it’s a solid mass below ground on the inside of the wall as opposed to mostly empty space (ie a

room) above the DPC? Or is the primary reason for using aircrete

blocks simply to negate the thermal bridging of the bottom most block sitting on the foundations?

 

I hope my naivety will be useful for others!

Edited August 29 by SBMS

[JohnMo]

20 minutes ago, SBMS said:

At the risk of sounding daft (I don’t mind!).. if the cavity below ground is filled with insulation, how is this thermal bridging any different to above ground - ie the same makeup with a block wall and then 200mm of insulation? Is it because it’s a solid mass below ground on the inside of the wall as opposed to mostly empty space (ie a

room) above the DPC? Or is the primary reason for using aircrete

blocks simply to negate the thermal bridging of the bottom most block sitting on the foundations?

 

I hope my naivety will be useful for others!

Couple of basics,

 

Heat always moves towards cold, in any direction. Sideways, up or down it doesn't matter.

You need to be able to draw an unbroken line around any cross section of the building, the line follows the insulation. This line determines if you have a thermal bridge. Below the red line is broken by the concrete block.

 

So with the cavity fully filled with insulation. And no aercrete blocks, you have a gap in you insulation. The heat inside the room will pulled down toward the ground. Insulation within the cavity has no effect on this. Having insulation in the area I have cross hatched in yellow serves no gains.

 

[SBMS]

4 minutes ago, JohnMo said:

Couple of basics,

 

Heat always moves towards cold, in any direction. Sideways, up or down it doesn't matter.

You need to be able to draw an unbroken line around any cross section of the building, the line follows the insulation. This line determines if you have a thermal bridge. Below the red line is broken by the concrete block.

 

So with the cavity fully filled with insulation. And no aercrete blocks, you have a gap in you insulation. The heat inside the room will pulled down toward the ground. Insulation within the cavity has no effect on this. Having insulation in the area I have cross hatched in yellow serves no gains.

 

Got you. I think the thing I couldn’t make sense of is why I’ve read in a few places on here that members have taken their insulation to the bottom of foundations. In my head in only makes sense to take it to the same depth as the slab insulation (dotted red line below) Am I correct on that?

 

So in summary, I just need to change the two courses of inner block immediately below DPC (marked blue below) to aircrete blocks and that’s it?

 

 

 

[JohnMo]

4 minutes ago, SBMS said:

Am I correct on that?

Yes correct.

5 minutes ago, SBMS said:

just need to change the two courses of inner block immediately below DPC (marked blue below) to aircrete blocks and that’s it?

Just about and take the dpm up inside the cavity to keep the aircrete blocks dry. Or even paint with a bitumen paint on the cavity side would work.

[SBMS]

Thanks @JohnMo got it now. 
 

Out of interest how would one typically ‘stop’ the EPS beads 225mm above the concrete cavity fill? A cavity tray?

 

FYI this is the architect detailing showing the DPC/cavity tray in red dotted line - does this catch the EPS beads?

 

Edited August 29 by SBMS

[Oz07]

I dont think you need to stop the beads they are not going to wick. Cant they sit on the conc infill? I had dritherm bats so managed to keep 3" off conc with the ties but pretty sure eps could go down to bottom. 

 

I did the aircrete blocks kept dry on my current place. You dont nessecarily have to wrap the dpm back in although it would be neat. You will have another dpm at finished floor level anyway

[Dunc]

12 hours ago, SBMS said:

Did you do similar detailing re DPC as suggested? Were you on PIR for cavity insulation or beads?

Yes: radon membrane under slab, continues horizontally under the aircrete the turns up inside the cavity join the horizontal DPM throgh the wall. 

 

I'm at the design stage too and haven't specified the insulation for the cavity, so reading this with interest to see what options are available. Mine is twin wall timber frame with cellulose fill, so my above ground cavity insulation is a separate consideration. Because of this, I'll probably run the radon membrane under my foundation cavity insulation then up the inner face of the external block to keep both insulation and aircrete block dry.

 

General question for you & others: I'm not sure on the value of 2x300mm aircrete blocks? With insulation above the slab the top block is pretty much continuous with this and limits both the horizontal and vertical bridges. What does another lower insulating block add?