Thoughts & suggestions on converting a garden outbuilding

[SilverShadow]

Hi there,

 

 

I'm after more of your wonderful wisdom. Appreciate there's loads of similar threads floating around on, so apologies if this is stomping on well trodden paths...

 

We'd like to convert our outbuilding at the bottom of the garden into an office, and wanted your sound advice on the type/approach of interior insulation, please. We've researched this a little so have a basic idea of what is required, but wanted to glean your knowledge if possible - the advice we're after is mostly for the walls, but any general advice is always more than welcome 😁

 

 

A little bit about the building::

• 6m long  x 3m wide x 2.5m high with metal sloping roof & solid concrete floor
• Appears to be single skin 100mm breeze-block with (i think) painted on the inside & with monocouche rendering on the outside.
• It has 3 upvc double glazed windows and upvc door
• Used to be a workshop (has plenty of power and it's own fuse box/consumer unit)
• From what i can see, the ceiling is already insulated (to an unknown depth), so our prime focus is walls & floor
• It has a wood burner with metal chimney leading to the outside

 

 

A little about it's desired usage:

• Primarily an office - so used 8 hours or so during the day , throughout the year
• We'll add electric heating as required
• If at all possible we'd like to keep the log burner (if this isn't going to cause flammable issues with PIR/Insulation?)
• We'd like to be reasonably budget friendly, so get the most comfort/heat for whatever reasonable price we can afford

 

 

Mock up of proposed wall insulation

I've enclosed my initial plans for the wall insulation

• Vertical wooden battens 1.2m apart, using 50 x 50mm wood
• Item A- 50mm thick PIR boards (foil backed), sized 2.4m x 1.2m, arranged vertically
• Item B - 37.5mm thick PIR boards (25mm + 12.5mm plaster finish), sized 2.4m x 1.2m, arranged horizontally
• No air gaps between wall/item A or item A/item B
• Blue item - electrical tube trunking for sockets, cut between 50mm PIR boards cut through 37.5mm, where sockets will be
• As yet - unsure whether to install a damp proof membrane (if required with foil backed PIR?), and if so whether to insert for at 1 (between wall and 50mm PIR) or 2 (between 50mm & 37.5mm PIR) on the diagram?
• No idea what to do with the log burner (if it can stay without an undue fire risk, which i strongly doubt)

 

 

This is something i'd really love to get to grips with in the new year, but realise it needs some strong advice to get off on the right footing

 

Many thanks as always for your time

Shadow

Edited September 27 by SilverShadow

[Redbeard]

I have never (I think) seen battens that far apart. I worry that at times when it is let  to get cold (weekends in the winter?) the cold plasterboard will gather moisture and 'sag out'. I can see no reason for a DPM, and indeed wherever you put it, it would block off the 'out' route that is the only one left for water vapour in a VCL method of insulation. The foil on the insulation is a VCL, though the bonded boards have the classic issue that you cannot tape the VCLs on the boards together because the plasterboard gets in the way. Some would say taping the 50mm layer would be OK to tape, but some wouldn't.

 

Be aware that 50 x 50 is rarely 50 x 50!

[elite]

Continous layer of PIR with battens at 600mm in front of it giving you a service void? Need to consider ventilation, reveals need some thought too

[Gus Potter]

11 hours ago, SilverShadow said:

We'd like to convert our outbuilding at the bottom of the garden into an office, and wanted your sound advice on the type/approach of interior insulation, please. We've researched this a little so have a basic idea of what is required, but wanted to glean your knowledge if possible - the advice we're after is mostly for the walls, but any general advice is always more than welcome

Now do you want to go the whole hog and make it building regs complaint and get planning approval for it? If you do so it could add a lot of value to the house?

[SilverShadow]

11 hours ago, Gus Potter said:

Now do you want to go the whole hog and make it building regs complaint and get planning approval for it? If you do so it could add a lot of value to the house?

 

 

Hey Gus - tbh, it shouldn't need building regs (as it complies with rules exempting it). I'd not really considered it, as it felt like a lot of hassle for potentially little return. But i confess to being no expert, so don't really know how many hoops i'd need to go through and for what ££ benefit.

 

I was aiming to use it primarily just as an office, rather than up the spec to something a person could sleep in (for example)

[SilverShadow]

17 hours ago, Redbeard said:

I have never (I think) seen battens that far apart. I worry that at times when it is let  to get cold (weekends in the winter?) the cold plasterboard will gather moisture and 'sag out'. I can see no reason for a DPM, and indeed wherever you put it, it would block off the 'out' route that is the only one left for water vapour in a VCL method of insulation. The foil on the insulation is a VCL, though the bonded boards have the classic issue that you cannot tape the VCLs on the boards together because the plasterboard gets in the way. Some would say taping the 50mm layer would be OK to tape, but some wouldn't.

 

Be aware that 50 x 50 is rarely 50 x 50!

 

 

 

Thanks RB - i had initially thought about 600mm wide battens, but as the boards we're 1.2m apart it felt logical to keep the batten distance the same to reduce the amount of cutting required. I wasn't aware that the boards could 'sag' as such, so good shout 👍

 

Thanks for confirming no need for a DPM - i presume if i used boards without VCL this would become a necessity?

 

As a general suggestion then: if i swapped to using 600mm wide boards (of same thickness & arrangement), moved batten intervals to match the 600mm board width, then added tape to my 50mm thick PIR (between boards and PIR/wood), then this should be ok do you think? 🤔

 

[SilverShadow]

12 hours ago, elite said:

Continous layer of PIR with battens at 600mm in front of it giving you a service void? Need to consider ventilation, reveals need some thought too

 

Thanks Elite - i wasn't sure at all about ventilation (where & what). Do you suggest having a void between the 2 layers of PIR, by way of using something like 70mm wood battens (to create a 20mm air gap between 50mm & 37.5mm boards)?

 

I presume the 'reveals' are things like consumer unit and power sockets, etc?

 

Apologies for the fundamental questions - this is probably quite obvious to the trained eye, but a big mystery to those without prior experience. All the videos on YouTube about conversion seem to conveniently skip this sort of consideration & many do it quite differently by the look of it. There doesn't seem to a generic agreed method/diagram for how to construct the layers 🙃 

Edited September 28 by SilverShadow

[SilverShadow]

Apologies folks, if i'm causing more problems than ever, with these continual questions

 

If it's easier: please feel free to point me to some diagrams/videos, that show how the PIR/battens/voids/vents/taping are all arranged and done for a similar sort of conversion (one without needing building regs, ideally) 👍

[Redbeard]

1 hour ago, SilverShadow said:

I presume the 'reveals' are things like consumer unit and power sockets, etc?

 

No, reveals are the 'turn-rounds' to windows and doors, where the walls on each side turn in to form the window/door opening.

 

Will look for some pics of the 'warm batten' (service void) method as described above. The 'service void' is as deep as the battens used, so normally 25mm deep. That may (but may not) mean that you can mount sockets on the plasterboard face without the back-box piercing the foil which (maybe) you are hoping to use as your VCL.

1 hour ago, SilverShadow said:

Thanks Elite - i wasn't sure at all about ventilation (where & what). Do you suggest having a void between the 2 layers of PIR, by way of using something like 70mm wood battens (to create a 20mm air gap between 50mm & 37.5mm boards)?

 

No ventilation (external air movement) between layers of insulation!! If you do (deliberately or inadvertently) introduce ventilation you are leaving part of the wall 'sandwich' 'out in the garden'. Keep layers tight to each other.

 

I have only had a quick look at later posts after my first, but so far I don't think anyone has mentioned ventilation. Yes, you want to keep the place warm, but you need to keep the occupants warm, but you need to keep them healthy (and indeed alive!) too.

 

You have also made no mention (I think) of floor insulation. I guess the door arrangement may make this difficult but I would strongly advocate for some, rather than none (for example 25mm PIR (Kingspan/Celotex or similar) taped at all joints and perimeters as VCL or (my preference) with a strong VCL over - so you can tramp all over it without piercing it when you are laying the boards - then glued T & G OSB - I hate chipboard - as a 'floating floor' - no fixings) even if you have to have an un-insulated area for the 'swing' of the door.

If you do keep the stove (a) it needs *permanent* non-closeable ventilation and (b) you need a good hearth and fireproof separation from walls which may now be insulated with a product which burned horrifyingly well in a much-publicised fire (not the only flammable element, I accept). At very least follow Bldg Regs guidance, and also look carefully at the specifics of the exemption. I cannot remember but if, for instance, it says it's exempt from bldg regs if it does not have permanent heating then you either take out the (permanent) stove or do as @Gus Potter suggests. Consider also whether you want to burn anything, which could (/will) cause pollution and nuisance. If I were you I think I'd lose the stove and look at a cheap air-to-air Heat Pump.

[Temp]

Tape the joints in the foil covered insulation with metal foil tape to form vapour barrier.

 

[elite]

10 hours ago, SilverShadow said:

 

Thanks Elite - i wasn't sure at all about ventilation (where & what). Do you suggest having a void between the 2 layers of PIR, by way of using something like 70mm wood battens (to create a 20mm air gap between 50mm & 37.5mm boards)?

 

 

 

Ventilation comment was for the room, not for the walls

 

I was proposing a single continuous layer of PIR, then battens. So your build up from outside in would be:

 

1. Existing block wall
2. PIR, gaps and joins foamed and taped
3. Battens
4. Plasterboard

[SilverShadow]

T

On 28/09/2024 at 22:38, elite said:

 

Ventilation comment was for the room, not for the walls

 

I was proposing a single continuous layer of PIR, then battens. So your build up from outside in would be:

 

1. Existing block wall
2. PIR, gaps and joins foamed and taped
3. Battens
4. Plasterboard

 

Many thanks, Elite 😁

 

Probably a daft few questions incoming.... 🫣

 

• Item 2 - would you dot n dab the PIR directly onto the painted breezeblocks?
• Item 2 - i could go all out and buy 100mm PIR, but hoping i could get away with 50mm or 75mm maybe and the heat not dissipate too quickly (seeing as budget is a big factor!)
• Item 3 - How would you fix the battens? I could use long screws for fixing the battens through the PIR into the wall, but seemed like making holes in the VCL could potentially cause vapour issues, so maybe adhesive grab is better
• Item 3 - presumably 25mm battens would be enough between the PIR and plasterboard? Saying that, I'd probably choose 40mm, so i could use 25mm socket knockout boxes with 15mm wood to fix the knockout boxes... 🤔

 

Most helpful buddy - definitely gives me a good insight into how to go about this

 

Shadow

 

 

 

 

[Canski]

Mine was easier as a new build but I built 4x2 studs filled with 90 mm PIR then attached 25mm PIR in front with 50x 25mm battens on the inside with all joints foamed and taped to give a service void for cables. It’s as warm as toast. We had a site meeting in there last night and had to turn a small fan heater off after about 10 mins. Mine is a similar size at 5.3 x 3.3 internally. 

[SilverShadow]

On 28/09/2024 at 13:37, Redbeard said:

No, reveals are the 'turn-rounds' to windows and doors, where the walls on each side turn in to form the window/door opening.

 

Will look for some pics of the 'warm batten' (service void) method as described above. The 'service void' is as deep as the battens used, so normally 25mm deep. That may (but may not) mean that you can mount sockets on the plasterboard face without the back-box piercing the foil which (maybe) you are hoping to use as your VCL.

 

No ventilation (external air movement) between layers of insulation!! If you do (deliberately or inadvertently) introduce ventilation you are leaving part of the wall 'sandwich' 'out in the garden'. Keep layers tight to each other.

 

I have only had a quick look at later posts after my first, but so far I don't think anyone has mentioned ventilation. Yes, you want to keep the place warm, but you need to keep the occupants warm, but you need to keep them healthy (and indeed alive!) too.

 

You have also made no mention (I think) of floor insulation. I guess the door arrangement may make this difficult but I would strongly advocate for some, rather than none (for example 25mm PIR (Kingspan/Celotex or similar) taped at all joints and perimeters as VCL or (my preference) with a strong VCL over - so you can tramp all over it without piercing it when you are laying the boards - then glued T & G OSB - I hate chipboard - as a 'floating floor' - no fixings) even if you have to have an un-insulated area for the 'swing' of the door.

If you do keep the stove (a) it needs *permanent* non-closeable ventilation and (b) you need a good hearth and fireproof separation from walls which may now be insulated with a product which burned horrifyingly well in a much-publicised fire (not the only flammable element, I accept). At very least follow Bldg Regs guidance, and also look carefully at the specifics of the exemption. I cannot remember but if, for instance, it says it's exempt from bldg regs if it does not have permanent heating then you either take out the (permanent) stove or do as @Gus Potter suggests. Consider also whether you want to burn anything, which could (/will) cause pollution and nuisance. If I were you I think I'd lose the stove and look at a cheap air-to-air Heat Pump.

 

 

Great advice there, RB - many thanks 😁

 

Good point about the floor - i'd roughly guessed something like 25mm PIR and use Osb3 above it. Sounds like maybe adding another VCL layer between PIR and Osb would be your preferred method.

 

Id thought about maybe 40mm battens, so i could sink the power outlets knockouts neatly into the plaster? I was confused (see other reply), about how these battens would be fixed above the PIR without piercing the VCL though....🤔

 

Another great point about the building regs, re keeping the log burner. I'll probably do away with it, if it requires building regs & stringent rules. Am i right in thinking i won't need building regs if i use a couple of wall mounted electric radiators? 🤷‍♂️

 

Thanks again

Shadow

Edited September 30 by SilverShadow

[SilverShadow]

3 minutes ago, Canski said:

Mine was easier as a new build but I built 4x2 studs filled with 90 mm PIR then attached 25mm PIR in front with 50x 25mm battens on the inside with all joints foamed and taped to give a service void for cables. It’s as warm as toast. We had a site meeting in there last night and had to turn a small fan heater off after about 10 mins. Mine is a similar size at 5.3 x 3.3 internally. 

 

 

Sounds great, Canski 😁

 

Think using continuous PIR/foam/tape, then battens on top (for void), then plasterboard (as elite suggests) would be fairly straightforward.

 

I've heard folks talk about leaving a gap at the bottom just above the floor - i presume this is is slide the floor PIR underneath?

 

The devil's always in the detail with this sort of thing...

[elite]

On 30/09/2024 at 18:25, SilverShadow said:

 

 

• Item 2 - would you dot n dab the PIR directly onto the painted breezeblocks?
• Item 2 - i could go all out and buy 100mm PIR, but hoping i could get away with 50mm or 75mm maybe and the heat not dissipate too quickly (seeing as budget is a big factor!)
• Item 3 - How would you fix the battens? I could use long screws for fixing the battens through the PIR into the wall, but seemed like making holes in the VCL could potentially cause vapour issues, so maybe adhesive grab is better
• Item 3 - presumably 25mm battens would be enough between the PIR and plasterboard? Saying that, I'd probably choose 40mm, so i could use 25mm socket knockout boxes with 15mm wood to fix the knockout boxes... 🤔

 

 

 

 

• Either insulation fixings or plasterboard adhesive foam 🤷‍♂️ - make sure you tape over any fixings Work
• Go for as much as you can afford / space allows
• Good question - I'd want a mechanical fixing in there, maybe foil tape over? I'd do some research on best approach. The wall I have done this on has a stud wall in front of it, so no penetrations as fixed to ceiling and floor
• Work out the biggest depth you need, might be easier with fast fix boxes, but I'm never as happy with them as a proper backbox

[SilverShadow]

12 hours ago, elite said:

• Either insulation fixings or plasterboard adhesive foam 🤷‍♂️ - make sure you tape over any fixings Work
• Go for as much as you can afford / space allows
• Good question - I'd want a mechanical fixing in there, maybe foil tape over? I'd do some research on best approach. The wall I have done this on has a stud wall in front of it, so no penetrations as fixed to ceiling and floor
• Work out the biggest depth you need, might be easier with fast fix boxes, but I'm never as happy with them as a proper backbox

 

 

Thanks again, Elite - some top tips in there 😁

 

In response to your response 😉

 

• I think i'll go use fixings for the PIR and tape over - this way they're easier to remove, if i ever need to get in there later at some point for whatever reason
• Roger, roger - i think i'll do 75mm PIR minimum. With the electric heaters and it being in a sheltered spot, it should suffice
• Mechanical fixings sound best - i've read that taping around all 4 side of the batten where the screw fixings go through should minimise vapour escaping
• Great shout with the fast fix boxes - i would prefer this to needing extra wood behind the box to fix it in place

 

 

I've made a quick sketch of what (i think) your solution will look like, below: breezeblock > PIR > batten gap > plasterboard

• Item 1 - batten gap between PIR and plasterboard. Using screws through PIR to fix batten, and wrapping tape around batten at points where screw goes through
• Item 2 - electrical trunking, with foam and and taping required. Screws for fixing PIR will be taped over

 

Seems like a great approach for me to implement - much obliged 🍻

 

 

 

Edited Wednesday at 08:53 by SilverShadow

[SilverShadow]

Incidentally, i was going to use these as my tape (for joining PIR & around screws/fixings) and fixings - obviously i'd need something like 125mm or more fixings for attaching the battens:

 

https://www.screwfix.com/p/diall-insulation-board-tape-silver-45m-x-50mm/4806V?tc=KC4&gad_source=1&gclid=EAIaIQobChMIi5OtkKnviAMVapFQBh0nhAOoEAQYASABEgJxRPD_BwE&gclsrc=aw.ds

 

https://www.screwfix.com/p/timco-insulation-fixings-110mm-x-8mm-100-pack/405KF?tc=KC4&gad_source=1&gclid=EAIaIQobChMIlLaenKnviAMVmKRQBh3HrzE9EAQYASABEgJol_D_BwE&gclsrc=aw.ds

Edited Wednesday at 08:57 by SilverShadow

[elite]

Just on your diagram - I'd keep services in the space between the plasterboard and the insulation if at all possible, just make sure your cables are run in the correct zones. One tip, if you have at least one socket on each wall, you can run the cable horizontally around whole room, making it easier to add extra sockets in the future. 🍻

[SilverShadow]

18 hours ago, elite said:

Just on your diagram - I'd keep services in the space between the plasterboard and the insulation if at all possible, just make sure your cables are run in the correct zones. One tip, if you have at least one socket on each wall, you can run the cable horizontally around whole room, making it easier to add extra sockets in the future. 🍻

 

Ahh, many thanks again buddy 😁

 

So, make the PIR one continuous layer all the way around and have the trunking in the PIR/plasterboard void 👍

 

I guess the tubular trunking could be held by plenty of tape to attach them to the PIR, rather than using more screws through the PIR (the easy fit knockout boxes should hold the outlet firmly to the plasterboard to prevent cable movement in the void). Plus, with a continuous layer of PIR, i probably wont need very much insulation foam i guess 🤔

 

Regarding sockets - yeah, the outbuilding was an old workshop, so has a double outlet every 2 metres around the internal perimeter. I guess i could cap 'every other/2 out 3' into boxes in the void, and only have the occasional outlet presented through the plasterboard for use.

 

I probably also want to think about how i'm going to cap off the old burner chimney. Maybe keep the chimney on the outside (as its well sealed against the monocouche, and cut it down on the inside/fill with a couple of layers of PIR, cut to fit the vent profile (then PIR board over it) 👍

[SilverShadow]

Quote

One tip, if you have at least one socket on each wall, you can run the cable horizontally around whole room, making it easier to add extra sockets in the future.

 

 

Nice advice! 😁

 

From what i can remember, the cable runs around the top of the walls/ceiling, and drops down from there to each socket.

 

If i cap redundant sockets, i probably want to trim back towards the ceiling (or remove the wire from each junction box, if possible). That way there is no danger of hitting a power line if anyone drills into the plasterboard 😅

[Alan Ambrose]

Suggest giving the whole services in the wall thing a miss. Use surface mount plastic sockets and plastic conduit or metalclad and plastic or metal conduit. You might want the  sockets above desk height for ease of use and it's possible you might want a group on a big switch so they can all be turned off at once. Sure in a ring main around all the walls. Philips square led panels for my money - but screw straight onto ceiling.

 

Ensure all insulation and heating is going to work properly - you don't want it to cost a bomb to heat. Ditto windows - it may be cost effective to install double glazed and better frames. Maybe a quick stab at the J Harris spreadsheet to check. Wall mount remote controlled fan heater for quick heat up before you start work. Whatever heating you're comfortable with after that. You could fix up the wood burner if you really like it, but it'll take up quite a lot of effective space once you keep everything flammable away from it. Strictly, the install/recommsiiosning should be (or if BC is not required, for your own peace of mind) Hetas or BC-notified. Don't ignore solar gain either - my workshop gets crazy hot in the middle of summer.

[SilverShadow]

2 hours ago, Alan Ambrose said:

Suggest giving the whole services in the wall thing a miss. Use surface mount plastic sockets and plastic conduit or metalclad and plastic or metal conduit. You might want the  sockets above desk height for ease of use and it's possible you might want a group on a big switch so they can all be turned off at once. Sure in a ring main around all the walls. Philips square led panels for my money - but screw straight onto ceiling.

 

Ensure all insulation and heating is going to work properly - you don't want it to cost a bomb to heat. Ditto windows - it may be cost effective to install double glazed and better frames. Maybe a quick stab at the J Harris spreadsheet to check. Wall mount remote controlled fan heater for quick heat up before you start work. Whatever heating you're comfortable with after that. You could fix up the wood burner if you really like it, but it'll take up quite a lot of effective space once you keep everything flammable away from it. Strictly, the install/recommsiiosning should be (or if BC is not required, for your own peace of mind) Hetas or BC-notified. Don't ignore solar gain either - my workshop gets crazy hot in the middle of summer.

 

 

Great advice too - thanks Alan 😁

 

I was considering surface mount cables, but would prefer the neater look of them tucked away in the void. However, it'll really come down to how fiddly that is, so it's definitely one to consider when i get into it. I'll take a look at the Philips square LED lights for the ceiling - definitely worth a shout (the existing ones in there currently work for now)

 

Thankfully the door/windows are already double glazed (upvc i'm fairly sure), which for sure could probably do with updating. However, money is fairly tight with just moving in to our house, so i don't think £2k or more on those would meet my budgetary remit (i could always do them later i guess)

 

I'd love to keep the log burner, but i agree it'll be a lot of hassle to make it compliant - so likely it'll have to go.

 

Thanks for the tip about the J Harris spready - this is definitely getting into the advanced side of things (which i may need). But it seems to be aimed more at satisfying building regs, rather than just a bog standard conversion, would you say? 🤔