Passive slab - is this sketch correct

[Big Neil]

Firstly I had no idea the already terrible sketch would scan so badly (my apologies), however for anyone who can make out the scribbles, have i got this more or less correct? the one thing I forgot to add whilst building this up was a DPM. Should this be immediately on the cross hatched area?

 

Second and third pages are the same but relate to how I understand an ICF structure should go atop such a foundation. Is this also correct?

 

 

Foundation.tif

[ToughButterCup]

Hi, you might ,like to re-save the image in jpg format and upload that too, @Big Neil, more people will be able to view your image. At the moment, its a '.tif ' (tagged image format)

Ian

[willbish]

Its quite difficult to make out your scan.

 

Here's a drawing I did of my passive slab. The hardcore layer needs to extent beyond the edge of the insulation which isnt shown.

 Part of the reason I chose Isoquick was because the concrete has 300mm under all areas. Some other systems have less insulation around the ring beam.

 

Foundation Section.pdf

[willbish]

Page 8 of the AFT catalogue has some section drawings which may be of use to you

 

Particularly if you want to build a solid outside skin

 

AFT(29515)Groundshield_Installation_V2.pdf

[Big Neil]

3 minutes ago, recoveringacademic said:

Hi, you might ,like to re-save the image in jpg format and upload that too, @Big Neil, more people will be able to view your image. At the moment, its a '.tif ' (tagged image format)

 

I will give it a try when i get home. At work at the moment and no way of saving it. We just have a crap low grade edition of adobe pdf reader so i can't save. Why that whacking great idustrial grade  printer/scanner doesn't just do it straight to PDF's is beyond me...

[Big Neil]

boooom - any better for people???

 

[Russell griffiths]

I would like to add a point about icf and a slab that has underfloor heating pipes in it @willbish and @Big Neil

i have just done a pour for my icf walls and found the propping system for the walls required over 120 holes to be drilled into my floor to hold the props up. 

Make sure you have thought of a way to hold these props up if you have pipes installed 

[Big Neil]

thanks @Russell griffiths. Would there have been any way of bracing your props without doing this, but which you might have discounted for whatever reason?

[Big Neil]

To add to the original post, if the cage part of things is correct, could one forego the need for mesh over the rest, by use of fibre/micro re bar? This way @JSHarris, could one then staple to the insulation and THEN pour. Might even reduce the likelihood of abrasion.

[Russell griffiths]

I did think of many ways, scaffold boards fixed on the floor with props on top ???

in the end I opted to leave out the ufh and install it on top in an additional screed

it was just the way I chose to do it, I’m sure there’s a thousand other ways, i was just happy that with all those screws I knew I had no pipes to hit. 

And after pouring yesterday I can confirm that those props need to be very very secure. 

[Russell griffiths]

2 minutes ago, Big Neil said:

To add to the original post, if the cage part of things is correct, could one forego the need for mesh over the rest, by use of fibre/micro re bar? This way @JSHarris, could one then staple to the insulation and THEN pour. Might even reduce the likelihood of abrasion.

You won’t have a problem with abrasion unless you leave the pipe in place for too long, fit it and concrete in a week or two and no problems. 

I belive you get better heat transfer and a quicker reaction time if the pipe is in the middle of the slab. 

Tbh, at the end of the day it will be down to your structural engineer to design your slab according to you soil survey and site conditions. 

[Big Neil]

15 minutes ago, Russell griffiths said:

I did think of many ways, scaffold boards fixed on the floor with props on top ???

in the end I opted to leave out the ufh and install it on top in an additional screed

it was just the way I chose to do it, I’m sure there’s a thousand other ways, i was just happy that with all those screws I knew I had no pipes to hit. 

And after pouring yesterday I can confirm that those props need to be very very secure. 

 

I assume doing it this way gave you the freedom to use a thinner level of insulation to lay your UFH onto? Could you in this case for example opt for those slotted boards? how are you going to go?

[Big Neil]

Also Mr G, is there any particular benefit to doing it the way you are? It's something i've considered in the past. I assume for example given it being an ICF build, you would't need to put expansion strips on those walls?

[TerryE]

@Big Neil, designing a passive slab is not something that should be done on the "back of a fag packet".  If you get it wrong then you might never get the BInsp to sign off on it, and more to the point your house might be structurally unsound.  You really need to have this done by an SE experienced in slab design.  So you need to find such an SE to do this.  If you want then a few of us have constructed our houses this way and can give you our SE details.

 

There are too many variables here for there to be a stock answer.  For example our SE required us to do a soil survey because the type of base materially effects the under-slab preparation: whether it is clay or sand, and the sub type, moisture contents, proximity of trees, etc.  Any differentials across the site can be critical to whether a passive slab can be used and how deep the foundation layers need to be.  The last thing that you want to happen is that you get any differential slump across the slab during pour.  This can and has happened.  My slab had to support an outer stone skin; Jeremy's didn't: we had to have internal load bearing cross members.

 

The slab has to have a design than is demonstrably compliant to B Regs; it has to work both structurally and thermally.

Edited December 18, 2018 by TerryE

[scottishjohn]

1 hour ago, willbish said:

Its quite difficult to make out your scan.

 

Here's a drawing I did of my passive slab. The hardcore layer needs to extent beyond the edge of the insulation which isnt shown.

 Part of the reason I chose Isoquick was because the concrete has 300mm under all areas. Some other systems have less insulation around the ring beam.

 

Foundation Section.pdf

were you foreced to have the raydon sump?

i don,t see any purpose for it if you have a barrier under the whole house.

theres no where for it to gather up in the foundations the way you have your barrier -

.personally  left to my own devices i would have put it below everything next to the compoacted sub soil and wrapped it round your nice insulated raft.

the worry with old type founds was it would gather up below the suspended floor

.or at least thats how i understood it

round here its all granite anyway 

[ToughButterCup]

1 hour ago, scottishjohn said:

were you foreced to have the raydon sump?

i don,t see any purpose for it if you have a barrier under the whole house.

 

 

While I agree with the sentiment, its absence is an easy win for the BCO. No sump? Oh dear....

Pick your fights: this isn't worth fighting about

 And all for £14 and a trench

 

 

 

[scottishjohn]

2 minutes ago, recoveringacademic said:

 

 

While I agree with the sentiment, its absence is an easy win for the BCO. No sump? Oh dear....

Pick your fights: this isn't worth fighting about

 And all for £14 and a trench

 

 

 

thats why I asked if he was forced --cos  they limit space under houses here --don,t like you having an area where the gas can build up ,which is a bugger cos who wants a crawl space under house --you want to be able to sit up at least .

If i put that sump under new house here they would want a fan  on a timer to purge it if you got they nasty BC man on a bad day 

lots you can,t even crawl under 

[Pete]

We just put in a Radon barrier in our Passive slab and B/insp was ok with that. No sump/fan.

[willbish]

2 hours ago, Russell griffiths said:

I would like to add a point about icf and a slab that has underfloor heating pipes in it @willbish and @Big Neil

i have just done a pour for my icf walls and found the propping system for the walls required over 120 holes to be drilled into my floor to hold the props up. 

Make sure you have thought of a way to hold these props up if you have pipes installed 

 

I've left a 75mm perimeter gap between inside skin of ICF walls and first UFH pipes. I'm going to fix scaff boards down at one end for the props to sit on. Like Russ says many many different ways to do it. The added faff of messing about with scaff boards out weighs cost of UFH in separate screed in my case.

Edited December 18, 2018 by willbish

[willbish]

1 hour ago, scottishjohn said:

were you foreced to have the raydon sump? 

Yes, high radon area and full protection measures required.

I agree the effectiveness of the sump is questionable in that location. I didn't want to cut a hole and a channel into my insulation to accommodate just below the slab, so I proposed placing it in the Type1 to Building Control who were happy.

 

37 minutes ago, recoveringacademic said:

Pick your fights: this isn't worth fighting about

 And all for £14 and a trench

Exactly ?

[scottishjohn]

41 minutes ago, recoveringacademic said:

 

 

While I agree with the sentiment, its absence is an easy win for the BCO. No sump? Oh dear....

Pick your fights: this isn't worth fighting about

 And all for £14 and a trench

 

 

 

but if thats what they ask for ,then are just plain wrong ,the whole point is to stop radon gas building up --so  if membrane  is in right place no need for a sump or anything else .

rolling over to thier wrong decisions will make them even worse dictators

 

[Jeremy Harris]

Pointless fitting a radon sump and breathers if you live in an area where there isn't any granite under the house, as radon only forms as a breakdown product of naturally-occurring thorium and uranium, and you only find those metals in areas of the UK where granite outcrops.

 

For example, here in the South we have 40m of blue gault directly under the house, then ~10m of running greensand, then maybe 50m or so of Purbeck limestone.  There's no granite anywhere near the surface, so there is zero radon risk.  Much the same applies to large swathes of the UK. 

[Gone West]

This will show you where there's radon.

 

https://www.ukradon.org/information/ukmaps