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The Vapour Control Layer That Stops Mold & Rot | Garden Room Build #10

Makers Manual

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Full Playlist → How to Build a Garden Room - Complete Buil...
🔹Build Plans - Cold Roof 2.45m https://makers-manual.co.uk/products/...
🔸Build Plans - Warm Roof 2.8m https://makers-manual.co.uk/products/...

In Part 10 of this garden room build series, we install the vapour control layer and fix a major flaw found in many warm roof designs.

If you're building a garden room, garden office, or insulated timber frame building in the UK, this detail is critical. I show how to correctly fit a vapour control layer for a continuous airtight seal — including butyl tape, sealing penetrations, and properly connecting the wall and roof membranes to prevent hidden condensation and long-term rot.

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🛒 FEATURED TOOLS (affiliate links)
As an Amazon Associate I earn from qualifying purchases.

→ Timber Merchant (5% Discount Code MAKERSMANUAL5) https://tidd.ly/3J9ckNv
→ Insulation & Membranes https://geni.us/Insulation4Less
→ Butyl Tape https://geni.us/ButylTape
→ Staples Stainless Steel https://geni.us/StaplesSS
→ Aluminium Foil Tape https://geni.us/AlumFoilTape

CHAPTERS
-------------------------------------------------
0:00 Intro
1:06 Vapour Control Layer
5:55 Part 5 Follow up
12:20 Calculations Confirming Theory
17:30 Alternative Solutions

#GardenRoom #VapourControlLayer #WarmRoof #ColdRoof #GardenOffice #BuildingScience #DIYBuilding #TimberFrame

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20 Comments

@MakersManual Creator 6 months ago Pinned

Full step-by-step build plans for this 4.2m x 2.4m Garden Room:
🔸Warm Roof (2.8m overall height): https://makers-manual.co.uk/products/garden-room-warm-digital-build-plans
🔹 Cold Roof (2.45m – Permitted Development friendly): https://makers-manual.co.uk/products/garden-room-cold-digital-build-plans
These are the exact plans developed and refined while building this garden room office — not generic drawings. Clear structure, logical build order, full cut lists and detailed guidance if you’d like something solid to follow alongside the video series.

@t1812o 1 month ago

This needs more views! Thank you for all your hard work.

@jayt2334 1 month ago

Thank you for your really clear explanation. Just watching all your videos now - they are very helpful.

@rodgerq 6 months ago

The tanking liquid is such a simple solution, can't believe I hadn't thought about that before!

@patrickloftus 6 months ago

Excellent video, well presented, clear diagrams! I love these more detailed videos on garden rooms.

@Trydiyb4ubuy 6 months ago

Great vid. I saw the accredited detail you mentioned when I did my room and reviewed the two roof scenarios. What I felt (when I looked at my sketch with 2 membrane layers like yours) i would end up with a cold roof……but with two layers. So for safety sake I built a cold roof. The architect alternative you mentioned had a continuous wraparound membrane, but not a continuous insulation layer, I looked at that as a sketch but to insulate the perimeter was not possible and condensation would have settled somewhere in the fabric without it for sure. That said I didn’t fit a single ply membrane (seen too many failures) I covered the OSB with a high quality breather fabric, crossed battened for ventilation and used a metal sheet. The roof is bone dry after 5 years. Well impressed with your video great work.

@yulaw5110 10 days ago

I have an extension on the house, Flat roof, I have a 50mm airgap then 100mm insulation between the joists I then foamed and foil taped, I then put up a waterproof VCL and then another 60mm pir with plasterboard , Someone has put the fear of me that the VCL i used was wrong that vcl you put up sure looked waterproof to me

@beckyheath-j3t 2 months ago

Hi, these videos are absolutely brilliant, thank you. I am thinking of using wood on the outside to clad it, would the same approach for vapour control work? Thank you so much

@terry3420 7 days ago

What about simply reversing it? i.e imagine the cold roof upside down. Negates the need for massive joist space below.

@tobysherring1369 6 months ago

I understood the foil facing on PIR to be a vapour barrier, requiring tape to join the over battens and noggins. Perhaps your continuous barrier is better - certainly quicker than taping every stud and noggin.

@TheTartanPanjammer 2 weeks ago

Well explained & has answered a lot of my queries. My spark has put the cables through the middle of my studs leaving me only room for 50mm PIR in turn leaving a 40mm gap at the back, is this sufficient or too much of an air gap? Is it worth doing what you've done & drill a hole further back & moving the cable back towards the OSB or will this weaken the stud?

@benhowellthomas 6 months ago

I want Rockwool to stop the rain noise too!
Managed to get an ai to find and do the calculation... aaand I probably can't have it, bother 😅

@stevefromuk 1 month ago (edited)

Hi thanks for the video It's really helpful.
I've just had a outhouse installed and the outside is composite cladding with a breathable membrane then plywood with the stud frame behind that.
So on the inside the studs are 3x2 all around,
Am I right in thinking I completely fill the void with 75mm PIR insulation board & aluminium joint tape then a full vapour barrier on the walls and roof (just a apex shed type roof).
It's just I've read to only fit 50mm PIR boards and leave a gap behind otherwise the 75mm could condensate when in direct contact with the plywood.
Thanks

@bradley-dot 2 months ago

Likewise with your VCL concern, mine is on the breathable membrane. People do the walls but stop at the sides of the joists / overhangs, but I don't know what the difference is. I just wonder if moisture can be brought through the joists between each VCL. Do you think this could be an issue ?

@mikeymcmullen1 4 months ago

Have to say i am very impressed mate. I have spent literally every free minute the last 2 weeks tryjng to find and get my head around so much fuss by apparent professionals on the tightness of the vcl on the roof and walls when there is this massive (in my case a 9 inch joist) void above the ceiling and 2 vcl's not connected. I hope to build in the next few weeks. So i was thinking of how i could connect the 2 the easiest way. I have an overhang of 300mm on sides and 500mm in front. Before applied the roof joists i would cut a 1.5m strip of vcl and drape over the top plate all around. Contruct the roof and bring this vcl around and over the top of the osb/vcl on the roof and tape. This would then be taped on the vcl after pir applied to walls on the inside. What you reckon?

@dominicbrown6415 6 months ago

Really appreciate your approach in these videos. Thank you.
How did you seal the vapour control layer on the lower edge? I assume just seal to the timber subfloor? Did you consider sealing to the DPM which you previously cut away in part 4?

@jonah84 5 months ago

Is there anyway of contacting you directly as would love your advice on a project coming up. Loving your content 👍

@WandererTom 6 months ago

Great video thanks! Please can you explain a bit more why an intelligent/smart VCL is not suitable for a garden room? From what I read these were well suited to a timber framed building so I'm just curious as I'm hoping to build in the summer and want to get it right.