Garden Room Workshop Extra | Insulating A Concrete Slab

[@matthaslam5799]

Ok, that was the most useful flooring insulation video I've seen. Thanks for that!

[@macrozanski]

Great video, thank you. Would a 150kps PIR be suitable for Option 1 for wall/roof load bearing if I wanted to build a 5mx3m garden room with a bifold door 2400cmx1950cm. I would like to place two 6x2 timber over the bifold with a flitch plate.

[@azmirfakkri91]

Great video for someone who has no idea on how to insulate a floor! Thank you!

[@innerechomusicltd6340]

Hi Ali, started watching your workshop built. I've got a concrete slab but cannot use all of it as need space from the fences to maintain the boundaries. If I was to use 3'x2' as a base to give myself the thermal insulation on top of the slab but then build the walls from 4'x2's, would that work? I like the idea of minimising the base to get more internal head height but how can that be done entirely without the base if my slab is bigger than my built? Hope the question makes sense. Thanks

[@mikeforwarduk]

Really, really helpful and wonderfully brief. Good job!

[@qshed]

over the summer we built a sun room(3×2.5m) on the back of the house. As one of the kids will use this as a bedroom when back from university we opted for the 100mm concrete over 100mm of insulation. Electric underfloor heating (1200w) then a 65mm screed. It takes a couple of hours to warm up but stays warm for 8 hours. so great for overnight. next job is solar panels to offset the heating costs.

[@AliDymock]

Nice! Good choice for a bedroom.

[@patrickloftus]

That's interesting to know about the warm up times there Jo. I was mulling over the idea of underfloor heating. Can I ask what flooring you have on top of the concrete? Laminate or carpet? And, you've ONLY got that 1200w underfloor heating, and no wall mounted heaters required? I guess the underfloor heating has to get that thermal mass of the concrete up before it seriously starts to radiate into the room.

[@MDMD-uz7bu]

Finally a video that explains options to avoid confusion

[@aesopshair6690]

Thanks Ali, great video! The use of 'thermal mass' is a default option that has arisen due to the way houses have been built over the past few centuries; it is now possible to control the internal environment more effectively and efficiently which reduces the requirement to have thermal mass for temperature stability purposes. Interestingly, water is the material with the highest specific heat capacity (over 4x that of concrete), so having a well insulated water tank is an effective way of storing heat energy for use at a time when required, rather than constantly heating the floor slab of a thermally leaky building.

[@AliDymock]

Yeh I found that fact that water is the best material in terms of thermal mass. I compared the temperature swings for 4 different cities at around 51 degrees north (latitude): Plymouth and Vancouver by the sea have very moderate temperatures vs Winnepeg and Nur-Sultan have very large swings as they're landlocked. I left it out of the video as thought it would be too much. I never thought about the a water tank working that way inside a house though - good point. It's fascinating stuff!

[@aesopshair6690]

@@AliDymock The issue of insulation is intrinsically linked to achieving and maintaining a comfortable internal environment. There is quite a lot of positive appraisal for underfloor heating, but is often tied to the assumption of having an internal concrete slab which requires being heated to remain comfortable - doing away with this thermal heatsink and having a better insulated floor opens up the possibility of other methods of heating/cooling that can be more responsive and efficient, but they are often overlooked.

[@rikgeartv1893]

Thanks for the video. I'm drawing up plans for a 7.5m x 3m workshop in my garden (central England), my first thought was option 1 and found this while searching for other options. The considderation of the slab being a heat sponge during the initial heating I hadn't considdered. However, There's some heavy equipment going in that will need bolting down (like a fly press) so options 2 and 3 are out. Also, as the building is going to be within 2m of the property boundry (right up against the fence on 3 sides), I'm limited to 2.5m hight, so the lower the floor the better. I've been using some old gym floor matting (about 15mm thick dense rubber) in my rented space as fatigue matting and this could work to dampen the heat transfer issues as well as dampen sound. And if converted to a garden room at a later date, would make a good underlay for laminate. Heating wise, my first choice is a cheap deisel heater and this will probably be installed anyway as it's a cheap option for a usable space asap. What I'm thinking now though is when fixing the roofing sheets down, install some brackets at the same time for future installation of solar panels to power ceiling mounted radiant heater panels or convector heater to take the edge off, and use the deisel heater as a booster for extreme cold weather...

[@rikgeartv1893]

Maybe lay some electric underfloor core into the concrete when pouring and use the slab as a storage heater.....

[@Reef_Club_]

Genuinely brilliant videos - so helpful

[@mikedeman5351]

Nicely done. I think it would be good to see a video where the insulation of floors, walls and roof is discussed in an integrated way, in order to eliminate thermal bridging. Maybe an idea for a future video. You have a knack for clear presentation.

[@AliDymock]

Hmm you’ve given me a good idea 💡, thanks Mike!

[@greenambles]

Thanks! That'll be option 3 then. My cement pad going down in the next couple of weeks 🙂

[@stephenbetley9596]

I have a typical early 1900s terrace with 2up 2 down. The ground floor front room is suspended flooring. The backroom however is a concrete base with what looks like a ~20mm liquid screed on top covered by tacky lino. Looking to bring the building up to thermal regulations. Adding insulation on top, then flooring, then laminate will potentially add 60-75mm to the floor level adding a step above the suspended floor level. Potentially raising that level when adding insulation below the flooring would have a knockon effect at the door frame level for front & back door. Any suggestions?

[@habzray2978]

Hi Ali, I already have a slab a DPM underneath it but no insulation atm (I do need to insulate the floor) and I am looking to make an outbuilding with composite panels (rock wool core) Can you advise the best way to prevent water / damp/ rodents getting underneath the building or inside the composite panels? I cannot have the wall panels right on the edge of the slab . The panels can sit in a galvanised U - channel but I'm not sure if its best to bolt the channel directly onto the slab or on a course of blocks with dpc on it first ? Does it need ventilation if on a course of blocks? and what insulation type do you recommend and in what order?( ie. DPM membrane first or insulation first then chipboard)? Many Thanks

[@peanutbutterry6364]

Hi, Iam thinking of putting a dpm(polythene) on top of existing concrete, then carpet underlay and lastly carpet on top. Is it ok? Thanks for your reply.

[@mattpage9451]

Been looking forward to this since you mentioned it a few videos back, thanks for uploading

[@AliDymock]

You remembered! Hopefully it lived up to expectations 👍

[@mattpage9451]

@@AliDymock As always. In the middle of house renovations at the moment so giving me plenty of food for thought whilst planning the next step, being the garden room!

[@patrickloftus]

Same here - also been looking forward to it, as concrete slab is currently what I'm looking at for my garden room. Great video, thank you @Ali Dymock Ah...watching it now....you just mentioned you'd avoid using the method I was thinking of: insulation under the slab. Although, thinking about the pro's/con's you've mentioned, still might be best choice for me: frequent/daily use (home office and gym). One disadvantage of option 1, is you really should insulate the sides of the slab too - and ideally your blocks on option 2 I think.

[@georgecorden9526]

Absolutely not geeky! This is a life saver really. I’m building a shed which is going to be more of a utility room with washer and dryer and various other appliances along with tools. I’m going for option 1 as it’s a shed and I think it will be ample

[@alishaomahoney2144]

Hi, not sure if you will see this comment... thanks for the detailed video on insulating. What method would be best for securing the base of the structure to the concrete? I have seen ground screws as well as timber screwed into the concrete.

[@AliDymock]

If you check Part 4 of the main series you can see I tried out two different ways of attaching my floor joists to the concrete blocks. Masonry bolts were the winner which involved drilling into the concrete and screwing them in. If you haven't poured the concrete yet you could instead have upside down threaded bolts along the edges and then drill holes in the bottom plate of your walls which will then fit on to them. Then secure with a washer and nut :)

[@tufty5801]

Love this video! I have gone for option one but I will probably add some thermal underlay just to top it off. I need to get some pictures over at some point!

[@AliDymock]

Sounds good!

[@MikeFromDublin]

Hi Ali, I'm working on a variation on options 2 and 3 for a workshop that I want to heat occasionally. I'm thinking of using foamed concrete instead of normal concrete, which will give me much greater insulation value. My main challenge is the design of the perimeter which has to hold the weight of the building - probably a shallow foundation and 100mm half-blocks. Great series, really useful, thank you.

[@gregkilpatrick9690]

Hi. Could you please advise on a 14x8 foot shed resting on flags. Pent roof with fall front to back. They are coming with 22mmx44mm bearers under the floor. Will this be efficient enough, for air to pass through and stop the floor from rotting from either lack of airflow or rain splash? Should I replace these small bearers with bigger ones?

[@AliDymock]

Hey Greg, that should be enough air flow for the floor joists but I can't really say if this is the way I would do things without knowing more about the site, build, cost etc but generally it sounds okay for a shed.

[@rikstar.]

Is there a weight limit to the 3rd option? Only asking because im about to build a extension and the 3rd option in much cheaper the the first to. Also i have a big aquarium to go on the chip board floor

[@DeadBodDan]

I'd like to know the same. Planning from a gym currently and while I'm no Arnie I think a concrete top layer would work best.

[@themuttsnutts4603]

what are the depths of each of the stages in this video, i putting a 3m x 4m base using option 1

[@davidryan8754]

Is there any benefit in trying both options. i.e Insulation beneath the slab for better thermal mass performance and insulation above the slab for comfort?

[@AliDymock]

Oh interesting question. I'd probably say no, after some thought, because the top layer will be so good at insulating that the heat probably won't make it through the concrete to reach the insulation beneath it rendering it a bit pointless.

[@paulholt4831]

Hi Ali I opted for option 3 but with floor joists (more money I know )anyway I'm using 50mm pir boards within 4 inch joists so it leaves a gap of 50mm would you float them so they are flush with joists or push them down to the concrete leaving a 50mm gap I'm not sure due to condensation etc could you help....

[@AliDymock]

I'd push them down so you get a ~45mm gap above. This air gap allows the foil to radiate infrared heat back towards the building (I can't currently explain exactly how this works but know that it does) whereas no gap means you lose that function for the most part. Then I'd put down a polythene vapour barrier over the top of the joists because with the insulation pushed down you cant aluminium foil tape them 👍

[@paulholt4831]

@@AliDymock cheers..so what would be the dis advantage of having the insulation flush with the joists and the gap below

[@DeadBodDan]

@@paulholt4831 possible condensation in the void and bridging of this to joists would be my biggest concern even if you are laying a dpm over the entire slab.

[@grassabrutta]

I am renovating a house which has stem/band foundations (which support the walls of course). I will replace the floating slab and install some deeper roadbase beneath it. Option 2 looks like the right option for us and here's my question : Should I bring the screed just to the top of the foundation ('block' in your diagram) or have a deeper (higher) screed which extends all the way to the wall ? I prefer to stay lower bc we don't have a lot of ceiling height, but I was worried about a floating floor on top of which we have our floor boards (which will sit on both the screed and the foundation (block)). Would the boards become unseated on the block side and potentially warp or crack when we walk on them ?

[@matthewcaldwell575]

Fantastic video for me who is trying to work out whats the best option whilst having zero experience and no clue to what i actually want

[@AliDymock]

Glad it helped. I'll just say all 3 options are good, I was trying to demonstrate the idea of thermal mass when pointing towards option 3. Might go for option 1 on my next build.

[@egold33311]

Great video, I have a large concrete area where an old garage used to be, I want to use some of it to build a garden room as in option 3, do you think I can build straight on to the concrete as its not raised that 5cm mentioned in your other videos? Thanks

[@AliDymock]

As long as it doesn't flood now from the surrounding ground (surface runoff) then it'll be fine I'm sure. Have a think what will happen to the water on the rest of the slab though.

[@egold33311]

@@AliDymock Thanks, it doesn't currently flood and used to have an old garage on there so hopefully wont be an issue, I will look at more of your videos thanks

[@frankklein4872]

Please could you make a short video clarifying the correct use of DPM VB and Breathable M. Builders seems to confuse the three and the best locations to use them, roof walls floors. Sticking to UK temperate climes please. Your video shows sheeting as well as foiled celotex. Great videos as ever, thank you.

[@AliDymock]

I'll put it on the list!

[@sphinx464]

I'm a little confused by options 2 and 3. In the drawings shown, the vapour barrier only meets the blocks or walls. Surely these will allow vapour through. Are the drawings simplified, in that the vapour barrier for the walls (not shown) should overlap with this flooring vapour barrier?

[@MrJohnnynapalm7]

Great explanation, Ali - thanks for sharing. I’m looking at doing a garage conversion - the garage concrete floor and single brick walls are very dry; would option 2 be best do you think? Many thanks in advance.

[@jamiemahmutyazicioglu5897]

Hi Ali, love your videos, I'm finding the rational for decisions the most informative. Question, I'm planning on building my 14m2 office building on a flat paving slab base. How do you advise I go about starting the office floor in terms of maximising roof ceiling height (up to 2.5m), fixing to the slabs (if at all) and general waterproofing? I'm finding information for paving slab bases really difficult to come by. TIA!

[@matthewvarley7672]

Hey Ali, was just trying to check out the website, I'm stuggling to connect, just wanted to have a look at Nicks Build, cheers for the most in depth series in diy

[@AliDymock]

That’s weird, mine was struggling too but working now. Here’s the direct link: www.alidymock.com/nick-somerville/

[@matthewvarley7672]

@@AliDymock cheers Ali, I found it picked up again, maybe was just some down time, cheers for the quick reply

[@leigh051985]

These are a great series of videos! what would you do if you was replacing a shed that is on a concrete base with no DPM?

[@AliDymock]

Hi Leigh, just pop a DPM over the concrete before your flooring or insulation if that’s what you’re going for

[@luuksta1027]

Thanks for the video, this is exactly the "geeky" topic I'm thinking about lately. I get why a garden room would benefit the most from option 3, but for a house, or when you want to keep it heated trough out the year, I would definitely go for option 1. Even when you are in a moderate climate (cant have enough thermal mass?). I would even go so far as making the sand blinding layer higher, or am I missing something? Also placing insulation the sides of the foundation looks like an wise improvement in option 1 drawing.

[@AliDymock]

Hey Luuk, yeh have a read of the description, I mention everything you have. All options are good for both a garden room and house. I sort of make a mountain out of molehill to demonstrate the point of thermal mass and really only make my picks to help people decide. :)

[@johnmit]

It's definitely possible to have too much 'thermal mass', in particular when it is linked to your heating system (e.g. a concrete slab with underfloor heating). Generally speaking, thermal mass stabilises the temperature in a house. Which means if you want to change the temperature quickly (e.g. you've just come back from holiday and want to warm the house up), then thermal mass works against you. This can become very problematic where that thermal mass is used as a radiator to heat the home (e.g. where a concrete slab contains underfloor heating pipes) - the more thermal mass the longer the control feedback loop - to the point where you have to change the thermostat several hours before you want to feel the benefit. If you overshoot and it becomes too warm then it will take several hours to cool down again. As a rough rule of thumb, you want to stabilise the temperatures across a single day/night cycle - but not any longer. That means for option 1 you need to be careful with how thick the concrete slab is if you plan on using it with underfloor heating - option 2 with a thinner screed gives you better control. And option 3 with wall hung radiators gives you the quickest response, which makes it well suited to garden rooms. And if you want to be really geeky - 'thermal mass' is a bit of mythical unit. Generally speaking people/architects/builders think it is similar to density, in that heavy materials are "better" (concrete/brick/block), however the key material property is actually specific heat capacity. For example water has roughly 4 times the heat capacity of concrete, but less than half the density. Plasterboard/Plaster are also good 'thermal mass' materials - and surprisingly so. As is wood fibre insulation.

[@luuksta1027]

@@johnmit Hi John, Thanks for your response. With the specific heat capacity you make a really good point and a counter intuitive one for me I must admit, really interesting. For a garden room I totally agree with you that you can have to much heat capacity. I must clarify that I watch the video's of Ali with a house in mind and try to apply the pro's and cons to may own situation. In a house I think that you can't have too much heat capacity. See my reasoning below, I'm curious what you or other people think. My current vision on house temperature is that with a modern (extra) well insulated house you never want to change the temperature "quickly". To have a comfortable home I would say that the temperature should stay as constant as possible, thus not regulate on a day or hour scale, but more a monthly/yearly cycle. When establishing that there are inputs in the system like day night temperatures, wind, sun, activity in the house, opening of doors that should be compensated for, the systems described below are possible (I have put them in extremes). - By a very good control system and low heat capacity, quickly heating or cooling when events happen. This means you need a lot of extra capacity on both heating and cooling to stay in control. - Or have a very slow system with a high heat capacity and a slow control system. The last sounds cheaper, and more comfortable to me. Having a slow system means you can use energy when its cheapest, or available (think of renewables). This makes the system less dependent on specific circumstances. I would not design a house for the 3 weeks of holiday per year I'm really away, especially because its in the summer and keeping every thing closed in the house during the day will help the most in keeping the heat out. Having the floor heating then disposing of its excess energy during the night via a outside radiator could be a nice addition to the system.

[@johnmit]

@@luuksta1027 You're right in that behaviours are a big part of this as well - if you have an underfloor heating system with a high thermal mass you need to expect and plan around a constant temperature. A lot of the problems come from people who treat it like a traditional hot water radiator system that can bring a house up to temperature in 30minutes. The problem with very high thermal mass in your slow heating system (which you're right, should be cheaper and more economical to run), is the control feed back loop. If for example you have underfloor heating at 30degC, which heats up the slab and then switches off when the room is up to temperature (say 21degC), the slab will continue to emit heat and the room will over heat. The control system needs to account for the heat that is in the slab and not yet released into the room.Turning the underfloor temperature down helps, but heating blending valves then also start struggling to maintain an accurate temperature

[@BSKiteHaps]

Loved learning more about insulting a floor. Thanks

[@sheppodiddly]

Very good. Clear and informative. Will be using your advice, thanks!

[@ronaldtrott5728]

Well explained only I couldn’t see whereyou explain how you were able to span your roof joists 5m.

[@robjennins8087]

Great info here, many thanks! I will probably go with option 1 to maximize internal height. Is a continuous thickness (150 - 200mm) slab OK for a 7.5 x 4m timber room or should it incorporate a thicker RC beam at the edges to support the additional weight of the walls?

[@AliDymock]

Sounds like a good choice. I would have thought that as the majority of the depth is underground that it wouldn't require any beam or rebar but some people do put in mesh in the middle of their slabs.

[@robjennins8087]

@@AliDymock Thanks for your response! Appreciated. Keep up the good work! You should be a public speaker/presenter!

[@muscleprotein]

When you say 150mm-200mm you referring to the depth of the complete construction?

[@paulstaniford8622]

Good Morning. I’m just in the process of converting part of my garage at the bottom of my garden to an office. The garage has a concrete floor and I’m trying to keep the floor build-up to a minimum. At the end of the video you recommend a thin floor insulation. Can I ask what product you would recommend as this will allow me to add maybe 25mm and not 80mm to the floor build-up height.

[@AliDymock]

I'm not sure about a specific product but check out Screwfix: www.screwfix.com/c/building-doors/underlay/cat850024. They all differ. I used the fibremat green stuff, maybe you could double that up (10mm total + 12mm laminate =22mm) to get a bit better thermal separation from the slab?

[@Asheldrake]

Any ideas on what to do with an existing slab?... I have an existing concrete slab for a small shed which i'd like to make bigger for a garden room. I have no idea if the existing slab has a DPM. Is it best for me to just smash it up and use as hardcore for a new base, and then follow option 3? Also, would you choose option 1 if you don't want the room to get too hot in the summer? And also if you use it daily for garden office?

[@AliDymock]

Hi Adam, if it were me and I was going for least trouble free and low cost and the concrete is in good shape unlike mine, I'd put a DPM over the slab, then insulation, then chipboard then underlay+flooring. This is what i plan to do in my garage (new house) where the concrete floor is substantially lower. Option 1 definitely will have a more solid feel though. If you do smash up the concrete into hardcore make sure you have some good dust/fine sand inbetween it all otherwise your sand blinding can sink into the hardcore and create a void under the concrete which can put it under tension.

[@Giedre3325]

​@@AliDymockhow would you fix the chipboard to stay in place?

[@robertquinn7412]

Great videos! Quick one: any advice on laying a new concrete floor between existing walls? Wondering about damp more than anything.

[@HidingZebraTube]

Hi Robert. It sounds liike we have a similar job on our hands that you did. I've got at 1930s semi with a concrete floor thats got no damp proofing. Thinking of ripping out and starting again. Did you end up laying a new concrete floor? any advice?

[@shanejohnson1692]

Another great video, keep them coming. I’ve been considering option 3 for my workshop but the concrete base is a little uneven. Do you think the celotex would ‘soak up’ any imperfections in the floor?

[@AliDymock]

Yeh it will along with the chipbaord and floor underlay but if it's quite uneven you could pour a liquid levelling screed that will flow into the low spots and make everything level and only lose a few mm at most 👍

[@w84it69]

What are the consequences of not insulaing the concrete floor, I am drilling the plate directly onto the slab. 6"

[@bellazroddaz]

Do I need a vapour barrier on top of the foil backed pir board ? Before the caber flooring goes down

[@AliDymock]

yes, but very simply joist tape the joins of the PIR with aluminium foil. Very easy 👍

[@leethacker1302]

Learning as I go along, I've used option 3 during my build but that's more because I just thought that was 'the way' and didn't realise there were other options. Working out well though!

[@Vass881234]

No bearers or air on option 3, first time I've seem this option, would help me greatly thank you

[@beardedpianist]

Hi Ali. This is another very helpful video, thank you. You showed someone else's floor, consisting of a vapour barrier, insulation, and then the boards for flooring. The third of your slab and insulation examples, which I'll probably use, has a DPM AND a vapour barrier. Would you recommend having both?

[@AliDymock]

Hi Michael, you're welcome. check out the accompanying websites under 'Your Builds' to find the one you're talking about. So that one which I agree with had a DPM which is essentially a VB then insulation which was taped (so another VB) then chipboard for flooring. The DPM is to stop damp coming up from the concrete below (optional but if you don't know if there is a DPM UNDER the concrete it's wise to put one down and the one above is to stop water vapour entering the insulation from inside 👍

[@beardedpianist]

@@AliDymock Was the example floor at 4:33 fixed to the concrete slab? Was the chipboard floor not fixed?

[@drsquirrel00]

The only downside with No. 3 is the possible usage of the room - if you're creating a gym or something with other high impact activities (maybe a workshop with large tools on castors) then will need quite a lot on top to keep it stable. Sadly learnt the hard way...

[@AliDymock]

That sounds right. Regardless of the thermal mass point, it's not how I would want an extension on a house built but for a DIY garden room I think it has some great advantages but if you're dropping weights from height you probably want no.1 due to the high compressive strength of concrete. Good to hear from your experience 👍

[@plummetplum]

Good point, I was about to go for option 3 but then saw your post. I want to make mine a workshop. What thickness of PIR and Concrete would you recommend for a 3x4m room ? Thx

[@gman1087]

Ah Ali I was wondering what u been up to...good to hear from you.

[@IncusBoneSound]

Excellent as always 👌 Thank you.

[@simonkee51]

You also need to insulate between the walls around the concrete pad to prevent thermal bridging. Not quite worked out whether this goes inside or outside the DPM though (about to lay floor in next few weeks for my block and stone games room).

[@AliDymock]

Yeh this was a simplified sketch. Send me some pics on how you do it, I'm interested to see.

[@andrewhall9581]

So I'm thinking of something similar. My favoured design is a concrete slab and wanting to maximize insulation but have the 2.5m challenge or go for planning permission. Are there alternatives to concrete slab then bearers then joists then floor that are not cold? As these all add up quite quickly and I'm finding I need to reduce the insulation in the floor and roof to keep a decent internal wall height.

[@simonkee51]

@@andrewhall9581 DPM, 75mm PIR, 100mm slab with mesh, breather membrane,18mm ply or chipboard, final surface. You could actually do away with the PIR dependant upon ground conditions, final finish (such as underlay and carpet) and not needing building regs permission.

[@mudflat]

I guess another question is, if you are going with a block perimeter - possibly give that a concrete trench foundation, and then just fill in the inside with something like MOT sub base, and work from there. Or rather, why bother with a concrete slab at all if you don't plan on using it for thermal mass?

[@mudflat]

I suspect people will answer to the effect that it keeps water out.

[@mudflat]

It's annoying that foamed glass isn't cheaper in the UK, because it could be so useful in cutting out plastic insulation in the base.

[@christhomas9818]

Great, informative video as always Ali. I too am opting for option 3. Interesting to hear you would just lay the PIR and lay flooring on top, I’ve been swaying between this and laying some 4x2 down for my floor to fix onto, hoping for a more stable and quiet floor. Do you think the PIR joints would rub eachother and squeak?

[@AliDymock]

Hey Chris, based on my warm roof which I had OSB over the top of and walked on quite a bit I never heard any squeaks and the PIR boards were butted up right next to each other so you shouldn't have a problem. I'm not a purest when it comes to construction. There are many different methods and ways of achieving similar results and what I would choose is not gospel so if you want to put 4x2 down then go for it, but ideally you'd want to raise the timbers off the slab to allow ventilation underneath which adds more height and effort.

[@christhomas9818]

Fair point about raising the timbers for ventilation! You may have just given me enough to move forward with just a PIR base! I’ll keep up my research, but this was just the video I needed as I’m getting my concrete slab down this side of winter ready for the build in spring.

[@SMMottershead]

Video idea: a pre laid concrete base from the 1950 with no dpm. The base might also be shared with a neighbor garage or shed so this makes it difficult with rain. Great video by the way :)

[@AliDymock]

Ooo tricky. I'd put down a boat load of silicon or other sealant on the concrete where the diving wall goes up, then DPC lapping up the outside wall and then bottom plate squishing the silicon. Then put down a DPM over your side of the slab running up the internal walls, insulation, chipboard, flooring. The alternatives would be to take one of those brick/concrete cutting tools to the concrete so water can run down to the ground or at least have a channel away from the building OR you could building with block and then use bitumen on the outside to weatherproof that bit 👍

[@buyabouzouki]

Hi all, I wonder if anyone can help me. I have an existing concrete base which stands 20cm high from the ground. I want to build an insulated garden room but I'm struggling with the 2.5 meters max height. Anyone have any ideas please?

[@mudflat]

The UK is an odd beast. We get a few months, probably November through to March where the Sun just doesn't offer much heat. In our greenhouse outside those periods it gets pretty warm. Well too warm for humans. There's a nice crossover where it's just a little warmer and pleasant inside when it is still a tad nippy outside. Annoyingly you want the summer heat in winter. But you have to work hard for that.

[@gazparadise]

Thanks Ali. Nicely explained.

[@muscleprotein]

Option one for me as using as a gym with heavy weights and this structure best suits that

[@bobbymooreok]

How would you fasten the chipboard floor for option 3?

[@shaunmorrison]

Yes, I would like to know the answer to this too. I can't figure it out. The two ideas I can think of are to (1) use plasterboard wall plug in the insulation and have some very accurate screwing, or (2) dig an 18mm deep small area out of the insulation and place some 18mm OSB/ply here to give you some purchase when screwing in. Both of these things won't stop sub/floor warping in some way.

[@satchman2000retro]

Really enjoying these videos mate; I’m considering converting my garage and doing it myself as I reckon I can get all the materials for less than a quarter of the quoted price from a builder :) keep em coming please :) oh.. the videos/builds you mention at the end- do you have links to these? Thanks!

[@AliDymock]

You definitely can! Yeh sorry I should have linked it somewhere, here you go: www.alidymock.com/your-builds/

[@vanepico]

How much concern is there for heat loss through the sides of the concrete slab in your first option? Would you put insulation to the side of the slab, and what might be a good way to protect/finish the insulation?

[@AliDymock]

Fantastic question. Yes the model I showed is a bit simplistic as you will want insulation on the sides of the concrete. Quite how to do it so it's not at the edge and subject to water ingress, I'm not quite sure, though you could lap the DPM up the outside as long as your cladding will hide it, that will work. It's something i'm potentially going to be doing next year so will be looking into it!

[@northeastcorals]

Yes you are supposed to lap the DPM up the wall & ideally tuck it into the mortar of the wall on a brick/block wall although this isn't always practical on existing walls so I think most people just run it up the wall & cut it bellow the top of the skirting boards. You then put at least 25mm thick PIR all around the perimeter of where the concrete slab goes to reduce cold bridging, easiest way is to fit the 25mm strips down the sides of the PIR boards laid on the ground so they are fixed in position by them.

[@afnankhokhar5578]

Great video Ali - please do more of these geeky ones as I find them the most interesting! Also, do you not have a patreon or buymeacoffe page?

[@AliDymock]

I'll get back to them at some point for sure as I enjoy them too. I do, right here: www.alidymock.com/support/ ☺

[@eliinthewolverinestate6729]

Another option is no concrete and using compacted crushed rock instead. Then glue and screw insulation. You can even spray foam the crushed rock.

[@AliDymock]

Interesting!

[@toekneesee]

Can anyone offer suggestions for the thicknesses of each layer, please, or point me in the direction of guidance? I'm going for option 2. I'm thinking of putting electric underfloor heating mats in the screed layer. Anyone envisage a problem doing this? All suggestions welcome. Many thanks in advance!

[@toekneesee]

Anyone?

[@tomek]

2:53 I thought building regs said extensions need insulation under foundation? (not certain though)

[@AliDymock]

I'm not 100% sure for extensions but I know you can do no. 2 with no issues as that's how my mum's extension was done and it was all passed by BR.

[@Danny-vx1wc]

Kingspan K103. Best board for floors.

[@jasonjayalap]

Thanks. But here's a couple of challenges (that I think you'll appreciate): (1) If your insulation has a vapour barrier on both sides, you're betting on it never getting wet, right? I think for option 3, you should let it dry to the interior. (2) In the USA at least, the sand is falling out of favor. Do read "BSI-003: Concrete Floor Problems", by building science loudmouth celebrity, Joseph Lstiburek. I've finally caught up on your vids. Time for me to stop researching and do the dang thing.

[@AliDymock]

Thanks Jason I'll read that. What's the upshot though, that the sand falls through and leaves void below your concrete? As for the VB I do think it's sensible to have the one above to stop moisture getting in -probably more important than from below. With option 3 another way of doing it is to have the concrete top finish below ground level, use blocks on the side and then insulate over the slab to bring the floor up to around ground level. Best of luck with yours!

[@Njao6q4vrv73]

Cool

[@Pete.Ty1]

.😊👍