Foundation design

[Kelvin]

Hi

 

I have finally got the SE pack back (well 4 drawings plus details) it's only taken them 9 weeks. Long story.

 

The house is a CPS timber kit (302mm) The blockwork foundation  detail is 325mm blockwork with a compressive strength of 7N/mm2 and a minimum built density of 750kg/m3. I was hoping to use an insulated block but they all seem to have a maximum density of 730kg/m3. Any suggestions? I also have concerns about thermal bridging which I've asked them about. 

 

 

 

[Carrerahill]

8 minutes ago, Kelvin said:

Hi

 

I have finally got the SE pack back (well 4 drawings plus details) it's only taken them 9 weeks. Long story.

 

The house is a CPS timber kit (302mm) The blockwork foundation  detail is 325mm blockwork with a compressive strength of 7N/mm2 and a minimum built density of 750kg/m3. I was hoping to use an insulated block but they all seem to have a maximum density of 730kg/m3. Any suggestions? I also have concerns about thermal bridging which I've asked them about. 

 

 

 

I think Celcon do a 750Kg - I read it in one of the journals, less cement used, higher strength and lighter were the key points. 

[Kelvin]

I was looking to use Celcon but their highest density is 730 kg/m3

[Kelvin]

More googling and I've come across this:  Mannok Lite Block 440 x 215 x 100mm 7N (Each) 

 

Gross Dry Density 760kg/m3

Average Compressive strength 7.5 n/mm2

Thermal Conductivity 0.19W/mk

 

 

[Thorfun]

you're having those awesome twin stud wall (I believe called Larsen truss?) which I presume is being filled with some form of cellulose pumped in? if so, why have you not considered a passive insulated slab? would get rid of all of your thermal bridge concerns at the slab level.

[Kelvin]

It's a good question. 6 months ago I knew nothing about any of this. I've been following architect's advice (timber kit company is Heb Homes) this is their standard foundation design and typically Scottish apparently. Since then I've become a bit more informed and now understand the consequences of some of the decisions better and started looking at passive insulated rafts. I have a meeting with HH on Monday to discuss all of this. 

[JohnMo]

At the moment you have a nice thermal bridge, I would also be concerned about condensation forming at the sole plate.

 

I have a typical Scottish strip foundation, with blockwork coming up from the foundation.  I modified mine with full structural approval.  Slightly different ICF build but the principals should be the same.  Your soleplate fixing could no longer be central, but be staggered between the inner and out blocks.

 

Total width 365mm

Inside block 100mm thermolite

Cavity filled with 100mm PIR - size to suit width required.

Outside block 150mm concrete block.

 

Inside all that I used a 70mm PIR upstand from the floor insulation, inside that is concrete screed 100mm deep.

[Iceverge]

Some details show twin wall timber frames supported on one side only allowing continuous insulation from the floor to the wall. Either insulated raft on the inside or supported on a block and strip foundation on the outside. 

 

Maybe you could explore this. 

 

 

 

[Kelvin]

Thanks. 
 

It doesn’t look to me that the kit load point is on one side though. 
 

I’ve spoken with Isoquick and they’ve come back with a couple of options and a cost estimate although they need the load points too of course. 
 

I’ve also spoken with the timber kit supplier (the MD in fact) and we’ll follow it up on Monday. They have done Isoquick foundations for their builds in the past but my guess is that is likely to have been with their SIP kits rather than the CPS we are doing. He did say that a thermal bridge like this wouldn’t get past building control anyway. He made a curious point (to me anyway) He said that the SE won’t consider insulation as part of the structural design as he’s only focused on holding the building up. It’s the architects job to figure out the insulation detail. That seems odd to me as surely one drives the other? 

Edited October 8, 2022 by Kelvin

[Kelvin]

21 hours ago, JohnMo said:

At the moment you have a nice thermal bridge, I would also be concerned about condensation forming at the sole plate.

 

I have a typical Scottish strip foundation, with blockwork coming up from the foundation.  I modified mine with full structural approval.  Slightly different ICF build but the principals should be the same.  Your soleplate fixing could no longer be central, but be staggered between the inner and out blocks.

 

Total width 365mm

Inside block 100mm thermolite

Cavity filled with 100mm PIR - size to suit width required.

Outside block 150mm concrete block.

 

Inside all that I used a 70mm PIR upstand from the floor insulation, inside that is concrete screed 100mm deep.

 

That sounds more like I was thinking. You don’t happen to have it drawn you could share? 

[JohnMo]

Section of my structural drawing attached.

 

The 140 thermolite was changed to 100mm, as at the time there were no 140mm wide ones available, insulation width increased.

 

The screed depth was also increased to 100mm, as without taking 35mm off each block the stub wall buildup didn't work out correctly.

 

[Kelvin]

Cheers. I can see a way how to build up the block wall to achieve the same outcome with my foundation. I’ve also found an insulated block that meets the design strength spec. 

[Thorfun]

8 hours ago, Kelvin said:

He said that the SE won’t consider insulation as part of the structural design as he’s only focused on holding the building up. It’s the architects job to figure out the insulation detail. That seems odd to me as surely one drives the other? 

time for them to get a new SE? 

[markc]

5 hours ago, Kelvin said:

Cheers. I can see a way how to build up the block wall to achieve the same outcome with my foundation. I’ve also found an insulated block that meets the design strength spec. 

Don’t call it insulation, say you want to use a revolutionary lightweight structural block called EPS, give them the mechanical properties sheet (compressive strength) and say the idea is the isolate the structure from the horrible corrosive cold ground.

if that fails go with @Thorfun and get another SE

Edited October 8, 2022 by markc

[Gus Potter]

On 07/10/2022 at 10:32, Kelvin said:

The blockwork foundation  detail is 325mm blockwork with a compressive strength of 7N/mm2 and a minimum built density of 750kg/m3. I was hoping to use an insulated block but they all seem to have a maximum density of 730kg/m3. Any suggestions? I also have concerns about thermal bridging which I've asked them about. 

Hope this helps give some pointers that will help you frame your questions in the way you want.

 

A 7.0 N/mm^2 unit strength dense concrete block wall weighs about 1800 to 2100 kg per m^3. An aerated (say Celcon ) concrete block wall weighs about 400 to 900 kg/m^3 and has a varying unit compressive strength. Remember that the light weight blocks contain air pockets.. the more air pockets the less concrete material to resist the loads so lower compressive strength but better u value. To make an aerated block the same unit strength as a dense block you need to use more cement for example and that is one reason why they are more expensive. The density is not usually a primary issue, it's the block strength that is considered first.

 

In principle there is no reason why you can use an insulating block provided it has the same compressive unit strength.  Just check you are not in aggresive ground for example.. always check with your|SE before you swap out materials. Also, check the mortar mix in case the blocks need a different spec. If the mortar is too strong it plays havoc with aerated blocks.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[Kelvin]

It’s these blocks. Mortar is M6 mix. However, there’s something that appears contradictory in the SE notes but it might be my lack of understanding. It states two different minimum densities. 

 

[JohnMo]

Looks like a bit of cut and paste engineering which hasn't captured all the changes needed.  I would ask him to clarify.

[Kelvin]

Yes what I assumed. I’ve emailed them about it. 
 

It’s another source of frustration. I don’t know if it’s normal but the lack of attention to detail is shocking to me. Since the start every single document I’ve been sent is full of errors. Ironically the SE drawings were delayed in being sent to building control as the kit company wanted to scrutinise them to make sure there were no errors (because of my moaning about all the other errors). What concerns me though is while I can easily spot these kinds of errors I don’t have enough technical knowledge to spot anything that’s technically incorrect. Nor should I of course but it is sapping my confidence. 

[JohnMo]

Our structural engineer missed the inner support for the lower part of the roof.  There is a support from the portal frame to hold up 2x 450mm glulam beams.

 

Neither the architect nor the structural engineering had any details of the shown on any drawings.  Ended up designing it myself, steel fabricator came back and site welded it together.

[Kelvin]

Here's an example of a typical construction drawing. It's not entirely clear from the keynotes but it looks like they fit 100mm of perimeter insulation above and 150mm below floor insulation. 

 

[Mr Punter]

A course of Marmox Thermoblock or Foamglas Perinsul blocks under the sole plate should largely eliminate the cold bridge.

[Iceverge]

Or you could use the denby dale detail but with a narrower cavity so that both side of the twin wall TF landed both sides of the insulation. 

 

 

Mind you from your drawing it look like you have a 50mm service cavity. 

 

15mm plasterboard, 

50mm SC

overlap the sole plate by 10mm 

 

 

you could have a 75mm perimeter insulation. 

 

This would perform excellently. 

 

 

 

[Kelvin]

The Foamglas stuff isn’t dense enough. The sole plate is 300mm wide so what would the blockwork build up look like using Marmox? 
 

The service cavity is 25mm. 

[Thorfun]

57 minutes ago, Mr Punter said:

A course of Marmox Thermoblock or Foamglas Perinsul blocks under the sole plate should largely eliminate the cold bridge.

but wouldn't the cost of a course of that lot around the perimeter be similar price to a truck load of EPS?! I had a few Foamglas blocks which I got from @ToughButterCup for a cheap price as he had them leftover. but to buy them new would've been eye-watering for the perimeter of a building.

[Kelvin]

Exactly my thoughts. The Marmox stuff is very expensive.