Inner leaf concrete by mistake

[WWilts]

Builder has put in 3 courses of dense concrete blocks around about a third of the foundation perimeter. Finished floor level about 700mm above footings. Thickness from blinding sand up through DPM, slab, insulation etc is about 315mm.

This means the inner leaf concrete block will abut part of the floor insulation. Cavity wall insulation will be bonded EPS beads, 100mm cavity with 100mm inner and outer (Brick or reconst stone) leafs.

Aircrete 7N is intended to be the inner leaf sitting atop 2 courses of foundation concrete blocks.

 

What will be the U value of the wall section with dense concrete block replacing aircrete 7N? Assuming thermal conductivity of 0.18 for aircrete 7N and 1.3 for dense concrete. Trying to decide whether it will make much difference to thermal performance, correcting the error over a third of the perimeter.

 

 

[tonyshouse]

_{Terrible! knock them off and replace as planned}

[WWilts]

12 minutes ago, tonyshouse said:

knock them off and replace as planned

Trying to work out the U value of the offending parts of the wall, so that I can weigh how hard to squeeze

Inner leaf concrete will abut screed too, 65mm thickness

Edited August 31, 2021 by WWilts

[WWilts]

Just realised the underfloor heating will escape through the concrete inner leaf and be heating the cavity instead of the room. So, replacement with aircrete needed.

[DragsterDriver]

Is there an insulation upstand around the screed?

[PeterW]

8 hours ago, WWilts said:

Just realised the underfloor heating will escape through the concrete inner leaf and be heating the cavity instead of the room. So, replacement with aircrete needed.

Leave them.. and just make sure you use 25mm of EPS around the screed and you’ll be fine. 

[dnb]

Back of envelope suggests that the junction performance will leak around twice as much heat with concrete blocks vs aircrete assuming a 25mm EPS upstand. Depending on the perimieter vs area, this might be significant for your performance goals. It was for mine.

[WWilts]

9 hours ago, DragsterDriver said:

Is there an insulation upstand around the screed?

Yes, 25mm

[WWilts]

7 hours ago, dnb said:

junction performance will leak around twice as much heat with concrete blocks vs aircrete assuming a 25mm EPS upstand.

Agreed solution. 50mm thick upstand, and for pragmatic reasons (materials onsite already) to continue with 7N concrete blocks for third course above footings. Point about underfloor heating leaking into cavity accepted. 

Aircrete is said to suck in moisture too, so concrete below damp proof course is preferred by builder.

Edited September 1, 2021 by WWilts

[PeterW]

17 minutes ago, WWilts said:

Agreed solution. 50mm thick upstand,

That may cause issues - 50mm will protrude into the room from below the board / skim / skirt so will make it problematic for gripper rods etc. If you want to improve the thermal performance use PIR. The thermal losses even doubled are tiny, and unless you’re building Passiv standards you won’t notice. 

[WWilts]

1 hour ago, PeterW said:

50mm will protrude into the room from below the board / skim / skirt so will make it problematic for gripper rods etc

Noted, tks. Not Passiv standards.

Ground floor UFH, very unlikely to have carpets. Does the concern about 50mm perimeter upstand remain?

EPS is said to be more compressible than PIR, allowing the screed to expand/contract.

Edited September 1, 2021 by WWilts

[Iceverge]

11 minutes ago, WWilts said:

Ground floor UFH, very unlikely to have carpets. Does the concern about 50mm perimeter upstand remain?

EPS is said to be more compressible than PIR, allowing the screed to expand/contract.

 

I calculated this when I did ours and I came to the conclusion it was worth it. About 0.04(W/mK) improvement in the thermal bridge. For example on a 0 deg day with 50m perimeter it would be delta t 20deg* 50m *0.04 about  or 40w on our heating load. However our heating load is only about 1400w so it was a 3% improvement. Ours is constructed to passive standard. 

 

If we had built to the building regs it would still be 40w but only about 0.07% improvement  or 7 parts per 1000 improvement. Practically nothing. 

 

We used a PIR upstand. No issues. We wet plastered inside (20mm) and have a 20mm skirting board so any more than a 40mm perimeter strip would have been an issue for carpets or exposed concrete. Tiles or timber no issue. 

[Iceverge]

Our foundation detail. Note our insulation in the cavity extends a good distance below the floor level and this design mitigates the worst of the bridging on it's own. In a narrower cavity omitting the thermal blocks is likely to be more significant. You could add a thin layer of EPS below ground level outside the wall if you're concerned, or even top up your roof insulation to compensate. 

 

However the difference to the total heating load of the house is still likely to be in very low single digit percentages. It'll be completely negligible compared to airtightness and your choice of windows etc.  On a non passive house you'll never notice. 

[WWilts]

Thanks, great advice.

Would increasing the overslab insulation from 100mm to 150mm (Celotex lambda 0.022) be a cost-effective option? Need to watch the budget.

Noticed that heat escapes downwards through the inner leaf. Does Passiv standard recommend insulation horizontally to reduce that? Probably trivial loss in the larger scheme of things.

 

Also, does anybody add internal insulation just above the usable wall area only?

Edited September 1, 2021 by WWilts

[ADLIan]

Moving from aircrete to dense concrete will increase U-value from approx 0.25 to 0.28 (exact wall construction not known). As an area weighted average for the whole wall area the impact is probably negligible. The effect at the floor/wall junction can only be estimated if the psi-value is known and this will depend upon the thermal properties of the materials and the exact geometry of the junction. Only if you know the psi-value that was used in the original energy assessment can you estimate the effect of the different block type.

[WWilts]

17 minutes ago, ADLIan said:

Moving from aircrete to dense concrete will increase U-value from approx 0.25 to 0.28

Thanks.

What would be the U value difference for the perimeter upstand + inner leaf only please, aircrete vs concrete? Probably a negligible point, but interesting to nerds like me.

 

The mortar of the inner leaf probably offends as much as anything else.

Edited September 1, 2021 by WWilts

[ADLIan]

It's not a U-value at that point it is a linear thermal bridge or psi-value that is calculated using 2D thermal modelling. Doubt the thickness of the perimeter upstand insulation would have any effect. That detail looks to be very similar to the Accredited Construction Details giving an 'approved' psi-value of 0.16 W/mK. You need to check what psi-value was used in the energy assessment

[PeterW]

1 hour ago, WWilts said:

EPS is said to be more compressible than PIR, allowing the screed to expand/contract.

It doesn’t expand or contract - it is an utter myth ..!!!  The 20-30°C variation in the temperature on a 7m slab is hardly measurable. 

 

The reason you can get away with 3mm foam is it stops bonding the concrete /screed to the concrete blocks and that causes stress cracks elsewhere. 
 

 

[DragsterDriver]

Always pir upstand for me  

[WWilts]

13 minutes ago, ADLIan said:

That detail looks to be very similar to the Accredited Construction Details giving an 'approved' psi-value of 0.16 W/mK. You need to check what psi-value was used in the energy assessment

Accredited details used for SAP calculations.

[WWilts]

15 minutes ago, PeterW said:

The reason you can get away with 3mm foam is it stops bonding the concrete /screed to the concrete blocks

3mm foam? Not familiar with that idea

Edited September 1, 2021 by WWilts

[Iceverge]

1 hour ago, WWilts said:

Would increasing the overslab insulation from 100mm to 150mm (Celotex lambda 0.022) be a cost-effective option? Need to watch the budget.

Noticed that heat escapes downwards through the inner leaf. Does Passiv standard recommend insulation horizontally to reduce that? Probably trivial loss in the larger scheme of things.

 

Also, does anybody add internal insulation just above the usable wall area only?

 Increasing insulation is always cost effective but it depends over what time period. If you plan on moving house in the first 10 years its likely to never return your investment improving from BRegs. From playing with PHPP the floor was the slowest for me to return the investment. 

 

Passivhaus standard doesn't specify any construction details rather that the psi value at every junction be  

 

Or

 

That the (total internal surface area heat loss+thermal bridges) is less than the (total external surface area heat loss)

 

Our house isn't certified so maybe someone with more knowledge could help more. 

 

Internal insulation backed plasterboard is a common detail in Ireland on our new builds over the entire wall area. However is expensive, wasteful of materials, and very difficult to get a satisfactory airtightness result. 

 

 

 

 

 

 

 

 

 

[ADLIan]

The ACD figures for linear thermal bridges are worst case so would use dense block inner leaf at the floor/wall junction so the 'mistake' is covered in the energy assessment. The use of aircrete would improve things slightly.