Garden Room – Cladding, Roof system & Insulation Again.

I know yet another garden room thread, we were always planning to build a garden room as a space for the kids as we have very small bedrooms and a not massive open plan living room / kitchen. There is a lot of earth works involved so apart from the constant torential rain lockdown is the perfect time to get a load of work done.

The garden room will be 3.7m x 6m and I will build it myself. I have read loads of threads on hear over the years regarding sheds, cabins garden offices but still have a couple of questions. It is being built to a budget so material costs are key.

The structure of the building is made from 4″x2″ timber, I will insulate the floor, walls & roof the current plan is to have a skin of OSB board possible a membrane and then the cladding. On the inside we will plasterboard and skim the joints.

001 by Ben Greenland[/url], on Flickr

1. We are looking to try and achieve some sort of Yorkshire board cladding. Originally I was looking at 6″ x 1″ gravel board. I like the look of the gaps between the front boards to be Black but I don’t think I can paint the gravel board as the wood is often still green very rough but maybe we can. Would the OSB board painted be weather proof enough with the cladding fixed on battens to create a small air gap and gaps through to see the painted OSB board.

004 by Ben Greenland[/url], on Flickr

005 by Ben Greenland[/url], on Flickr

If you were going to do this would you put a waterproof membrane between the stud walls and the OSB board.

This was my original plan for the cladding probable with some horizontal battens to create a better air gap.

003 by Ben Greenland[/url], on Flickr

2. Is regarding the roofing do I go with the old school cold roof, i.e. studwork frame insulation between studwork, OSB skin, battens to create an air gap and the corrugated metal attached to the battens. Or is the warm roof of the studwork with solid sheets of insulation attached above and then the corrugated metal screwed through the foam into the studworks a better system. On either roof is a waterproof membrane necessary.

3. The floor and roof will be insulated with 100mm kingspan type foam. Is it worth the extra cost of insulating the walls with kingspan type foam again 100mm or will rockwool type be easier and cheaper. It would seem much easier to run the cables in the walls through rockwool than kingspan type foam. I intend to run all the cables and install the sockets and switches and will get a spark in to make all connections and test and certify everything.

002 by Ben Greenland[/url], on Flickr

The CAD models aren’t complete yet, just trying to work out enough details to get some materials quoted more accurately at this stage.

Wall build up is framework, osb, wrapped in membrane, battens, cladding. Gaps between cladding boards is not ideal as the membrane degraded with uv and it makes a nice hone for wasps and beasties. Only you can decide if you want rock wool or rigid foam insulation in walls, look at comparative u values. A cold roof with corrugated metal sounds like a great way to create condensation if not done properly. You will need a continuous vapour control layer internally to keep moisture out of the walls and roof. If you are running cables in insulation check the rating with your sparky. Warm roof buildups are generally better but take up more room so will reduce internal head height to keep you under your planning eaves height. Make sure roof timbers are big enough for the span and snow loading. They look like they would be tiny from your model.

Ajc thanks for the comments. Will put a vapour barrier on the inside between the stud work and the plasterboard, forgot to mention that bit. I hadn’t sized the roof truss’s. The snow loading in Devon is very low but will probably use 170 X 50mm timbers. Cables will be sized correctly although I am wondering about doing the cabling in trunking on top of the plasterboard.

Thanks for the advice.

I designed a garden office very very similar to yours to great detail for an old colleague. I even did a 150 page step by step ‘ikea style’ construction instructions for him to self build so I might be able to help a little bit.

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1) with weatherproof paint your OSB would be weatherproof enough as long as you coat it well. A friend designed a similar garden annexe with just painted OSB on the rear elevation to save some money as nobody would ever really see it. You’ll get much more life out of it if you use a breather membrane (Tyvek type thing). You typically don’t need to worry about UV damage if your cladding has at least 50% coverage (found this out from Tyvek on a building we designed a few years ago for Southampton university). So if you have 150mm boards with 40-50mm gaps you’ll be ok.

2) I would recommend waterproof membrane on any roof, make it nice and continuous down onto the walls as well. I think you’ll be alright with insulation between studs, that way you’ll be able to make your insulation at wall/roof junction meet nicer. Here’s a drawing of the roof on the one I did. You can see the wall build up we used in that drawing as well

3) Either insulation will be efficient enough. I would say main downside with rockwool style insulation is it will slump eventually and leave gaps at the tops of the cavities. Would you not put OSB on the internal side of studs, then batens which gives a 25 or 50mm cavity to run cables/pipes/whatever, then your plasterboard?

Oh and we used 220x135mm (3 x 45x220mm beams screwer/glued together) for the main structure but we only had 5 main beams then 45x220mm purlins connecting them. This is an image of the floor plate but the roof looked pretty much the same. We needed bigger spacing on posts and beams as there were some large openings on the front elevation.

(we did involve a structural engineer who probably oversized this a bit as they usually do)

If you haven’t already, I suggest watching Ali Dymock and Oakwood Garden Rooms YouTube channels as they are full off useful info. Liam, who runs Oakwood GR, also runs a brilliant Facebook group that is great for getting advice like this. Search on FB for ‘self build garden rooms, offices and home bars’.

For your cladding I would definitely put a black UV proof breather membrane to protect the structure of the building. Tyvek do one specifically for Rainscreen cladding. Without this the structure will get wet and start to decay, seriously shortening the lifespan of the building. Also you will need battens behind the cladding and before the membrane so water can escape. It is uncommon to have vertical gaps in the cladding as rain will enter but it can be done.

Do a warm roof if possible with the height you have. Even if it means applying for planning permission to go above 2.5m. Cold roof and metal sheets is a recipe for condensation and damp. You don’t need a waterproof membrane as such as your outer roof layer is the waterproof bit (metal,EPDM, GRP etc). What you will need is a vapour barrier immediately on the warm side of the insulation to stop water vapour passing through the structure and condensing on a cold surface. Look at Steve Roofer on YouTube for info on roofing. Also, do not scrimp on the roof structure (joist sizes, vapour barrier etc) or you will have expensive issues later.

Make sure you have agreed with a sparky your planned approach in advance, as you will struggle to find one that will sign off work they have not seen and pre-agreed.

Rigid PIR boards are better insulators per mm than rockwool but can be a pain to fit. Many people do a mix in the walls with 50mm PIR and 50mm rockwool to get the best of both. The problem with just rockwool is it will sag over time and leave cold spots. You could maybe go with 50-70mm PIR for the walls which will be cheaper and easier to work with than 100mm and still give plenty of insulation for the walls. 80-100 is good for floor and roof. Note that if you are using 4×2 regularised timber that it is only 95x45mm so 100mm PIR will not fit.

Main tip is to do lots of research and planning and understand the details as mistakes will get expensive! Get planning, permitted development, building regs right too!!

Good luck and enjoy the build. I’m 90% through mine and it’s been very rewarding.

Ginkster & AnyExcuseToRide thanks very much for the help and advice will check out the links you have sent. I am within permitted development and as it’s more than 2m from the boundary I have 3m height to play with so a warm roof is possible.

Have toyed with this off and on, so looking at it for bookmarking purposes as much as anything.

However, when I looked at it I was looking at potentially using SIP’s for speed and ease of the two skins and insulation between.

https://www.sipbuilduk.co.uk/what-are-sips/

Are you sure you can have 3m eaves at 2 m from boundary? A few years ago when I did mine I think it was 3-2.4m on mono pitch and 4-2.4m on dual pitch roof within permitted development. This may have changed though.

I am within permitted development and as it’s more than 2m from the boundary I have 3m height to play with so a warm roof is possible.

That’s great. General consensus seems to be go for a warm roof if you can as it removes the risk of condensation. It’s also easier to build than a cold roof. Wish I could have gone 2m from boundary or had applied for planning as I had to stay within 2.5m. I opted for a cold roof and had a few condensation problems in the snow as the new building was drying out. Seems to be sorted now and all dry even when it snows.

Are you sure you can have 3m eaves at 2 m from boundary? A few years ago when I did mine I think it was 3-2.4m on mono pitch and 4-2.4m on dual pitch roof within permitted development. This may have changed though.

This is correct – although it is 2.5m & not 2.4m.
There is a maximum height of eaves of 2.5m, regardless of distance to boundary.
This is the Permitted Development Guidance :-
https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/830643/190910_Tech_Guide_for_publishing.pdf

I put membrane around mine.

Cedar finish

For longevity, studs, sheathing, tyvek w/taped seams, rainscreen plus insect screen, siding.
You could do board and batt style in reverse to create your 2 tone gap look. The siding is the primary weather barrier, not sure I’d compromise this.

Roof wise, a cold roof needs the deck venting and you absolutely need a water management layer..in which case you’re venting under the deck between the rafter bays above the insulation. Factor in how you are getting that air in and out too.
You can do an unvented cold roof but that would need a continuous membrane on both side of the roof deck. Spray foam below, peel and stick or torch on above.
Rubberised asphalt Peel and stick and you can secure the metal roof direct to the deck.
Vapor barrier inside (heated side it the insulation)
Rockwool is simple and friction fits way better than fiberglass ever did. Done right I don’t see sagging being an issue… But then I’ve not opened up any Roxul walls but have torn out plenty of sagged and ineffective fiberglass. I’ve also seen some shockingly installed new insulation that literally been trwown ad a wall in the hope it’ll stick. Rockwool is also super easy to account for wiring etc. Last build inside with some awkward corners I ended up doing the trouble spots before sheathing the studs. Gave much better access to the cavities in those places.
There’s plenty of newer building evolutions for framing and methods of removing thermal bridging of stud walls while still using batt type insulation. I like the less is more approach with stud work too.

As above best to board on board clad and not rely on membrane for primary waterproofing. All the battens and every staple for the membrane would get a soaking every time it rains. When open rains screen is done it is normally horizontal boards with a chamfer to make a drip on the bottom of each board and also to protect membrane from uv.

Bookmarking.

Some of you guys garden rooms are better specced than the average new build.

Lotta money being thrown around for minimal gain.

My garden Room …..sorry I mean shed

Roof wise, a cold roof needs the deck venting and you absolutely need a water management layer..in which case you’re venting under the deck between the rafter bays above the insulation. Factor in how you are getting that air in and out too.

Has this. With a metal roof and no condensation issues and it’s heated. Its actually really warm even without the heat .. . 100mm rock wool in the walls and roofs. Only seen it slump where either it’s got wet or folk have tried to run continuous wall lengths rather than having dwangs in their frames and compartnenting their insulation. Including removing our extension that had been up for 20 years. No sign of any slump. Your shed will be gone before you have issues with slump of rockwool provided you get the ventilation and waterproofing right.

Lastly it’s a shed. Don’t over think it and don’t over complicate it.

Some nice ‘sheds’ there 🙂

I do think cladding these things in OSB is incorrect. The OSB is glue impregnated chips, it is pretty impermeable to water. The membrane around the outside once you’ve clad in OSB is essentially doing nothing. As I understand it the point of the membrane is to allow any moisture to escape from your insulation, putting OSB around this prevents it. It’ll probably be ‘fine’ but it’s not the optimum.

I built mine in the following order from the inside of the room.
Plasterboard
Vapour barrier (aka polythene sheet)
2×4 treated frame work (was cheaper than untreated!)
100mm rockwool in frame void
Semi permable membrane
25mm air gap created by timber battens
16mm shiplap

https://www.youtube.com/channel/UChqWFbSX8STP_c8W0RVW1Xw/videos

Osb underneath external membrane is totally standard in timber frame and helps with racking. This may not be needed if there is enough structure but a good idea especially as quite a few of these garden rooms have a whole wall or even wall and corner of glazing.
In regards to comments about being over specified compared to building regs. That’s not exactly hard is it. 100mm insulation is hardly pushing the boat out, and a good idea if you want to use it year round.

Mine is well over specced in most areas but when you are doing it yourself it is easy, plus it was a hobby for me and a lot of the fun was in the building. if I was a builder I would have lost a lot of money…
I do think you can disappear up your own backside with “regs” based trivia though….

Our lockdown shed project, nearing completion, has wall structure from inside out:

11mm OSB interior sheathing, 450mm spaced CLS stud timber frame, foam board insulation inside studs, membrane, horizontal featheredge

The roof was equally simple:

600mm spaced trusses from CLS studding, foam board insulation between, 18mm OSB deck, felting.

Board insulation has enough gaps to vent. Would probably have gone EPDM if feeling more spendy. Super spendy option would have been another layer of OSB for exterior wall sheathing + battens for the featheredge.

But, ours is a shed not a garden room, we didn’t overkill the structure, it just needs to be dry, not warm/habitable with fancy decoration.

Board insulation has enough gaps to vent.

So the board’s doing very little ?

Osb underneath external membrane is totally standard in timber frame and helps with racking

I know lots of people do it on YouTube, but what is the membrane doing if you’ve put a layer of impermeable material directly behind it? The external cladding should prevent any racking issues.

I know lots of people do it on YouTube, building sites/detail drawings for building warrent

That said….I don’t disagree with your point.

It is a weatherproof membrane to keep water out of the fabric of the building and to help stop wind blowing through the insulation. Not to be confused with a vapour barrier that is on the warm side of the insulation. If you don’t use sheathing externally and just rely on a membrane all you are doing is making a lovely house for all the mice living in the area. They will make a nice warm nest in the insulation in your wall.

The wind is never going to get to the insultation if there is a piece of 18mm OSB there. Also, insultation works by trapping air – wind is not going to penetrate (either closed cell foam or through rock wool). You do want a flow of air on the back of your insulation tho to extract out the moisture. The rodent issue is real, but should be dealt with by stopping them getting into it (e.g. with mesh in the cavity), not just trapping in moisture.

In the diagram above, there is an air gap between the cladding and the inner fabric – a lot of what is discussed above, is no air gap, just a sandwich of cladding, membrane, OSB. In that combo, the membrane is doing nothing, as the OSB is also effectively a membrane. The air gap is an important part of a timber building.

If you’ve got OSB -> insultation -> vapour barrier, then the moisture that is in the air in the insultation when you’ve built it is going to be trapped there and not regularly exchanged. That will result in condensation at times, which isn’t good.

All that being said, it won’t matter for a garden building in reality. The studs are probably treated 2×4, the OSB is pretty rot proof, certainly within the life span of the building.

So the board’s doing very little ?

Probably. Maybe I should just shove it up against the OSB.

Well yes an air gap is fundimental to most things.

That’s why your external.cladding goes on battens. Your just trapping moisture against the back of your cladding by not doing it.

But as I said earlier don’t over think it it’s a shed.

Just ensure adaquate waterproofness / ventilation and insulation. In that order.

Re shoving the board up agains osb. Still won’t do much if you have air getting round it.

Re shoving the board up agains osb. Still won’t do much if you have air getting round it.

I assume he means gap above the foam not gaps between the foam edge and studs.
A 1-2″ Gap between the foam and sheathing would be correct assuming there’s a way for that air to be exchanged from sofit to ridge.

In a new build residential space, air tightness is a critical part of the building envelope. OSB is in 4×8 (or 10) sheets here so there’ll be a gap unless your building is only 4ft tall…as you’ll be wanting to run these perp to the studs. So, the tyvec is an important part of the building envelope to prevent moisture being wind driven into the structure.

https://support.egger.com/hc/en-us/articles/360001072478-OSB-Moisture-Exposure-

https://www.apawood.org/buildertips/pages/M300.html#:~:text=APA%20recommends%20a%201%2F8,expansion%20and%20buckling%20can%20occur.

https://register.certmark.org/certificates/Kronoply/SK_OSB_manual_AUS_2017_04.pdf

I agree with your conclusions and will eagerly look forward to your future updates. Just saying thanks will not just be enough, for the fantastic clarity in your writing.

I’d happy to share my thought to choosing a right Roofing Contractor in Edinburgh where i live, They are very proffesional work done!

Ronald G Graham Roofing and Building Ltd
20 Gordon St, Edinburgh EH6 8NA
+44 131 555 5121

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