Insulated Concrete Slab Garden Office - Questions

[Iceverge]

Floor level 150mm above ground. 

 

Otherwise you're into a basement type detail really. 

[Ticky]

What does raising my current design by that amount achieve? I appreciate I wouldn't need the brick/block work and the timber could be fixed directly to the slab, so that would solve the thermal bridge bit I presume, but what else would it mean?

 

Thanks

[JohnMo]

Why not a simple strip foundation blocks on that, nothing to complex needed inside the blocks, harcore,  sand DPM, insulation with upstand around perimeter, with concrete screed or floating wood on PIR.

 

Do the blocks below ground mostly, have 50 to 100mm showing above ground, build from there.  Gravel around the external perimeter to limit rain splash. Job done. It's not a 100 year design life, like a house.

[Ticky]

Something like this @JohnMo ?

 

Am i right in thinking that the blocks would be the same depth as the timber that would be fixed on top of it?

In my head this seems like a much simpler approach and less to worry about regarding exposing the insulation to heavy load/moisture

[JohnMo]

You can get the blocks 100 or 140/150 wide. But you may be able to have block less wide than the wall and carry up the perimeter strip to the base of the wooden walls, to remove the thermal bridge as the block comes out of the ground.

[Ticky]

Yep, I had that as an after thought about the perimeter strip. Thanks @JohnMo

 

I guess with this strip foundation design I could interchange the position of the slab and insulation?

 

@Iceverge does this help me overcome the concerns you were pointing out? as my floor level will remain close to ground level.

[Iceverge]

28 minutes ago, Ticky said:

does this help me overcome the concerns you were pointing out? as my floor level will remain close to ground level.

 

Will do a model in an hour or so if I can for you ....

[Iceverge]

 

 

 

Some thermal simulations of what you've drawn. 

 

Think of keeping heat in like keeping sheep in a field. The will get out eventually but you have to try to make their progress as slow as possible. 

 

Higher R value materials like Mineral wool and polystyrene are like denser bushes in the fence that will really slow the sheep ( heat!). You need less thickness to slow them to an appropriate level. 

 

Lower R value materials like concrete and steel are like really non dense bushes that the sheep can move through easily.  When you have a low R value material like brick/concrete connecting the inside and outside it's a thermal bridge. It's so significant that it renders all the other insulation almost ineffective.  For my analogy it's equivalent to letting the gate of the field open, the sheep are going to run straight out there and the thickness of bushes ( insulation ) elsewhere is irrelevant. Demonstrated by the Flux vector ( arrow) diagram lowest above.  

 

 

Edited March 11, 2023 by Iceverge

[Iceverge]

Here's the reason that any timber products are kept well out of the rain splash zone. 150mm is recommended as a minimum. 

 

It can even be a problem for single skin masonry too. The splashed water can find its way to the inside via mortar joints etc. 

 

 

 

 

The problem is made worse by having a surface that will allow water to pool or bounce easily. Gravel is a good way to stop this.  Make it deeper and put some drainage pipe there and you have a French drain that will prevent the local water table ever rising up and pushing up water inside the floor, even if the DPM fails. ( Assuming the French drain can always flow to a lower plain)  

 

 

 

 

[Iceverge]

 

 

Here's a quick sketch of a robust detail.  

 

Note the 65mm upstand which joins the insulation between 150mm studs to mitigate the thermal bridge. 

 

I would consider tanking/rendering the outside of the foundation blocks to stop rain water getting into them as over time the freeze thaw action may crack them. 

[Iceverge]

 

 

 

 

Something like this is what I'd look at if going below a floor level of 150mm. The door would still need to be above the DPM termination in my mind so you'd be stepping up to step down. 

 

I would tank the floor to the walls with a waterproofing kit  and put in a drainage sump for when the floor floods too. 

[Ticky]

Thank you @Iceverge
Looks like you put some time into that so I really appreciate it.

[Iceverge]

 

This might be easier to build. Note the 50mm insulation outside the concrete, the 50x100mm wall plate supporting 150mm studs to give a much better junction thermally. 

 

Something like EPS 100 should be fine for a garden room. I've thickened the slab 50mm at the edge for some strength but it's educated guesswork really. Some reinforcing mesh would make it much stronger too. 

[IanR]

4 minutes ago, Iceverge said:

This might be easier to build. Note the 50mm insulation outside the concrete, 

 

We've gone full circle.

 

 

Edited March 13, 2023 by IanR

[Dan F]

We used this approach to get a 2.8m high garden room with good internal headroom (the ground level was higher at the back of the building).  (We just used standard low u-value blocks for the retaining wall, nothing special)

 

 

 

[Ticky]

Thanks again.

 

I'm starting to think that the easier options will be my best approach to which I'd have to accept the slab being 15cm off the ground as in @Iceverge's last drawing.

 

A couple more questions about that.

 

1. How would the concrete board be fixed to the upstand? In an earlier comment you said EPS doesn't need any DPM between it and hardcore as long as there's a perimeter drain. How does the layers below the EPS differ to my drawings of hardcore with a layer of sand.

 

2. With the wallplate being shorter depth than the studs does that mean that the studs have the corners cut so they sit around/over it? I'm just trying to picture it.

[Dan F]

This is another approach, that we used on house, that doesn't involve a retaining wall on the slab.   We did this because the rear of the house has a level threshold and we wanted the rear patio at the same level across the whole back of the house.

 

(The drawing is slightly wrong because external GL is actually the same as internal FFL)

 

 

 

 

[Ticky]

Thanks @Iceverge @IanR

 

I'm gonna go with that (full circle) design

 

One thing I've been thinking about it the best way to fix the faceplate to the concrete. What I don't want to do is blow the sides out of the slab by drilling/screwing too close to the sides and as 50mm of it will be made up of EPS, is the best way for me to do it, to set some bolts into the concrete facing up, and then fix the faceplate to those. Any tips on that?

 

Also, what would be the depths of the hardcore/type-1/pea shingle layers? 

And what's the difference between hardcore and type-1? Is hardcore much bigger stone/bricks etc?

 

My day job has delayed my progress this last month, but keen to start pushing forward now. I've already started digging out the space I'll use, first big job is removing the roots of the tree that was in the way, which was taken down last month. Got the guy coming back to grind the stump down to about 500mm below ground level, but I need to clear the way for him. I've got a skip coming this week to fill with the ground I'm removing. I started filling rubble bags but it was taking too long, plus I can use the skip for other crap I need to chuck.

[Temp]

2 hours ago, Ticky said:

is the best way for me to do it, to set some bolts into the concrete facing up, and then fix the faceplate to those. Any tips on that?

 

Whats what I did. To stop them falling over before the concrete set you can fix them into some wood beforehand and just push them in until the wood is on the surface of the concrete. 

 

Put scrap waste pipe over them once set to avoid impaling yourself if you trip over. 

 

My bolts were long enough to go up through two courses of brickwork and the wall plates. I made them from threaded rod and nuts.

 

Don't leave it too late to install them as I found the concrete set up faster than expected. 

 

Edited April 10, 2023 by Temp

[Ticky]

On 10/04/2023 at 15:34, Temp said:

 

Whats what I did. To stop them falling over before the concrete set you can fix them into some wood beforehand and just push them in until the wood is on the surface of the concrete. 

 

Put scrap waste pipe over them once set to avoid impaling yourself if you trip over. 

 

My bolts were long enough to go up through two courses of brickwork and the wall plates. I made them from threaded rod and nuts.

 

Don't leave it too late to install them as I found the concrete set up faster than expected. 

 

Good safety advice, thanks

 

Did you have a DPC? Would I need to cut holes in it to allow the bolts to pass through?

 

[Temp]

3 hours ago, Ticky said:

Good safety advice, thanks

 

Did you have a DPC? Would I need to cut holes in it to allow the bolts to pass through?

 

 

 

Yes I laid two courses of engineering bricks then put a DPC on top and then the wall plate.

 

I cut an X with a knife in the DPC and just pushed it over the bolts. Then trimmed off the bits that stick up with a stanley knife. It was overlapped at corners and where the end of the roll occurred. The double thickness in a few places didn't cause any issues. 

 

The holes in the wall plate were drilled quite a bit oversize just to make their position less critical and allow it to drop over the bolts easily. I put big washers on top under the nuts. 

[Iceverge]

On 10/04/2023 at 13:31, Ticky said:

gonna go with that (full circle) design

 

Keep us up to date with pics please. We're all learning. 

 

On 10/04/2023 at 13:31, Ticky said:

One thing I've been thinking about it the best way to fix the faceplate to the concrete

 

Put the bolts in when you pour the slab as above. plenty of tape, or a bit of snug pipe slipped over the top to keep the threads free from concrete. 

 

When the time comes for a DPC I would prefer a paint on one ( less risk of trapped moisture and no gaps). You can do a more effective job of air sealing too this way. 

 

Lay the sole plate on top of the bolts. Use a hammer to hit the timber such that the bolts make an imprint. Then drill the imprints. Perfectly accurate without the need to oversize the holes. 

 

Lay the sole plate on a fat bead of permanently elastic sealant or FM330 foam to create a robust seal between the floor and the wall. 

 

 

 

 

 

[Ticky]

Thanks @Iceverge - again, great tips.

 

For the foundaton. What depth should each of the layers be that sit underneath the EPS?

 

And yes, I'll be documenting with pics as I'm going along

[Iceverge]

On 14/04/2023 at 10:59, Ticky said:

For the foundaton. What depth should each of the layers be that sit underneath the EPS?

 

 

Yikes. A piece of string question....

 

Ultimately you are aiming to have your foundation on a layer that doesn't move with changes in moisture or temperature. Or at least moves a minimal amount and evenly. 

 

Any soil with lots of organic matter is a No No. Likewise any soil that will swell and shrink because it will get wet and dry repeatedly. Tree roots can have the same effect. 

 

Then there's frost, deeper is better but it depends on the local conditions. I've heard 600mm below ground but I have no idea where. 

 

What kind of soil have you got? 

 

Have you got the spade out and dug a trial hole? . Say 300*300 and 600 deep. Do this and post some pics. 

[Nickfromwales]

So many ways to over-think this! 

On 13/03/2023 at 13:04, Iceverge said:

 

This might be easier to build. Note the 50mm insulation outside the concrete, the 50x100mm wall plate supporting 150mm studs to give a much better junction thermally. 

 

Something like EPS 100 should be fine for a garden room. I've thickened the slab 50mm at the edge for some strength but it's educated guesswork really. Some reinforcing mesh would make it much stronger too. 

Can't help coming back to this each time I re-read this. Deliciously simple. Thanks!

 

I was looking at 2x 3x2 stud walls, disconnected by a 2" gap, outer on 75mm EPS upstand ( instead of the 50mm you show ) and the inner simply sat on the slab. So an 8" void, less the sole plate and repeat cold bridging, and filled with 'rockwool' batts / loose stuff or foam where necessary. 9mm OSB3 biscuits will be installed x2 per vertical rise to join the internal stud to the external stud to stiffen things up a little, with near zero bridging.

 

I've already started taking down my beloved rear gazebo so I can get a micro digger and a powered barrow up steel ramps, up two rises of patios, to get to the top of the garden to start digging out for my new "studio" home office, which will then get PV on the roof. A home office has become a no-brainer, so glad I had the winter to rethink all of this before putting PV on the gazebo! ( It will now get rebuilt with a single mono-pitch roof, south-facing for 9 PV panels ( 21 then going on studio roof E/W split )).

 

For simplicity, I am thinking 300mm of EPS under slab to absolutely minimise losses ( cheap as chips tbh ) and an electric in-screed wire for heating the slab "just enough" so it's comfortable in there, plus a cheap as chips AC system for additional heat / cooling, only running that when absolutely necessary.

 

Roof would be pre-insulated powder-coated steel profile sheets, with a second layer of PIR bonded underneath. Easiest for an MCS compliant ( DIY ) PV mounting system.

 

I think that's a solid plan?

 

Single room MVHR most prob, or PIV and some one-way hi / low vents.