Heating system for small outbuilding

[MP1]

Good Evening,

 

I am building a new outbuilding in my back garden. I am trying to figure out what the best electric heating solution (CH + HW) would be and I would like to ask you for your opinions here.

 

The outbuilding will contain an office, a gym and a small bathroom with a steam shower unit and a sink as per the plan below. It will be used throughout the year on a pretty much daily basis.

 

I initially thought about using electric heating mats for my UFH and a small boiler for my hot water needs but now I think there may be a better, more cost effective solution.

 

I have seen a number of threads talking about ASHP, Ampec flow boilers and Willis heaters. It made me think about fitting a wet underfloor heating system and using one of the solutions but honestly speaking I am not sure what would be best as my knowledge and experience is limited.

 

I would be very grateful for any advice.

 

Martin

 

[Iceverge]

UVC on cheap electricity for water. Fan heater for heat.

 

Spend your time and effort on airtightness insulation and mechanical ventilation. Your heat demand will be so low then heating may not be needed.

[MP1]

Thank you @Iceverge.

 

I understand that with UVC, I will have to have a large tank which I am trying to avoid because of the space that it takes. The bathroom will be mostly used for the steam shower which will come as a unit and I understand that the steamer will have its own heating element. It seem that I will just have to heat some water for quick showers and for washing hands. You reckon it is still worth having UVC?

 

 

 

 

[Nickfromwales]

An UVC can be smaller and wall-mounted, the problem you'll have is that I've fitted a few of those steam cabinet shower affairs and the mixer shower needs a far bit of oomph to get the occupant wet. Does the unit you've selected have a rainfall head, handset and body jets etc or just a rainfall head and a handset? The litres per minute flow rate requirements will be quite high with the former. If you plan on short showers, just to wash before / after using the sauna function, then that's ok, just you'll need to accept that. If the primary use is for steam sauna then just fit a 60-80L wall mounted UVC ( instantaneous water heater ) and have that outside the bathroom, maybe make an airing cupboard just to the side of the bathroom door for towels / water heater / CU etc?

For heating FORGET wet UFH and Willis heaters and all the shart that goes to make up such a system. I've just done a similar sized annex with in-screed electric UFH wire ( NOT under-tile heating mat ) and it works brilliantly. Zero maintenance and running costs will be the same ( less than actually ) than wet UFH + Willis ( both will use direct electricity, but the wet system will have losses and two heating pumps ).

In the immaculate solution, I would fit an air to air heat pump for heating, and use that in the summer for its air-con capabilities too. The better you make this outbuilding, the more you will fry in the summer. I'm just in the process of buying a dual / split unit for my 6m x 4m workshop / home office so I can heat and cool. I am buying one that serves 3 internal units so I can fit one in my boys attic room, one on the landing of the house, and the other in the shed / office. One stone, 2 birds.  

[Nickfromwales]

Apologies. The above UVC would not be an instantaneous heater, it would store hot water. It would delivery high grade DHW instantly.

On 14/04/2021 at 00:23, MP1 said:

It will be used throughout the year on a pretty much daily basis.

UVC + no brainer. Or, if this is adjacent to the house, tap into the DHW from the house and pipe it out there.

22 hours ago, MP1 said:

I will just have to heat some water for quick showers and for washing hands

The steam cabinet will only accept heated water, unless you can find one that utilises an electric shower ( which would be a bag of shit ) plus you'll still need another multipoint ( instant water heater ) for the basin.

Don't forget with all these instant water heaters, you buy in at 12-15kW so will need a whopper of a supply cable back to the meter / DB1 and then a DB2 at the annex to take the various loads. 3kW to the wash hand basin would also be the norm.

The UVC will service both the shower and the basin, and will only need a 13a fused spur to connect to the electricity supply. The in-screed heater wire(s) will also run off a 13a fused spur, same with UTH if you tile and heat the bathroom floor. 

I would insulate the life out of the floor in the office area and lay an LVT over 2x layers of 18mm T&G OSB  atop the insulation. That will mean the floor will never feel cold, and the air to air air-con / heating will warm the place with ease, and very quickly too vs in screed wire which will take time to bring the room up to temp. Heating via the ASHP will also reduce the running costs significantly and you can operate them via an app or timer to simply bring the room up to temp 30 mins before use. You'll also save the cost of screed.

Forget a boiler, waaaay overkill, and annual inspections will be required increasing ongoing costs.

KISS. 

[MP1]

Thank you, @Nickfromwales for all the sound advice.

 

On 14/04/2021 at 15:05, Nickfromwales said:

If the primary use is for steam sauna then just fit a 60-80L wall mounted UVC ( instantaneous water heater ) and have that outside the bathroom, maybe make an airing cupboard just to the side of the bathroom door for towels / water heater / CU etc?

 

I have not decided on the unit but it will be used primarily for steam sauna and short showers. I like the idea of 60-80L wall mounted UVC. When @Iceverge mentioned it, I assumed I would need much larger storage capacity as I read somewhere on this forum that small UVC would be 180L.

 

On 14/04/2021 at 15:05, Nickfromwales said:

For heating FORGET wet UFH and Willis heaters and all the shart that goes to make up such a system. I've just done a similar sized annex with in-screed electric UFH wire ( NOT under-tile heating mat ) and it works brilliantly. Zero maintenance and running costs will be the same ( less than actually ) than wet UFH + Willis ( both will use direct electricity, but the wet system will have losses and two heating pumps ).

 

Yes, this makes perfect sense. I am going to stop considering any wet UFH. Do you mind if I ask what in-screed electric UFH you used?

 

On 14/04/2021 at 15:05, Nickfromwales said:

In the immaculate solution, I would fit an air to air heat pump for heating, and use that in the summer for its air-con capabilities too. The better you make this outbuilding, the more you will fry in the summer. I'm just in the process of buying a dual / split unit for my 6m x 4m workshop / home office so I can heat and cool. I am buying one that serves 3 internal units so I can fit one in my boys attic room, one on the landing of the house, and the other in the shed / office. One stone, 2 birds.  

 

I really like the idea of installing a heat pump and using its air-con function in the summer. Would you know, more or less, what its cost would be for such a small outbuilding?

 

I think I now have to decide between in-screed UFH and air to air heat pump... From what you are saying I would prefer a heat pump with a well insulated floor unless the cost of it is prohibitive for me.

 

Once again, thank you both for your help.

[MP1]

I have started researching air to air heat pumps and I came across electriQ Smart12HP 10000 BTU (2.93kW) Wall Mounted Heat Pump Air Conditioner that is said to be suitable for spaces up to 30SQM. I have seen some positive comments about the appliance on this forum. It cost around £700 which is much less than I thought I would have to spend. Am I being naive to think that this kind of appliance could be suitable fo my outbuilding?  

 

[Nickfromwales]

1 hour ago, MP1 said:

Am I being naive to think that this kind of appliance could be suitable fo my outbuilding?  

Nope. As long as you detail a bit of decent airtightnss and as much insulation as you can get, that is a winner. Electric towel rad and UTH for the bathroom and bingo.

The cost of insulation vs screed is a no brainer. If you can get 150mm-200mm of PIR and then another 36mm of deck board atop that, you wont need any screed.

In screed heater wire 

[Iceverge]

10 hours ago, MP1 said:

I have started researching air to air heat pumps and I came across electriQ Smart12HP 10000 BTU (2.93kW) Wall Mounted Heat Pump Air Conditioner that is said to be suitable for spaces up to 30SQM. I have seen some positive comments about the appliance on this forum. It cost around £700 which is much less than I thought I would have to spend. Am I being naive to think that this kind of appliance could be suitable fo my outbuilding?  

 

 

More than enough. 

 

 

 

 

[MP1]

Thank you @Nickfromwales and @Iceverge. You guys made my day.

 

 @Nickfromwales - to build the floor, would you just just install joists directly on the raft foundation (with damp-proof membrane laid down first), place PIR between the joists and then 36mm of deck board atop? I have not done it before so I am sorry if I am asking obvious questions....

 

Also, you both stress on airtightness and mechanical ventilation. Do you think that a single room heat recovery unit (https://www.fastlec.co.uk/blauberg-mini-air-single-room-heat-recovery-unit-with-wifi3) for the office area and a traditional mechanical fan (https://www.amazon.co.uk/gp/product/B01LYTOSBH) for the bathroom would be suitable?

 

 

Edited April 16, 2021 by MP1

[Nickfromwales]

@MP1

You simply bond the PIR ( you could just use 200mm of much cheaper EPS if you have the depth? Not much in it performance wise tbh ) to the slab with foam, weight them down whilst curing, and then you don't need joists. The 2 layers ( lay in opposite directions per layer with staggered joints eg not just overlaid ) will give you a rock solid floor with zero bounce, and zero cold bridging through the joists.

Winner winner, chicken dinner.

You need to shutter the slab in a timber shoebox so you can get the slab flat and level so there are no undulations to transfer through the uppers layers, ( that's where attention to detail is very important, but you're laying a foundation anyway so no extra expense other than time / eye for detail ) and you're good to go. For DPM, just put a sacrificial 25mm layer of EPS on the slab, then put your DPM atop that ( will prevent any puncturing ) and then put whatever insulation you want atop that. so 175mm PIR and 25mm EPS would be the 200mm sandwich. 200mm should be doable as that will allow a bit of a rain splash zone at the bottom of the external walls. If you can get 150mm or 200mm of insulation in, that would be the best outcome, but if the floor is not heated, then you could probably get away with 25mm EPS and 120mm PIR as that will still be quite a high spec for what it is. What are your plans for the walls and roof? Roof detail is of importance too.

MVHR? I wouldn't bother tbh, as how far do you really want to go? Trickle vents in the windows and doors and a regular extractor in the shower will suffice in the real world. You will end up getting dizzy balancing between A2A heating and cooling plus mechanical ventilation. Plus you cant really have a single room MVHR system and a regular extractor sharing the same envelope in such close quarters as there will be a conflict when both are running simultaneously.

A2A climate control, floor insulation to the max and save on the screed. A2A units will provide a degree of air quality and humidity / CO2 control when used in unison with manually set trickle vents. The trickle vent in the bathroom will typically need closing whilst using the steam room so the warm air from the office / bathroom spaces gets pulled into the bathroom vs cold air from outside, which will help with managing the damp / humidity in the bathroom. That will need a beefy extractor too, as the steam room will be a condensation bomb. Most steam units come with an integral fan which you duct to atmosphere, but I would use that, plus a secondary unit set to control humidity as well as 'stink removal'. I would look at the ICON range as they have shutters for when the fan is not in use, very handy to prevent backdraught without an annoying sprung 'flap'.  

[MP1]

Thanks @Nickfromwales for the extensive guide. It is extremely helpful.

 

5 hours ago, Nickfromwales said:

You need to shutter the slab in a timber shoebox so you can get the slab flat and level so there are no undulations to transfer through the uppers layers, ( that's where attention to detail is very important, but you're laying a foundation anyway so no extra expense other than time / eye for detail ) and you're good to go.

 

I already have the raft in place. I need to check if it is perfectly flat and level. If not, I presume I would need joists? Or do you think I can somehow level the floor on a higher layer?

 

5 hours ago, Nickfromwales said:

 What are your plans for the walls and roof? Roof detail is of importance too.

 

Flat Roof:

I am thinking of 6x2'' joists and 18mm external grade plywood. Then 100mm PIR between the joists and plasterboards from the inside. I don't know what I am going to cover the roof with. I started researching EPRM. Any ideas would be much appreciated.

 

Walls:

150mm solid wall (Thermalite blocks) with external insulation (50mm or 100mm).

 

Do you think I need more insulation on the walls and the roof? 

 

5 hours ago, Nickfromwales said:

I would look at the ICON range as they have shutters for when the fan is not in use, very handy to prevent backdraught without an annoying sprung 'flap'.  

 

Thanks for that. I am dropping my silly MVHR idea.

[Nickfromwales]

12 hours ago, MP1 said:

I need to check if it is perfectly flat and level. If not........

.....then simply use a single part self leveling compound to get it flat, uniform and level, and you're sorted. Fully bonding all of the floor layers during construction will be of paramount importance, especially where the gym equipment will reside. That said, I would boost the flooring to 2 layers of 22mm OSB3 for the few quid extra expense.

 

12 hours ago, MP1 said:

Then 100mm PIR between the joists and.....

.....30mm insulated plasterboards so you lose the cold bridging of the rafters. Heat rises, so pay the most attention there. Use foil tape to seal the 100's to the rafters and same to seal the insulated roof structure to the top of the internal vertical walls, immediately prior to boarding the walls.

 

12 hours ago, MP1 said:

I don't know what I am going to cover the roof with. I started researching EPRM. Any ideas would be much appreciated.

Rubber roof membrane or GRP, but rubber is the quietest in the rain. Nicer for an office environment / phone calls etc IMO.

 

12 hours ago, MP1 said:

150mm solid wall (Thermalite blocks) with external insulation (50mm or 100mm).

Plenty good enough for this. Again, you could use a 30mm insulated PB on the internal, but if this is a gym and office you may be better off insulating externally with just 75mm, which should be plenty, and paying a plasterer to fully wet coat the interior so you can screw shelving / equipment to the internal walls without them caving in. Thermal blocks are shite for any kind of fixing, so keep that in mind if you have anything serious to fix to the walls.

 

13 hours ago, MP1 said:

Do you think I need more insulation on the walls and the roof?

Roof, as above, walls good enough to not invest more IMHO. If nothing discernible being affixed internally, then 30mm insulated PB to the interior would further improve things.

[MP1]

Thank you very much @Nickfromwales for your tremendous help!

 

 

 

[markocosic]

+1 from me for air:air heat pump / air conditioner for an insulated outbuilding. Remember the condensate drain.

 

 

Thinking out loud on build:

 

 

I don't like the idea of trying to air-seal PIR foam to studs/joists using foil tape. It sounds like a monumental faff cutting and fitting and taping all that compared with using a dedicated air tightness/vapour retarder membrane on the inside then stuffing flexible insulation between the studs/joists. 

 

If you swap from block to timber frame then you've got 100+ mm extra depth available and won't need the high insulation values of the PIR. You probably want 195 mm joists for the roof for structural reasons so again don't need the high insulation values of the PIR.

 

 

Walls

 

Maybe don't build in blocks (slow, messy) then add a token amount of insulation externally. (and add a finish on top of that)

 

Try:

 

Inside: double plasterboard then horizontal battens to make 70 mm service cavity (lets you run cables, pipes inc waste pipes and A/C pipes, 205 x 60 mm ventilation ducts etc), mineral wool infill then an airtightness layer/vapour retarder membrane. (add extra horizontal battens where you want to attach really serious loads)

Structure: 145x45 mm studs on 600 centres. 150 mm mineral wool infill.

Outside: wind barrier membrane, vertical battens attached to studs, horizontal cladding

 

The plasterboard provides your shear strength in this arrangement. Swap the airtightness layer/vapour retarder membrane for taped OSB if you need more shear strength. Or put an extra sheet of OSB behind single plasterboard to give shear strength and let you screw anything to any wall anywhere but note that this won't be an airtightness/vapour retarder due to all the socket holes etc.

 

I'll be way faster, quite a bit warmer, and probably cheaper than building in block. A single storey frame that size goes up over a weekend with two blokes who know what they're doing.

 

And when you realise that the bog drain is in the wrong place you've got enough room to sneak a bog pipe down through your wall at the expense of a little insulation. Sink drains run in the 70 mm service cavity.

 

 

Roof

 

 

If you do what you describe with the 6x2s, PIR, and plywood then your plywood is never going to dry. Why?

 

Putting a sheet of plastic (air/vapour barrier) on top of grass gives you...moisture under the plastic from the moisture generating activities underneath. Do the same with your roof (EDPM or felt air/vapour barrier)...and it will rot. 

 

Your flat roof needs to be:

- waterproof layer (EPDM better than felt)

- support layer (OSB is fine)

- VENTILATION gap

- Insulated roof structure

- Airtightness / vapour retarder 

 

 

It's fine to have an unventilated flat roof on a shed...because the entire shed is ventilated. It doesn't work for habitable buildings.

 

 

For long life you probably want more like:

 

- waterproof layer

- 12 mm OSB (this is enough; go thicker if you'd like to spend more)

- spacers (use 20x50 mm battens if you are creating the "fall" in the structure of the roof; use something fatter cut to an angle if you are laying the structure of the roof flat)

- 195 mm C24 roof joists on 600 centres to span the full 4 metre width and avoid any beams inside (see span tables e.g. https://www.timberbeamcalculator.co.uk/span-table/flat-roof-joists?load=1&class=C16)

- mineral wool between these

- airtightness layer/vapour retarder (without holes through it)

- plasterboard

 

To save on height run all your cables and ventilation at high level in the walls, rather than in the ceiling. (anything that "crosses" joists should be run in the walls) Where they must go in the ceiling put them between the joists so that the plasterboard "pushes up" the membrane and the insulation slightly - fine for cables for lighting or alarms etc; don't try it for deep downlighters; and don't fit downlighters anyway wall washers will give you a nicer light)

 

 

Alternative?

 

Going for a shallower roof means a bigger fall (and letting it bend a lot) and/or internal structure (to support the shallower joists at midpoint).

 

If you could tolerate a post where your "shelf wall" is, and making the partition wall between bathroom and gym structural, then you could have a "ridge beam" running from left to right and a very shallow dual-pitch roof with joists that are only 2 metre span. Then you're into C24 45x95s on 400 centres (or 600 centres if ok with it being bendy) rather than 45x195s. 

 

You could go with structural panels at that point; as they easily span 3 metres but are dubious at 4 metres unless you go for particularly fat ones. 

https://www.kingspan.com/gb/en-gb/products/insulated-panel-systems/insulated-wall-panels

https://www.griffithsandson.co.uk/kingspan-insulated-seconds/66

 

They're fugly but if used in single pieces (no joints on the slope) you can go for a near flat roof that will outlast felt/EPDM, has U-value of between 0.15 and 0.5 (depending on which thickness you buy), and includes all the structure/airtightness that you need as part of the panel. Hide three sides from view by continuing your wall cladding up above the roofline. Either have the 4th side drain into a gutter on the non visible side or if you're braver build an EPDM box gutter on top of that 4th wall and again bring the wall cladding up to hide it.

 

 

 

Sturctural

 

Single sole plate and header plate on the walls. Line up your roof joists with the wall studs on 600 centres. 

 

Careful on BIG openings.

 

You will need a structural header above the big door to carry the roof joists. Maybe 2x 195x45s in C24 - use something like this to check https://www.timberbeamcalculator.co.uk/) 

 

Push this to the TOP of the wall so that all the load is carried at high level by two pieces like this ||. Then have another piece of 145x45 mounted like this _ above your big door but don't put any vertical sticks between this and the roof. Why? These pieces || provide the roof strength. They will bend as the roof is loaded up. This piece _ provides the in/out strength for the door but has all the structure of we spaghetti in the vertical direction. By separating the loads you will avoid jamming the door or busting a window under a large opening in the timber frame.

 

The other option is making a "solid" wall (with vertical sticks between your header and the the top of the window opening) then putting the window OUTSIDE the timber frame on an OSB or ply box. This way the wall can move under load, and the OSB/ply box will bend so that the window can stay the same size even as the size of the hole in the wall changes)

 

(most fixed windows have 5; maybe 10 mm of movement between glass and frame before you run out of room and start loading up (breaking) the glass; most moving windows/doors can only allow 5 mm of outside frame movement before they stop opening/closing/sealing correctly; big timber frame openings will move more under load than you might expect; and not all window/door people that are used to selling to brick and steel constructions will give you the right mounting details for a timber frame)

 

 

Floor

 

If you have a raft already then gluing some foam and structure on top as nick suggests makes sense. If you're using an engineered wood finish (basically tongue and groove plywood with some veneer on top) and you glue this to 22 mm OSB (rather than gluing it to itself and floating it) then you have the equivalent of a 40 mm kitchen worktop to walk on. It won't move and saves the extra 22 mm OSB.

 

Your bathroom might dictate otherwise. Are you going to have a drain in the floor there that you can hose down all the mud / sweat / human fur into? Does the steam shower need to drain into the floor rather than the wall?

[MP1]

Hi @markocosic

 

Thank you very much for your extensive and solid advice.

 

It all makes sense but there are some constraints now due to certain decisions that I have already made and committed to. Namely:

    - I have already started and almost finished building the walls using 150mm Thermalite blocks.
    - I have already bought some of the materials for the roof. These are 47 x 150 C24 joists  and 18mm hardwood external grade plywood.
    - I already have Catnic CN81C 4575mm above the big bifold door.

Walls

 

I now need to figure out how much insulation I need on the outside as I am already invested in the block walls. I think that the 75mm suggested by @Nickfromwales is reasonable.

 

Roof

 

I infer from what you are saying that in the roof I could have the plywood, then leave a ventilation gap (50mm), then have 100mm PIR, then airtightness / vapour retarder and then plasterboards. I cannot use wool because I would not have enough space for 150mm of it. I will have a waterproof layer on to of the plywood, of course.

 

Structural

Do you reckon I would still need a post and the ridge beam? Perhaps I should do some proper structural calculations to ensure it is all sound....

 

Floor

I like your idea of thicker engineering wood finish.

The drain in the bathroom is in the floor. I have not decided on the steam shower unit but I will pick one that drains into the floor. I intend to tile the whole bathroom.

[Nickfromwales]

6 minutes ago, MP1 said:

Do you reckon I would still need a post and the ridge beam? Perhaps I should do some proper structural calculations to ensure it is all soun

You're at 4m wide, no need for such OTT measures.

 

12 hours ago, markocosic said:

Inside: double plasterboard then horizontal battens to make 70 mm service cavity

I'm currently specifying M&E for a 400m2 passive house which has between 25mm and 35mm service cavities for a family of 6. 70mm for what exactly?

[Nickfromwales]

13 hours ago, markocosic said:

The plasterboard provides your shear strength in this arrangement

No it does not. Sheathing with OSB or plywood is recognised for racking strength. Plasterboard is friable and is not constructional, and is only mentioned as a beneficial reinforcement in most reference documents. A structure should stand on its own prior to internal 'decorative' finishes / laminations.

 

13 hours ago, markocosic said:

Or put an extra sheet of OSB behind single plasterboard to give shear strength

 

28 minutes ago, MP1 said:

 I will pick one that drains into the floor. I intend to tile the whole bathroom.

The steam cabinets are free standing in every unit I've ever fitted, so you can go out of the wall to an external drain which is less complication.

Edited April 18, 2021 by Nickfromwales

[MP1]

9 hours ago, Nickfromwales said:

The steam cabinets are free standing in every unit I've ever fitted, so you can go out of the wall to an external drain which is less complication.

 

@Nickfromwales Is there any particular model that you liked and you think it would be a good fit for my outbuilding?

I can still change the size of the bathroom as the partition wall is not there yet. I already have the drain in the raft foundation but I can still go with an external drain.

[Nickfromwales]

This was who my last customer bought from. 
They’re all much of a muchness. Chinese things with crap instructions etc but was ok when built. I remember having to clear CT1 a few more junctions than they indicated to be watertight. 
 

[markocosic]

@MP1

 

Sounds like you're already committed there!

 

Roof

 

If you look at span tables linked a 4 metre span wants 195 mm deep beams @ 600 centres. Or 145 mm beams @ 600 centres that are a little bendy but hey who cares if there's enough fall to avoid ponding or on closer centres if you really want the roof to be more rigid.

 

You've already got the 150x47s. Those on 400 or 450 centres with the 18 mm ply will be sturdy. Could have used 145x45s on 600 centres and 12 mm OSB if being cheap about it.

 

Set the roof beams with a fall, put your air barrier/vapour retarder on the bottom of the beams, drop your insulation in between the beams (which is not as thick as the beams; say 100 mm or even 120 mm), then pop your roof deck ply / OSB on top of that. The space above the insulation becomes your ventilation and your buildup is perhaps 175 mm (5 mm membrane, 12 mm OSB/ply, 145 mm beams, ball hair of membrane, 12.5 mm plasterboard)

 

Only if you wanted to drop to 95 mm beams, or wanted to use Kinsgpan type panels, to give more headroom under some height limit, would you need any support in the middle.

 

 

Wall

 

£ for £ (or mm for mm) the timber walls with 225 mm (or even just 145 mm) of mineral wool would be warmer than the block walls with 75 mm of insulation and I think faster / easier to finish. Given where you are already it'll be your final finish (render? cladding?) that dictates what you do here more than anything else.

 

 

Floor

 

IMO:

 

Get a few samples. First thing I did was dropped them on the ground, stabbed them with kitchen knives, scraped a brick across them, then hit them with a hammer. Fancy finishes...look screwed. Brushed finishes...look much the same afterwards. Some brands had supporting ply made of cheese. Others offered good support to the veneer and were unmarked.

 

You'll likely end up with random lengths if buying cheap. Unpack them, sort by size (and rejects), let them acclimatise. Plan your joints to ensure that you have lots of overlap. It's difficult if they give you a pack of short bits.

 

You'll want a 3 mm notch trowel and MS polymer adhesive. Don't get it on the face of the boards or in your hair. That includes kneeling in it. Lay your boards, bed them, and don't walk on them until it has cured. When done scuff the boards up and use a commercial duty two pack satin lacquer over the whole lot to add protection for the surface and avoid the inevitable red wine spills staining the edges by sealing it all up.

 

If a labrador does a "cartoon dog on ice" impression the lacquer will scratch. Magic sponges (slightly abrasive melamine sponges) polish this out and leave a similar finish to a satin lacquer.

 

The finished item gives more of a "dull thud" than a solid piece of timber on joists. It has some compliance but it's not the "laminate laid on some sponge on top of a solid floor" type feel. Difficult to describe but not unwelcome.

 

 

 

@Nickfromwales

 

Logic on the fatter 'service cavity' rather than fatter wall studs for achieving a given level of insulation was:

 

- Easier to raise the (lighter) 145 walls with 70 mm service cavity added later than say a 175 mm wall with 45 mm cavity or 195 mm wall with a 25 mm service cavity.

- You can order all the insulation @ 70 mm thickness rather than having to order mixed thicknesses

 

It isn't for fat services per se; though 204 x 60 rectangular ducting fits for ventilation at low velocities, rainwater pipe for ducting mini-spilt AC lines fits, and ; you're never short on depth for shower mixers, wall hung taps etc

 

The vapour retarder / air barrier winds up a little closer to the middle of a wall that's likely to be heated (wants vapour retarder inside) as much as it's going to be air conditioned (want vapour retarder outside) too.

 

45 mm works, and has the advantage that you can rip down leftover x by 45 mm timber from building the walls/roof to use as the internal battening. 25 mm works too until you want waste pipes buried without having to have half in the service cavity and half "eaten into the plasterboard" or shoved into the structural wall a bit. The 70 suggestion was for maximising the performance per £ / mm within the "150 mm thermalite plus 75 mm" kinda thickness outlined.

 

 

You could use diagonal 25x50 mm strapping on the outside for shat strength if you wanted to. For the purposes of a garden room / shed...sat on a raft...with a big roof diaphragm on top...and gets horizontal cladding to the outside of the studs...double plasterboard is more than just decorative and gives useful thermal mass.

 

OSB behind plasterboard (attached to the battens attached to the studs) is more useful to you than OSB on the outside or OSB between the studs and the service risers...because you can then woodscrew things anywhere you like on that internal wall. It's there for screwing things to wall convenience in the same way as you might ply line parts of a wooden shed. Not really needed in that structure.

 

For a pukka house that's not short and squat and it's be a different story. In this case it's all block anyway. Oodles of shear stiffness for the size. But more laborious to build/finish and not as well insulated once done.

[MP1]

Thank you @Nickfromwales and @markocosic for all your comments and recommendations.

 

Based on what you are saying, I came up with this picture of my roof:

 

 

 

Please, could you help me with the following:

 

1. For the rubber roof, what product would you actually use and what thickness. I am looking at EPDM Rubber Flat Roofing Supplies | Rubber4Roofs but I am not sure if I am looking at the right thing....
2. For the air barrier, could I use this product: Powerlon Air Barrier Membrane - 50m x 1.5m Roll | Roofing Superstore®? Or is there any better product you would recommend?
3. Do I need anything between the plywood and the joists / air gap? 
4. I planned on having spotlights / downlights as I thought they would be best for the office / gym area. However, I understand that I should not break the air barrier.... Do you reckon that wall washers would be really better? Is there any specific product you would recommend?

 

 

 

 

Edited April 19, 2021 by MP1

[MP1]

@markocosic

 

20 hours ago, markocosic said:

Given where you are already it'll be your final finish (render? cladding?) that dictates what you do here more than anything else.

 

I think it will be monocouche render to match what I have on the house.

 

 

[markocosic]

Kinda yes to your build up.

 

 

If it were mineral wool then nothing between ply and the top of the joists. It will all air dry. Leave those channels above the insulation "open" to the outside air by venting them into the soffits. That is how any moisture going through your roof will be cleared. Then vapour control layer where you've drawn it

 

If PIR boards and you mount these PIR boards level with the bottom of the joists then the foil tape over the gaps as Nick described. It will be cheaper/easier than a membrane. The barrier is still in the same location as you drew. You can't make holes in it which I think answers your downlighter question.

 

 

The "insulated" plasterboard will give you somewhere to hide any ceiling cables. (router / gouge out a little of the insulation so that you don't push the PIR boards upwards and tear the aluminium tape) That works for surface lights and for surface mounted spotlights.

 

What about slim downlights you ask? (e.g. 25 mm depth for 40 mm void) Careful. These usually need ventilation from above. They're meant for fitting into an uninsulated "service cavity" not a "pocket" in insulation and will cook if you try encasing them in insulation. Many also have remote drivers that you'll need to find space for too. For example here @ 40 mm:

https://www.expertelectrical.co.uk/aurora-enlite-slim-fit-shallow-led-downlights#product-info-dimensions

 

Maybe you can find something that doesn't need ventilation. Personally I hate downlights especially for an office. You'll have horrible hard reflections off all horizontal surfaces; such as your desk; and also off your angled screen. I fitted them once because she who must be obeyed wanted them and apart from vacuuming/scrubbing the floor - and the bathroom - they're never used. Spotlights will be better but you'll end up trying to bounce the light off walls rather than directly lighting objects.

 

This kind of light is soft and forgiving on the eye and on the workmanship:

https://www.ikea.com/gb/en/p/hektar-floor-lamp-dark-grey-60216549/

https://www.ikea.com/gb/en/p/ranarp-floor-reading-lamp-black-00331378/

https://www.ikea.com/gb/en/p/ranarp-wall-clamp-spotlight-off-white-50231324/

Install sockets* that are switched form a light switch to make turning them on and off easy. Lights inside shelving units; desk lamps (usually bounced off the wall /ceiling instead of facing the desk); also good.

 

If you'd like to see ALL the painting/plastering imperfections in the wall / ceiling then go for something like this:

https://www.screwfix.com/p/lap-charcoal-grey-led-up-down-outdoor-wall-light-2-x-350lm-5-3w/4732x

 

It's small and practical and gives a nice light to work / eat by but oh dear lord does it shadow at the slightest imperfection. Likewise anything that you would see in a shop that's meant to catch all the twinkles. 

 

 

*Sockets are good. If you're not sure which should be ring-main-sockets-for-stuff and which should be switched-from-light-switch then you can run the cables and blank the ends. Think double socket next to a single socket. Double socket is always on ring main. Single socket can be fed from a light switch. Or it can have a spur from the double socket next to it. Or it can be an Ethernet socket. Pull the cables before you close the walls and blank the unused ones if you're unsure how the space will be used / lit?

 

 

 

 

 

[MP1]

Thank you @markocosic

 

I reckon you are right about the downlights and their suitability for an office I need to really think what lighting would be best for my video calls too. Thanks for all the ideas and suggestions.

 

I still have my doubts about the rubber roof. I will have two 600x900 skylights. Do you think I should go for EPDM? If so, any particular thickness and brand you would recommend? Any suggestions would be much appreciated.