Passive Strip Foundations

[Iceverge]

Hi All, 

 

Welcoming opinions on the below detail. 

 

Plaster and Air tightness omitted for now. Aerated concrete blocks shown in light green. 

 

[PeterW]

Any reason for a 215mm inner block ..??

 

You could drop that to 100mm and gain 115mm inner space. 

 

Also, push the floor insulation up to 250 or 300mm and you’re at a really good spec all round. 

 

What is the significance of the light green blocks ..??? 

[Iceverge]

215mm leaf specified by Structural Engineer due to lack of buttressing walls and loadings from precast concrete first floor. Upstairs will be 100/250/100    block/cavity/block.

 

The footprint of the house was designed around these thick walls. Had we needed more space we could have added it in the plans. 

 

I ran PHPP with multiple options and was able to achieve 11 kWh/(m²yr) and 8 W/m2 with this amount so I didn't see the need for more.

 

They are lightweight concrete blocks (k value of 0.17 W/(mK)) to help reduce heat loss to the foundations. 

[Iceverge]

Here are some Therm Simulations I did. Apologies as it is a mirror image of the above diagram. The inside of the house is on the left in these drawings. 

 

 The final two pictures probably tell the story best. You can see that without the lightweight blocks the heat soaks much further down into the foundations and subsequently escapes to outside. I found little benefit in going beyond 2 courses of these. Carrying the insulation down into the cavity helped a lot however.   

 

 

1. WITH LIGHTWEIGHT BLOCKS

 

 

 

2.WITHOUT LIGHTWEIGHT BLOCKS

 

 

 

3.WITH LIGHTWEIGHT BLOCKS

 

 

 

4.WITHOUT LIGHT WEIGHTBLOCKS

 

[Miek]

My foundation design is very similar, the architect has specified foam glass perinsul as the thermal break on the inner skin (light green in your case). This means you then don't need to insulate below ground within the cavity. 

Edited August 28, 2019 by Miek
to add pic

[Iceverge]

While we're at it, I'll include 2 options for the threshold. One with covered fiberglass grating and one with compacfoam.

 

I'm leaning towards the GRP grating as it looks stronger and more tolerant of onsite shenanigans. Does anyone have any experience of these two choices or something similar? 

 

 

 

[Miek]

24 minutes ago, Sensus said:

Your DPC needs to be at least 150mm. above ground level (and it is normal practice to link it to the radon membrane/DPM and cavity tray.

 

At the moment, you have a path for damp over the top of the DPC to the junction of the base of the wall with the floor slab.

 

I'm not terribly happy with the DPC arrangement on Miek's drawing,  either - I would be referring that back to the Architect, if I were him.

could you elaborate? is the concern that the weepholes and DPM are at ground level with the outer leaf DPC 1 block above? 

 

[Miek]

Thanks for taking the time to explain, much appreciated.

 

Apologies to the OP for jacking this thread

 

[joe90]

Well we used the golcar passive foundation idea but with a twist. My builder (who has used this before) lowered the DPC to internal floor level which meant the weepholes were at ground level but draining into a French drain (no hard path) he then inserted a second DPC 150mm above ground level on the external skin and this stops “rain bounce” from soaking up the external brick work. My builder found that the Golcar internal DPC above skirting level created a slip plane and a crack line in the internal plaster above the skirting line.

[Miek]

1 hour ago, joe90 said:

. My builder found that the Golcar internal DPC above skirting level created a slip plane and a crack line in the internal plaster above the skirting line.

 

That's a good point, i hadn't thought of that issue arising.

 

[joe90]

Actually, just had a lightbulb moment, why bring the DPC into the inner wall? The inner wall is above the DPC within the floor makeup. Why not just wrap it over the sloping cavity insulation and put through the outer wall!!!

[Ferdinand]

1 hour ago, joe90 said:

French drain (no hard path)

 

I created a hard path on a drain by laying BFO 900x600mm slabs on loose gravel.

Also a way of managing a moving surface or tree roots if that is a problem.

 

F

[Miek]

8 minutes ago, joe90 said:

Actually, just had a lightbulb moment, why bring the DPC into the inner wall? The inner wall is above the DPC within the floor makeup. Why not just wrap it over the sloping cavity insulation and put through the outer wall!!!

Like this?

 

[joe90]

Yup!, no slip plane and DPC 150mm above ground level. I would like others opinion on this, may be missing something.

[Miek]

the only issue I see is that potentially if water got into the cavity and crossed through the insulation to the inner leaf and ran down it would end up on the inside of the building. This is a remote possibility but better to design for worst case scenarios IMO.

 

I'm not convinced entirely that there is a big issue with the original design, but its a detail which needs to be got right in the first place as it can't easily be altered afterwards .A perimeter french drain seems a  good idea regardless. 

 

 

[Iceverge]

Thanks v much for all the input. 

 

I hope that the blown bead will be pretty resistant to soaking water as well. 

 

How about this? Notice I have't extended the tray entirely across the inner leaf to avoid plaster cracking. Thanks@ @joe90