Insulated Concrete Slab Garden Office - Questions

[Ticky]

Hi all,

 

I'm new to the forum but already found some useful guidance on here, so thanks.

I'm also new to this level of DIY but I'm not shy at having a go, like to make sure things are done correctly (hence my questions) and have a brother who's handy at a lot of things to help (who's also a spark :))

 

So let me set the scene first.

The garden room I want to build will be 5m x 3m to stay within the 15m2 limit and needs to be close to the perimeter so I'm limited to 2.5m height - I'm 6ft 2in so need to maximise the headroom.

 

Because of this, I'm leaning towards an insulated concrete slab, with the insulation below the concrete. As it will be used as an office, I'd even consider something like polished concrete as the finish.

 

Ideally, my slab would be something like my attached diagram, with insulation below the slab but also 25mm to the sides as well. That would mean both the DPM and the separation layer would need to be folded over the side insulation and the slab with the idea that the wall timbers would rest on top. The reason I've put 5x2 as an option is that the timbers would need to overhang the slab to cover the 25mm of extra side insulation so just thinking of getting enough coverage to fix to the slab without potentially cracking the concrete.

Also, I appreciate that the wall timber will be close to the ground but I thought the OSB and coverings would add protection (happy to be corrected/guided)

 

I'm not planning on pouring the concrete myself but I'm thinking of doing all the prep work myself so it's done as I want it

 

Also, having the slab at almost ground level would mean I can get the most headroom before I hit the roof joists, but I'm conscious this brings more challenges with regards to damp etc

 

I'd be very grateful for your feedback on this. 

 

Thanks

 

Edited March 1, 2023 by Ticky
added more info

[Iceverge]

The norm is to keep any sole plate at least 150mm above the ground to prevent any splashback damage. 

 

The OSB will be a structural component of the building too and longterm fairs even worse than timber when exposed to wet.  I wouldn't have them that close to the ground. 

 

For the roof you could do a 100mm insulated panel at 5⁰ slope. 

 

Even with a 150mm floor that would give you 2m head height at the low side. 

 

To drop the floor any further I would have a very robust perimeter drain and a blockwork cavity outside the timber.

 

 

 

[IanR]

Here's my go at an over-engineered insulated raft, for a Garden Office.

 

As per @Iceverge, ground should be around 150mm below the finished floor level. No separation layer needed for EPS.

 

To do the chamfer detail on the upstand you'd need to make up a hot wire cutter. If not then a thinner upstand (25mm) will do. Upstand can be stuck and screwed to the base layer. You could, almost definitely, do it all in EPS100, rather the the EPS200 I've shown under the ring beam, as you won't be having a heavy roof finish on that 5° pitch.

 

 

 

Edited March 1, 2023 by IanR

[Ticky]

Thanks for your feedback and suggestions.

 

I suppose I'll just accept that the slab will be raised 150mm, as pointed out I should still get 2m if I choose the right roof design.

Thanks for sharing your design @IanR. I like the use of the lip below the cladding (along with the alu clad/render) to help direct any rain away from the base. However, I notice that your design doesn't have any DPM between the lowest layer of insulation and therefore your 50mm upstand is directly exposed to the ground apart from the aluminium clad.

Would my idea of folding over the DPM and separation layer be ok if I raised everything by 150mm?

Also, what would be the recommended depth for my concrete? Would rebar allow me to keep the depth to 100mm?

I'm thinking of using Celotex GA4000 PIR at 75mm, as aiming for a U value of 0.25 and that seems to offer it (based on their calculator).

 

[Moonshine]

This is what i did and didn't have a screed above the insulation, just UFH and laminate, its not too springy. I did it this way as in future the outbuilding may be used as a garage.

 

 

in relation to the roof to save floor to ceiling height, have a look at a hybrid roof.  This would be a big no no if it was a permanently occupied and in a cold area, due to the risks of interstitial condensation, but as its an outbuilding then it may be possible.

 

We have done it, and will see how it performs.

 

Have a look what Oakwood Garden rooms say on the matter, though there is some debate.

 

https://www.youtube.com/watch?v=EZFcHcxbFbE

 

https://www.youtube.com/watch?v=P_dBayq3_FA

 

[Temp]

On 03/03/2023 at 11:17, Moonshine said:

This is what i did and didn't have a screed above the insulation, just UFH and laminate, its not too springy. I did it this way as in future the outbuilding may be used as a garage.

 

 

 

 

That's virtually the same as what I did.

 

In case not obvious to @Ticky.. The blue lines are typically plastic sheet. The one below the slab stops water in the concrete draining out weakening the mix before it sets. Under mine I put hardcore and a sand blind. The one above is a DPM. There is also a DPC on top of the brick courses. The one in the wall is a Vapour Control Layer marked VPC.

 

On mine I made the slab slightly above ground and put small gravel filled trench around the outside edge. I also extended the cladding down to about 50mm above the slab to hide the engineering bricks I used.  

 

 

 

[IanR]

2 hours ago, Ticky said:

However, I notice that your design doesn't have any DPM between the lowest layer of insulation and therefore your 50mm upstand is directly exposed to the ground apart from the aluminium clad.

Correct there is no DPM, or any membrane under the upstand or the lowest layer of EPS. It is OK for the EPS to sit directly on the Pea Shingle. The Pea shingle (no fines) is free draining, so combined with the perimeter drain, removes any water from under the raft.

 

 

2 hours ago, Ticky said:

Also, what would be the recommended depth for my concrete? Would rebar allow me to keep the depth to 100mm?

 

 

Yes, 100mm is all you need with Mesh included. A deeper ring-beam under the external wall is unlikely to be needed with the loadings for a garden office.

 

2 hours ago, Ticky said:

I'm thinking of using Celotex GA4000 PIR at 75mm, as aiming for a U value of 0.25 and that seems to offer it (based on their calculator).

 

The setup I've shown is for EPS not PIR. I don't believe there's a PIR/PUR approved for ground bearing (happy to be corrected), so for this the concrete has to go under the insulation as per @Moonshine's option, and is no longer an "insulated" slab/raft.

Edited March 3, 2023 by IanR

[Ticky]

Hey everyone, thanks again for all the advice here. It's all a very steep learning curve

 

I'd really like to maximise the head height, so still favouring the insulation below the slab. Plus, if I kept the concrete slab to ground level but then used some form of brickwork or concrete block on the perimeter (> 15cm) then the timber walls are fixed to those, would that be suitable? I guess I'm then shifting load from the slab to the brickwork then?

Also, would I be ok just backfilling where the shuttering was with soil/gravel be ok as they'll be in direct contact with the DPM?

 

 

[JohnMo]

I'm keeping it super simple.

 

Slabs on concrete, 175x45 timber upstand, spaced 10mm off the slab, DPM inside and up the wall level, taped to DPC.  Inside 100mm PIR, 11mm x2. OSB glued and screwed (floating on PIR), 50mm battens (screwed to OSB at 40pmm centres) and 50mm polystyrene UFH grooved insulation panel at 115mm centres, between the battens.  Timber floor to battens.

 

Building will be 70mm logs, with breather membrane inside, fully tapped to walls and all joints, with an independent 65mm stud wall inside spaced 25mm from outer walls (not attached to walls or roof), with 90mm Frametherm 32 insulation vapour control layer, airtight glued to OSB in floor and the multifoil in the roof, them plasterboard.

 

Ventilation will be dMEV fan operating on humidity and PIV sensor, with back flow damper.

 

Replace the timber for your blockwork on a simple strip foundation, or concrete pad.  Surround building with gravel to stop splashing of rain water.

 

Roof

I'm actually going to use multi foil and PIR, as below but with 100mm PIR.

 

I need a vapour protection layer, so thought use multi foil the way it's supposed to be with the correct air gaps.  If it adds to the insulation value great as it's added no more height than I wanted, if not, I have vapour/air tight layer and was only going to use 100mm PIR anyway.

 

[Ticky]

Hi @JohnMo, I'm not 100% sure if that's an answer to my questions or a description of your own build? I'm thinking it's the latter.

 

I feel the foundations are gonna be the most important part for me to get right the first time round.

 

I'm really struggling to think of a simple way to keep the slab at ground level whilst somehow raising the timbers 15cm. Do most people just pour a 150mm concrete slab (above ground level) and then fix timber from there?

 

For once in my life I wish I was a lot shorter too! ha

[Ticky]

@Moonshine @Temp, is this just 2 x regular-sized bricks (on top of each other) - that would give me the 150mm clearance (great) but how would you secure the timber to the brick?
Would there be a certain type of brick/block to go for?

Also, how do I 'terminate' (maybe not the right terminology) the DPC if I had this brickwork above the slab? Do I fold it under like I have in my image? I guess that would screw with the securing the brickwork to the slab

[Moonshine]

34 minutes ago, Ticky said:

@Moonshine @Temp, is this just 2 x regular-sized bricks (on top of each other) - that would give me the 150mm clearance (great) but how would you secure the timber to the brick?
Would there be a certain type of brick/block to go for?

Also, how do I 'terminate' (maybe not the right terminology) the DPC if I had this brickwork above the slab? Do I fold it under like I have in my image? I guess that would screw with the securing the brickwork to the slab

 

Yes, two 65mm standard bricks and two standard 10mm beds of mortar gets your the 150mm.

Use cement screws to screw down through the wood into the block (into drilled holes).

 

This is how mine turned out

 

 

In mine the dpm is under the slab and comes out and terminates at ground level. There Is a second dpm that sits under the insulation, laps up the inside of the two bricks and gets joined to a dpc which is between the brick and the timber frame sole plate

[Temp]

14 hours ago, Ticky said:

how would you secure the timber to the brick?

 

Yes just two courses of engineering bricks (more waterproof than regular bricks).

 

I made some long bolts from 12mm stainless steel threaded rod and nuts and embedded them in the concrete "head" down after levelling it. The mix was stiff enough they stood upright on their own but might need support. The bricks had holes in so were easy to drop over the top of the bolts when laying them. Just had to cut a few bricks where the holes or mortar joints didn't line up. DPM then drilled clearance holes in the wall plate. Big washers and nuts.

 

Warning: I tripped and fell over and nearly impaled my head on one of the bolts when laying bricks, so make sure to put lengths of 40mm pipe over them as soon as the concrete is set until they are safe. 

Edited March 7, 2023 by Temp

[Ticky]

 

On 07/03/2023 at 09:41, Temp said:

DPM then drilled clearance holes in the wall plate.

Hi @Temp Can you add a bit more to what you mean here?
 

In my design I'm going to have both the DPM and the separation layer up the side of my slab. I guess I can cut the separation layer where it surfaces as that will be covered by the engineering bricks. But to terminate the DPM, could i continue it up and seal it to brickwork with liquid rubber? Something like this below.


Also thinking..... will I need to be pretty acccurate with the size of the shuttering/slab? My DPM is going to be coming up the outside of it so if I need to fold it over the exposed surface of the slab to terminate it on the brick I guess the more accurate the better.

Speaking to a colleague, he thinks that I will need brickwork/footings instead of shuttering as when the shuttering's removed I've only got insulation, protected by my DPM. Is he correct or is the DPM covered insulation going to be OK if it's packed in with the soil/grass of my garden? 

[Temp]

Sorry I meant I put DPC band on top of the brick course then the timber wall plate.

 

Its hard to drill 12mm holes in exactly the right place in the wall plate to suit the 12mm bolts cast into the slab as they may not be perfectly vertical. So I drilled oversize/clearance holes around 18-20mm in diameter and used large 40mm square washers under the nuts to bolt the wall plate down.

 

 

Edited March 10, 2023 by Temp

[Iceverge]

this will be a big thermal bridge. 

 

You might get mold + condensation on the inside. 

 

[Iceverge]

Have you investigated the water table?

 

Dig some holes in the ground in the area of the garden room and see after a period of heavy rain what depth the water settles to. 

 

This will inform a sensible level to put your floor at.

 

If it low enough, a good french drain and well detailed DPM should allow you to drop the floor as low as you like assuming water cannot flow in the door of the room. 

 

Maybe incorporate a small sump and space for a plug in pump if you go down this route.  

Edited March 10, 2023 by Iceverge

[IanR]

3 hours ago, Ticky said:

 

Taking a step back, before considering a finished floor, level with outside ground, what Insulation are you considering under the slab?

 

75mm is low for any insulation product, but my concern is that you are thinking of using a PIR/PUR insulation. If so, have you found a product that has the compressive strength to be load bearing as you have it shown?

 

It would be typical to use EPS/XPS for a loadbearing requirement and EPS100 or above can do what you are showing, but 75mm of EPS is a really poor level of insulation, even on an unheated floor.

 

With regards the Finished Floor Level, I feel you'd need to build it like a basement, up to 150mm above FFL, and you've still got some work to do to detail the threshold of any doors. For the basement option, I'd look at how an insulated raft with ICF basement is detailed. You'd "just" need one course of ICF formers around the perimeter.

Edited March 10, 2023 by IanR

[Ticky]

2 hours ago, IanR said:

what Insulation are you considering under the slab?

Celotex GA4000 PIR at 75mm has an R-Value of 3.4 (see link) and going through the calculator on the following site it gave me the u-value of 0.25 for a similar floor, and that figure is recommended from a building regs point of view. It looks like 100mm of EPS100 has an lower R-value at 2.78m2K/W. So I appreciate the PIR was more expensive but it's thinner and gives better insulation (as far as I can see).
Celotex GA4000 Thermal PIR Insulation Board | Saint-Gobain (insulation-uk.com) 
I see a number of mentions that PIR can be used under the slab. A couple of examples below.
IMA-Best-Practice-Guide-No3-Floors-Final.pdf (insulationmanufacturers.org.uk)

Celotex GA 4000 Floor Insulation Guide (insulationshop.co) (this one for the specific model too)
 

Any solutions around the thermal bridge @Iceverge?

 

 

 

Edited March 10, 2023 by Ticky

[IanR]

38 minutes ago, Ticky said:

I see a number of mentions that PIR can be used under the slab. A couple of examples below.

 

None of those show it as load bearing, ie. they show a "floating" concrete slab that stops short of (and insulated from) the perimeter wall. The loads of the perimeter are taken down to a strip foundation. The BBA Certificate for Celotex GA4000 shows the same.

 

You are showing it used for an insulated raft, where the raft supports the perimeter wall. Typically EPS/XPS would be used for this due to it's better long-term compressive strength and that it does not absorb moisture. It is less insulating, so a greater thickness of EPS is required to achieve the same U Value as PIR.

 

Edited to add:

 

A link from one of your links quote:

 

Quote

or low-rise buildings on poor ground, the floor slab can be designed as a ‘raft’ to act as the foundation. While Celotex PIR boards have a good compressive strength, they cannot be laid below a raft slab and take the structural load of the building. They can, however, be installed above the raft slab, in the same arrangement as described above. For non-domestic uses you should seek the advice of a structural engineer regarding the suitability of any insulation product in a specific floor system

 

Ref. https://insulation-uk.com/floor-insulation/ground-floors/concrete-slab-floors

Edited March 10, 2023 by IanR

[Dan F]

In order to get good head height, we had our slab below ground level on 2 sides.   A retaining wall was built on the slab (in turn on top of EPS) and then we uses EPS/DPM between the wall and ground level.  Blocks were also a special low u-level type as well, from what I remember.

 

Isoquick helped with the design and supplied the EPS.

 

 

[Ticky]

Thanks @IanR, that makes sense. I've literally just got the same answer back from the supplier.

 

It was the load of the building itself that I didn't take into consideration. Thanks for pointing that out

Looking at EPS - Would I want something like this? Jablite Jabfloor EPS150 100mm Polystyrene Insulation Board 2400mm x 1200mm - FWDirect (flooringwarehousedirect.co.uk)
This data sheet for it PowerPoint Presentation (flooringwarehousedirect.co.uk) says that to meet the u-value I'd need approx 100mm (my P/A ratio is 1.067) as all perimeter walls of my 5m x 3m room will be exposed.

 

[IanR]

4 minutes ago, Ticky said:

Looking at EPS - Would I want something like this? Jablite Jabfloor EPS150 100mm Polystyrene Insulation Board 2400mm x 1200mm - FWDirect (flooringwarehousedirect.co.uk)

 

On the proviso the floor is unheated, I'll hold my nose and say "Yes". If you plan an electric heat mat, then you really should double it, at least.

 

I would include a 50mm thick perimeter upstand, chamfered at the top, as I detailed in my original sketch.

[Iceverge]

@Ticky

 

 You must decide if you are willing to accept a 150mm floor level or build as you would for a basement with all the associated detailing and drainage works. 

 

Trying to get away with detailing a ground level floor the same as a raised one won't end well I'm afraid. 

 

 

 

 

 

[Ticky]

@Iceverge Do you mean the slab itself being 150mm above ground or just that the finished floor is?

 

What does raising my current design by that amount achieve. I appreciate I wouldn't need the brick/block work, so that would solve the Thermal bridge bit I presume, but what else would it mean.

 

Again, apologies if all this is obvious. I'm brand new to all this but willing to take advice of those that know a whole lot more

Edited March 10, 2023 by Ticky