Brick Garden Office

[navs1001]

Hi, i am looking into building a brick garden office / gym. Just electrics, no gas or plumbing. I was initially thinking of building it out of brick and block, so a double wall with cavity, but one of the builders mentioned i could build it with a brick outer layer and stud walls filled with insulation inside for a lot cheaper. The builders said the walls will be thinner too, 150mm, and i am a bit tight for space. Question though, is 150mm enough for this type of building?

Doing a bit of research online it looks like you need all the below, totaling at least 240mm
100mm brick
50mm air gap
breathable membrane
9mm OSB board
20mm air gap
50mm PIF insulation
vapour barrier
12mm plaster board
final plaster finish

Question is, is the above overkill for a garden office / gym?

Interested in peoples views.

 

[motorbiking]

Size and height is important.

If its under 2.5m height and 15m2 or less (within 1M from boundary) or 30m2 (with 1M from the boundary) its classed as an outbuilding and does not need to comply with building control / planning (usually).

You don't have to use brick at all. Celotex etc. has a build in vapour barrier so can be attached directly to the wall with foam and taped to seal. Then you can add plasterboard on top.

Depending on size, I would go:
timber cladding T&G treated and painted or Marine/water resistant ply if its a wall that can't be seen.
Waterproof membrane (optional)
Stud wall
Celotex or similar insulation between the studs.
Plasterboard

Or

Rendered block wall with DPM
Celotex plasterboard foamed on with plasterboard foam
All gaps filled and taped
Same again for the plasterboard, then finish as usual.

I'm about to start something similar and having originally done it the first way, I think it might be quicker to get a bricky to build the 3 sides and then finish with timber and glass (as per option 2).

You might find this video interesting.

 

[navs1001]

Hi, yes it will be up to the height allowed under permitted development. Total external size 4.9m x 3.4m with a flat roof, which means the floor area is less than 15m2, i read that floor area is the internal floor area.

Option 2 sounds good. I assume i can use brick if i want it to match my house colour and then use Celotex plasterboard foamed on with plasterboard foam.

Am i right you end up with two plasterboards with option 2?

 

[motorbiking]

It's actually cheaper to buy celotex boards (GA4000) ~50mm £25 and standard plasterboard (£6), but you can buy all in one (PL4000) 72mm £60. This note explains what I thought - that being you don't need an additional vapour barrier.

https://docs.planning.org.uk/20210105/40/QH4MZOCRLOI00/8rsu4n9e5k0n83wm.pdf

Ive used soudal plasterboard adhesive foam to attach both the boards to the wall and the plasterboard to the board. Then you put a few long screws in to secure.

With breeze block and 50mm celotex you will get a U value of around 0.25 given the air gap created by the foam. Going for the 2 board system allows you to make sure the celotex is properly sealed and all gaps filled with standard foam to avoid the additional vapour barrier.

 

[mrrusty]

Here's one I prepared earlier - might give you a few ideas. https://community.screwfix.com/threads/garden-room.256842/

Mine has PP as it is over 2.5m but no BR as >1m from boundary and <30sqm

 

[darkkent8]

Do you not think it will be quite cold in there? That video above was just block and plasterboard.
Might it be a better idea just to make it all from PIR and timber?

 

[motorbiking]

Personally I would go with the block and add 50-80mm celotex and then Plasterboard. The rock wool walls are not going to be great with a single skin block, but my guess is it keeps the cost down.. But still likely to under 0.5 U value. better than some houses. If you are tight on space then timber frame is better as you will get <0.2 U value for <120mm walls, where as the block walls will be +170mm allowing for render and 'dab' bonding. I'd also increase the roof angle to avoid needing two gutter runs.

 

[navs1001]

Yes, the video did show them use 50mm rock wool but they said that was to keep costs down, i wasnt sure about their timber studs up against the walls, unless the render is protecting against damp penetration.

I think i understand this better now, so this design calls for no timber studs in the walls
1. Brick outer layer (instead of block just for appearance)
2. Damp Proof Membrane, the material type i assume.
3. Celotex 50mm / 80mm GA4000 foamed on with plasterboard foam
4. Plasterboard attached to Celotex above - not sure how you said this would be attached
5. Skip steps 4 & 5 and use 72mm PL4000 instead
6. All gaps filled and taped
7. Dot and dab 2nd plaster board over existing plasterboard

Have i got that correct?

 

[motorbiking]

4. same foam as the first layer - GA4000 goes on wall with foam, plaster board goes on GA4000 with foam. screws/frame fixers to hold (4-9 per board).
2. not needed if you go for block and render, Render makes the block water resistant.
7. not needed 11.5mm is enough unless you need more than 30 mins fire rating for some reason?
8 skim joins or whole boards as required.

If you go for standard brick/no render I've no idea if you can avoid breathable membrane. and no idea how you will attach insulation to the material. If you look at the building control doc I attached, the render is functional not cosmetic.

 

[mrrusty]

As my post above, look at these SIP prices and tell me why you want to build it in brick with separate insulation. https://www.simplysips.co.uk/pricing/ 120mm SIP has a u value of ~0.2. Externally you can clad in timber, corrugated painted metal, cement cladding planks, render - all sorts

8x4 SIP is £90 To cover 8x4 area just in brick is about 140 bricks, at say £1 each is £140 before you add any mortar materials, insulation, inner framing etc.

 

[motorbiking]

That was how I did my first Garden Room office. Except I made my own. But I think the difference is close if you go with thermalite block. 1.2 x 2.4 ~ £65 in blocks, but then you have Render and Mortar, but save on the external cladding.