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 Friday at 11:52 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 12 hours ago 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