Insulated Concrete Slab Garden Office - Questions

[JohnMo]

41 minutes ago, Nickfromwales said:

Single room MVHR most prob

I just went dMEV, with presence sensor.  No-one in the garden room, ventilation is off. Someone in room for 90 seconds or more, fans runs for the time the person is there and an additional 30 mins.  Simple cost effective, ventilates only when required. No heat recovery but hay ho.

 

Plus points, only ventilated when needed.

Fan not running 24/7

No or little ventilation heat loss when ventilation isn't needed.

Very low electric consumption.

 

Cons

When someone is in room vent fan is running, with no heat recovery 

 

 

 

 

[Nickfromwales]

2 hours ago, JohnMo said:

When someone is in room vent fan is running, with no heat recovery 

Trickle rate with dMEV though?

 

Would you / did you consider single room MVHR, even though the heat recovery is crap it is better than sucking in raw freezing cold air? Cost is the con as these units aren't cheap, so needs some thought.

[sam]

In the context of a small single story lightweight timber frame and lightweight roof, what would be the purpose of the concrete ring beam? I note viking-house have many examples where the ring beam is separated from the floor slab.

 

I also like Moonshine's suggest of flooring directly onto insulation.

 

I'm wondering if it would be possible to put the timber frame directly onto EPS and do without any concrete.

[Nickfromwales]

32 minutes ago, sam said:

In the context of a small single story lightweight timber frame and lightweight roof, what would be the purpose of the concrete ring beam? I note viking-house have many examples where the ring beam is separated from the floor slab.

 

I also like Moonshine's suggest of flooring directly onto insulation.

 

I'm wondering if it would be possible to put the timber frame directly onto EPS and do without any concrete.

Having the ring beam separate from the slab would be way overkill, and would typically provoke much thicker wall profiles ( to ensure thermal bridging etc was controlled ). Not sure this would be relevant or warranted with this type of structure / duty / requirement.

 

It would have to be a considerable EPS sub-structure to accept the frame independently. All EPS structures shown previously here have a reliance, in part, for supporting this "frame", but none independently. The skins of the walls would usually be treated 'as one' and the load would be shared or taken significantly by the slab, in most instances.

[Ticky]

20 hours ago, Iceverge said:

 

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. 

 

I've already been digging the garden out this weekend to get to the main roots. I've not quite gone down 600 deep but these are about 450-500 below. Also uncovered what looks like a little pond (which I'm going to have to deal with as it holds water still)

[Nickfromwales]

Are you using a machine? Or doing it the "old fashioned" way?

[Ticky]

Been doing my own Nick. It's a nice little workout when the sun is shining (not so much today though)

 

Filled half an 8 yard skip so far with the wheelbarrow and spade

Edited April 16, 2023 by Ticky

[Ticky]

I do have someone coming on Friday with a stump grinder to get rid of the rest of the tree stump

[Nickfromwales]

1 minute ago, Ticky said:

Been doing my own Nick. It's a nice little workout when the sun is shining (not so much today though)

 

Filled half an 8 yard skip so far with the wheelbarrow and spade

Legend, lol.

 

I'm delegating, and at £800 a week for hire of a micro + powered ( lifting ) barrow, it's a no-brainer. 25m hike down my garden and through the side > front access alleyway = feck hand-balling. Would cost more than the machine hire.

 

Getting the farmer to drop off his 14 ton trailer @ £150 a load ( inert only ) for muck-away 👌

[Nickfromwales]

14 minutes ago, Ticky said:

I do have someone coming on Friday with a stump grinder to get rid of the rest of the tree stump

Another use for a small machine, which will pull out the stumps entirely, move a shit-ton of dirt, and yank out the majority of the roots to boot. Get a machine and not a stump grinder! Kill 3 birds with one stone ( cost ).

[Ticky]

The guys coming with the grinder are doing it as a freebie as they were the guys that originally took the tree down.
I did consider getting a little digger but decided it was doable with a bit of graft. As much as it's hardwork I quite enjoy it.

[Nickfromwales]

2 hours ago, Ticky said:

The guys coming with the grinder are doing it as a freebie as they were the guys that originally took the tree down.
I did consider getting a little digger but decided it was doable with a bit of graft. As much as it's hard work I quite enjoy it.

Crack on, lol. My frame is starting to creak so I've opted to pay and observe, ( to make sure nothing gets damaged more like ).

[Iceverge]

On 15/04/2023 at 17:42, Nickfromwales said:

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.

 

Yikes, super complex! 

 

How about a layer of EPS outside a taped layer of OSB on a 100mm stud with batt insulation. Zero bridging, especially around windows and doors.  Robust airtight layer. Cheap as chips and fast. 

 

Same for the roof, although I have no idea what the requirement of MCS are. 

 

On 15/04/2023 at 17:42, Nickfromwales said:

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.

 

 

A degree of energy balancing needs to be done, although your 300mm is gold standard it only works if everything else is sorted. What size is the building you are making? 

 

I am about( any month now) to install a Daikin FTXM split unit in our house. A bit dearer than a nobrand but I was talked into it by a mate who does heat pumps. Parts, lifespan etc he convinced me. Time will tell. Might be overkill for a small office. 

 

I am half way through my own 4m2 office. I'm weighing up the best ventilation strategy.  I probably have to do some sums to make my mind's up. 

 

Access sounds an issue for concrete etc. Would it be better to minimise it? 

 

[Ticky]

@Iceverge this hole was 600+ deep.

[Iceverge]

15 hours ago, Ticky said:

@Iceverge this hole was 600+ deep.

 

It's hard to tell really from that pic but I'll do my best. 

 

I'm unsure if it's virgin ground or made up. 

 

My best guess is to dig to slightly below the level of the red line I've drawn, below where the plant roots stop. Then add some free draining stone and you're good to go. It would be good to lower the excavations below the main level to below the height of the main dig and backfill with stone too. This will keep the area under you building drier. 

 

 

 

[Iceverge]

16 hours ago, Iceverge said:

I probably have to do some sums to make my mind up. 

 

@Nickfromwales

@JohnMo

 

 

 

Excuse the scribbling but here's my findings from playing with PHPP. This is for a theoretical garden room 4m x 6m for it's peak heating load. It has a 2.4m X 2.4m 3g door/window facing south. 

 

3 variables. Heat recovery at 75% and 0%, airtightness at 3ACH and 0.5ACH and average U values of 0.29 and 0.145. Bear in mind to get this lower average the floor walls and roof had to be close to 0.1w/m2k to balance the door. 

 

 

In summary the best bang for your buck was, as usual, airtightness. Then heat recovery and then improving the U values. 

 

Annual heating costs are a complete wild card as it's unknown how much anyone might use this space. These figures are for the very coldest day in the south of Ireland, no solar gain either. Your mileage may vary but a fair assumption is that on most normal winter/spring days you could just wear a jumper and bring the dog in with you and you'd need no heat at all. 

 

The only solid take away I can offer is to build really really airtight, do a good job on the U values and balance your need for MVHR with the lightly annual occupancy. 

 

Certainly don't spend much on the heating. Even a couple of ye olde incandescent lightbulbs would do. 

 

 

 

 

 

 

 

 

[Ticky]

5 hours ago, Iceverge said:

 

It's hard to tell really from that pic but I'll do my best. 

 

I'm unsure if it's virgin ground or made up. 

 

My best guess is to dig to slightly below the level of the red line I've drawn, below where the plant roots stop. Then add some free draining stone and you're good to go. It would be good to lower the excavations below the main level to below the height of the main dig and backfill with stone too. This will keep the area under you building drier. 

 

 

 

Thanks - I might be getting confused here.  I was expecting to be digging down to a level then dropping & compacted hardcore/type1 layers below the pea shingle? Is that not needed when using EPS?

 

5 hours ago, Iceverge said:

It would be good to lower the excavations below the main level to below the height of the main dig and backfill with stone too.

I'm also lost here too. Sorry.
 

Edited April 17, 2023 by Ticky

[Iceverge]

2 hours ago, Ticky said:

I was expecting to be digging down to a level then dropping & compacted hardcore/type1 layers below the pea shingle? Is that not needed when using EPS?

 

I would dig a to a minimum of below the layer of organic material. Then a layer of stone. I would steer away from anything with too many fines in it.(They may prevent drainage) Hardcore is good. Compact it well in layers. 

 

Then a layer of something that's more easily levelled before the EPS. Sand or limestone fines would work. I'm not sure about pea shingle as it wouldn't hold its "shape" very well under foot when laying the EPS. 

 

By just digging a perimeter trench to the excavations and filling it with stone you have a simple french drain. Think of it as thickening the stone at the edges. It'll ensure that the soil under the main foundation will be dry as the stone will allow water to pass to a lower plain easily.

[JohnMo]

8 hours ago, Iceverge said:

Excuse the scribbling but here's my findings from playing with PHPP. This is for a theoretical garden room 4m x 6m for it's peak heating load. It has a 2.4m X 2.4m 3g door/window facing south. 

 

3 variables. Heat recovery at 75% and 0%, airtightness at 3ACH and 0.5ACH and average U values of 0.29 and 0.145. Bear in mind to get this lower average the floor walls and roof had to be close to 0.1w/m2k to balance the door. 

 

 

In summary the best bang for your buck was, as usual, airtightness. Then heat recovery and then improving the U values. 

Thanks for that, good to see.

 

So for a like ACH there is circa 100W difference between MVHR and say a dMEV fan.

 

So take it we have a heating season of around 6 months.

 

.1 x 28 x 6 = 16.8kWh, assuming electric heating x £0.34 (electric price) that £5.70 per year difference.  If heating by ASHP, it would be a third of that so £2 or less.

[Iceverge]

It depends on occupancy rates. If heated 24/7 like a house PHPP puts the ventilation losses  closer to 300kWh. At 30p per unit with a direct electric heater it's about £90/year.  Worst case I know. 

 

 

To get a feel for the annual energy use total I took a realistic high use case. Say is that it'll be heated intermittently and used for a 35hr work from home week. That's about 1/5th of the time. However that time will be all during daylight hours when all of the solar gains happen.  

 

Annual heat losses is 2MWh for the dMEV, Average U value of 0.29 and 0.5ACH case.  So 1/5th of that is 400KWh. Similarly say 1/5 of the internal heat gains is 80KWh annually. Now assume 1/2 of the solar gain will available during occupied hours or about 380KWh. (All from PHPP sheets) 

 

The total is 400-80-380 or about 60KWh in positive balance.

 

It's a funny quirk of a well insulated, airtight and orientated building, occupied only during daylight hours is that it will probably need more cooling than heating! 

 

 

Perhaps someone who has actually done this, ( south facing, well built and airtight home office)  can confirm the absolute minimal heating needed. 

 

 

 

 

[Ticky]

Thanks @Iceverge

 

I've just educated myself about the different types of aggregate (you lost me on that too ha ha) and came across this blog and the guy seems to make a lot of sense (echoing a lot of what you said) Do You Need Gravel Under Concrete Patio, Slab, Footings? (plasticinehouse.com)

 

 Sound like using at least a 3 inches of gravel (20 - 25mm in size) should do the trick - would you agree?

 

One other thing that got me thinking.....

Both the designs from @Iceverge and @IanR  included a 'step' with the 2 layers of EPS, making the concrete deeper beneath the walls.

How would DPM lay over this 'bump in the middle' without there being lots of creases etc? Does the weight of the cement take care of any air gaps?

 

[IanR]

11 hours ago, Ticky said:

Both the designs from @Iceverge and @IanR  included a 'step' with the 2 layers of EPS, making the concrete deeper beneath the walls.

How would DPM lay over this 'bump in the middle' without there being lots of creases etc? Does the weight of the cement take care of any air gaps?

 

In my section I showed the DPM between the top and second layer, to get the ring-beam filled without any voids.

 

On 01/03/2023 at 19:27, IanR said:

 

 

 

The problem this can cause is the top layer of EPS "floating" up if you pour into the ring-beam first. The easy answer is to pour on to the top layer first, to weigh it down and allow the concrete to them flow into the ring-beam from the top surface.

 

You can see the DPM on my slab coming out from under the top layer of EPS, along the bottom of the ring-beam then up and over the upstand:

 

 

11 hours ago, Ticky said:

Sound like using at least a 3 inches of gravel (20 - 25mm in size) should do the trick - would you agree?

 

The sub-base layers in my section total 200mm from the bottom of the hardcore (which should be sitting on a geotex membrane) to the underside of the first layer of EPS.

 

For the top layer I wouldn't use anything as big as 20mm-25mm, I'd use 10mm pea shingle or granite chippings, but importantly, "no fines" to ensure it is free draining.  There should be a perforated pipe within this layer, around the periphery, to drain away any water that collects in the sub-base. EPS is fine getting wet, as in it doesn't absorb any moisture, but if it's sitting in water up to the DPM it may as well not be there in insulation terms. The layer itself doesn't need to be any more that 30mm - 40mm, thick, although make it a bit deeper at the edge where the land drain is so that the perforated pipe is fully enclosed in the pea shingle/granite chippings. There's no need to compact this layer.

 

The MOT Type 1 is also a relatively thin layer, just to level the hardcore and get it compacted. Around 30mm - 40mm thick again is fine.

 

40mm crushed hardcore for the bottom layer, around 120mm - 150mm thick.

 

The dig for this build up could be as shallow as 300mm deep from grass level, to get the FFL 150mm above.

Edited April 18, 2023 by IanR

[Ticky]

Amazing - thanks @IanR

 

On 18/04/2023 at 23:36, IanR said:

The MOT Type 1 is also a relatively thin layer, just to level the hardcore and get it compacted. Around 30mm - 40mm thick again is fine.

 

40mm crushed hardcore for the bottom layer, around 120mm - 150mm thick.

 

I'm struggling to see the difference between crushed hardcore and MOT type 1.

When I'm searching for prices on bulk bags they both seem to be coming up detailing crushed aggregate sizes 40mm to dust.

Links to the right stuff would be really helpful.

 

Also, would I still be using a compactor on the pea shingle that sits under the slab?

I'm also not having much luck finding 10mm pea shingle that specifies 'no fines'

Edited April 21, 2023 by Ticky
adding more info

[Mike]

38 minutes ago, Ticky said:

 

I'm struggling to see the difference between crushed hardcore and MOT type 1

MOT Type 1 is a specific Government standard for the sub-base of roads, with multiple requirements that must be met. To be precise, the specification is technically now 'Specification for Highway Works (or SHW) Clause 803 Type 1' - the specification was originally written by the Ministry of Transport, hence the snappier 'MOT' is still widely used.

 

Crushed hardcore is just that - it doesn't necessarily meet any specification at all. It could be close in composition to MOT Type 1, or not.

 

MOT Type 1 is frequently specified because there is no doubt that it will perform well long term. If you choose something different, you need to satisfy yourself that it's up to the job.

 

 

[Temp]

On 01/03/2023 at 15:58, Ticky said:

 

 

 

At the very least you want two courses of engineering bricks with a DPC between the slab and frame.

 

 I would do something like this..

 

 

or this might be better still. Use a 4" frame and line whole thing with insulated plasterboard.

 

 

Might also want a vapour barrier behind plasterboard.

Could also tank the outside face of the brick and concrete with a paint on tanking compound.

 

Do think about the door seal. Is there any risk of standing water forming in the area?

Pipe in the gravel for french drain?

Edited April 21, 2023 by Temp