Warm Roof vs Cold Roof. What’s The Difference? 

Thanks for this. Definitely the clearest and most easy to understand explanation of a cold roof and warm roof on RU-vid.

Very clear and helpful, nice video Rodger! We're always doing these sort of extensions and when the joiner talks to me about hot and cold roofs I'm always bamboozled 😂😂 look forward to see the next video on this project bud!🍻👍

Great as always Rodger. I have a 15-year-old log cabin. It's perfect inside but a bit dated on the outside. I spend around £100 per year maintaining it as well as the cost of heating. I am about to insulate it on the outside all around and have the products but not 100% sure of which way to have the insulation, membrane etc. I don't want to insulate the inside as I like my LOG cabin. I have 60mm Kingspan sheets (got them cheap so can't change) a breathable membrane, PVC cladding for the walls and box profile sheets for the roof. I was intending to stitch the membrane onto the outside walls then Kingspan held with 2"x1" battens and the pvc cladding onto this. There would be the 1" air gap behind the cladding vented at the top and bottom with mesh to stop wasps etc. The roof is 20mm t&g pine and is felted. The felt needs replacing so was intending on removing this stitch the breathable membrane on top, Kingspan, 15mm OSB then the box profile sheets directly on top. Do I need to change the membrane for a vapour barrier? If so where does it go? Do I need an air gap roof or walls? Any advice would be appreciated as I'm getting conflicting messages on other forums, everybody seems to be an expert

Interesting stuff Rog , what a massive pain in the area for 5 “ though ! 😳. Look forward to the build 🧱👍🏽

Great vid Roger, nice and simply explained.

Great video explaining in very clear way the difference between cold and warm roofs

Thanks Roger, excellent explanation to a bit of a head scratcher. Am going to replace my old plastic roof on my conservatory and had been trying to weigh up the best way of tackling it but you just explained it in less than 10 minutes!

San Francisco, California, homeowners should take note of Roger's sage advice!

Such a unique breadth and depth of knowledge from Roger, we are lucky that he is willing to share it with us. Thank you Roger! 🙏

You are the best! Thank you for the explaining.

Going to be doing a garage to office conversation soon, needed this lessen. Thanks Roger

Thank you very much for sharing, Master!

Brilliant explanation Roger

Excellent info, thanks 👍🏻

Excellent as always!

crazy how the neighbours would be annoyed if the roof was 120mm higher... I would NEVER EVER EVER be that kind of neighbour. Why would they object to 120mm? What difference will 120mm make to them? Crazy! Anyway, thanks for the video! Very helpful

A very informative video. Thanks for sharing

Thank you for a very informative and well delivered explanation. Keep up the good work.

It's time to thank you. I've used your videos to build 2 walls, insulate my garage and tank a wet room. You explain things so well that I understand and can question things in my head. Insulated summer house / office is next which I'm hopping to start in the next month or two. I still bring in the pro's where needed, but understanding what is going on really helps and I think just raises the standards as a whole. Thank you!

Awesome explanation, Thank you. I am considering this for a project and wanted to understand it.

Thanks, excellent video! Simple answer - badly installed cold roofs are bloody freezing when the builder who owned your house doesn't air seal between the Kingspan slabs and the joists. It's basically external temperature just above the plasterboard.... 🥶 Also just found out he didn't stick a vapour barrier above the plasterboard either. 🤦‍♂️🤦‍♀️😤

I wish all tradesmen were like you Roger.

Really informative video, Roger.

Learn something new everytime, thank you

Excellent job great explanation 👍

Thanks...great video

So well explained!

Good explanation for people that don't understand the difference. 1 thing that would be better for ventilating the dead spots created by the lantern would be to counter batten the roof the opposite direction of the joist to create a cross flow ventilation throughout the whole roof. Although you increase the deck height by 25mm you get rid of the need for unsightly mushroom vents everywhere 👍

Perfect timing. Will be starting on a garage conversion soon and need a new roof. Exactly what I needed. Warm roof it is. Cheers

Worthwhile advice!

Great vid.. very informative

Good explanation cheers

This is very useful :) I'm converting my garage to a room and I'll speak to the builder what he plans to do with the moisture issue :D

My thanks to Skill Builder and Roger - now all i need to know about warm and cold flat roofs. Infact my new extension Surrey side of Gatwick a semi-detached and all flats are cold. my new extension will be warm and work to change my current flat roofs to warm but will leave attached neighbours as is. with some magic (in the drawings) for the builder !!

Great info thank you

Great teacher, thx.

Never really thought about the subject before Roger but in many big old houses they had cellar and attic spaces with air grills below the floor space for the air to circulate. I was lucky when we bought our 300 year old 3 bedroom cob cottage with a thatched roof (cold roof). The 3 foot thick cob walls are build off a rubblestone plinth with no foundations or guttering, the ground is hard clay and stone. There was bad damp when I moved in because the walls were cement rendered and it had been a holiday home for about 15 years which had caused all the damp issues and rotted the timbers on the damp side of the house because I have a hill behind the house whose gable end faces into the hill and in front of that there is a 25 foot deep stone lined well, the water line is about 4 feet below ground, never higher and is only low during summer. Since I put in a French drain along the back side of the house and lime rendered the place we've have no more damp issues. The first floor ceiling is wooden lath & plaster (the walls are reed and plaster), a thick layer of haired course lime mortar and a single layer of thick top coat white lime skim 5 to 6ml thick. Another property of lime walls is they absorb moisture and CO2 and give you better acoustics than cement or gypsum, a friend in nearby Stogursey has plastered his internal walls with clay which has even more amazing qualities. It can be expensive to do but, done well it should last a life time whereas modern builds are designed for about a 40 year life, even a strawbale, single floor structure would last a 100 years. Above this ceiling line is the triangular shaped attic where, between the joists you can see the back of the ceiling and the folded nibs of that 200 year old, haired lime putty mortar and on top of that, between the joists are a few rolls of Rockwool insulation with a 5 foot air space before you see the underside of the straw roof. It's relatively dry. In some of the bigger houses they'd put a thick layer of tamped soil and sand which would be good for the sound proofing and insulation properties. During the night, summer and winter, we sleep with our bedroom window open and during the day, unless somebody is home, everything is shut up. Up the road is an equally old property which has a double-Roman, clay tiled roof, when I climbed up to look there was no roofing felt and it too was dry. As you said on one of your rants, there's no point having an air-sourced heat pump, if the home isn't well insulated inside. I'm house rich and cash poor and can't afford one but I would like a ground-sourced heat pipe, they do work, I would also like some solar panels please. It would help the planet if we all had solar, wind and thermal generating capabilities with storage capacity for any excess power with every home as standard and, if you have a small garden, a compost heap is great for all your organic waste. Ours is barely large enough for the herring bone pipe to the septic tank which is emptied every year (£250) and is cheaper than the mains sewage bill I'm told. We've go no government grants, none available, or help with lower VAT for sustainable buildings, and it was bought to be our family home and so long as it continues to be repaired with like for like materials it should last another 300 years. keep up your interesting topic please, it's healthy when we can all rant.

Highly interesting....more demonstrations would be very much appreciated...thanks

More videos like this Roger 👌🏼

Brilliant, thank you.

I am about to do this, bloody gold.

Excellent stuff. I'll now change what I was going to do with the roof of my new dormer.

Very interesting subject. Most people wouldn't give it a second thought. But it's important you know in case of dry or wet rot. Thanks rodger👍👍

Well that’s another new thing I have learned from you, Thankyou.

Thanks so much!

Really love watching your videos - that are really informative particularly for a novice. I am in currently undertaking a single story rear extension and I was let down but he builder. The flat roof basically was completed with trimmers, joists etc and a GRP layer was pasted on. The roof wasn't insulated so, I got someone else to finish the job and created warm roof on top of the GRP. I asked the roofer to add in a vapour on top of the GRP and beneath the insulation. was that the right thing to do? Will this have a detrimental impact on moisture internally ? Thanks Joanne

hi great explanation bang to the point

Great explanation

great explanation.

learned a lot. That’s why I love this channel. Keep em comin Roger.

Thank you for sharing that knowledge. In NZ we build a lean to extension with a mono-pitch roof. Plaster board 200mm joists, vapour barrier then corrugated Iron. The Insulation is Pink Batts skillion roof insulation. The insulation had to be specific due to the insulation loft limitation giving allowance for at least a 20mm gap - I now further appreciate what that gap is for. More so it gets me to thinking about flashings and how well they can release water on the inside without letting water in. Hopefully the building codes we built to are correct.

Good explanation.

Thank you!!

A RU-vid channel called Steve roofer shows a good ventilation detail on the flashing, well worth consideration

I used the 25mm multilayer foil insulation for a warm roof, supposedly equivalent to 225mm of fibre insulation, but adds no more than 40mm with the counter battens. It all seemed to work well. I'm planning an orangery type extension on the house I'm buying.

Thanks so much, just about to build a warm roof for my conservatory. Amazing to see how literally none of these "kits" do not include the ventilation for the cold roof.

Great job Nick 👍🏋️‍♀️🤺🤗

I’m stuck with a cold roof on a extension on a grade 2 listed building. 50mm air gap, 100mm of insulation and then a vapour barrier before the finished ceiling is the system I’ve gone for. Then added extra ventilation on the facias, but the vapour shouldn’t get up into the void to start with.

Jeez, that's interesting I did a substantial repair job on my roof myself (Cold roof) I did my research and I knew the principle of the through flow of air and I didn't really do a great deal of work so I assumed the existing structure would work in the way it was designed. I had serious condensation problem. The previous owner - my father in law had put fibre glass in between the joists laying directly on top of the ceiling plaster board. My analysis of the problem was the moisture was getting through the boarding, and then hitting the cold air beneath the tiles the condensation accumulating under the tiles and dripping into the fibre glass and eventually making contact with the plaster board. I felt that the fibreglass that was filling almost all of the cavity and was reducing the air-flow and replaced that with GRP. That then left a good gap for the air flow. Not had any problems since (4-5 years now) so assumed I'd done a decent job. I'm going to check that out when the weathers good. PS it's a very shallow angled roof 25 degrees. Good vid.

Nicely done, I did a little office using warm deck. I stopped the joists at the wall plate, but extended the OSB and 4 inch insulation beyond to create a 200mm overhang. Then sandwiched the edge with 2x4 so I could hold it all together. I only did this to avoid creating an ugly thick fascia that was 300+ tall, instead there's a neat 150mm fascia with the gut attached.

I think i would have gone a cross the 9x2s with tile batten so the whole roof is connected and reduce the number of roof vents. Not only is all them mushroom vents going to cost a lot especially as they have to be built into the roof covering, they will look facking ugly. Good vid Roger 👍

Excellent stuff roger, as a building control surveyor the most common mistake is a warm roof with no decking and VCL below. Happens all the time! Every PIR supplier will ask for an 18mm deck with the VCL

Very good video Roger 👍 My questions 👇 So what is your recommendation for roof construction / insulation for - a warm & humid climate with cool air-conditioning inside - a warm, wet & hot climate with “humidity control with cool air-conditioning inside” - insulating concrete slab roof & concrete walls, in hot & wet climates with cool air-conditioning with water condensing on walls & ceiling What about MVHR, with humidity control, as an addition to air-conditioning Look forward to your reply. It may mean a whole new video, I suppose 👍

Hi Roger, thanks for such a detailed explanation, I finally understand! We are building a cabin in our garden (for us to live in whilst we do our house renovation). The builder has done a cold roof, but not put OSB boards on top. He has a membrane directly above the joists (where the insulation will be), the battens, then our metal roofing will go on top of that. Im a little confused as I thought you would always have boards on top of the roof, but I guess this way you solve the issue of ventilation/allowing vapour to escape? For peace of mind can you confirm you dont need OSB boards on top? Thanks so much!

Hi lads, You know what, I am level 6 NVQ (not very qualified) site manager, level 3 carpenter and joiner, and intermediate qualifications in Brick laying, plumbing and plastering, (also a fully qualified Locksmith), with 33 years of mud on boots experience, I've forgotten more than most people actually know, but...... I literally LOVE your videos and pod casts, (only found you the other day, so alot of catching up to do),very informative entertaining AND engaging, even if you are shandy drinking Southerners. Keep up the awesome work 👌

When I put up my extension , It was a cold roof because of the design I wanted. I used foil back pb and foil taped all the underside of the joists to the foilbacked Kingspan so there could be very little chance of any moist air penetrating. Redbrick vents to the front and proprietary vent escape vents to the rear. Hmmmm, forgot about the dead spaces though, gulp ! That said, ten years in and no problems as yet.

Great explanation! In a conservatory where the roof is plastic and the beams are aluminium, as there’s no wood, would mould still be an issue in a cold roof scenario if you’re using something like recticel for the insulation, is the ventilation still as important?

Nice video. I've got a small extension poorly insulated. I need to insulate it with celotex. I was told to cut to size and slot them into the spaces and there should be enough room for them to breath. I don't there is a vapour barrier, will that be OK? Cheers

I did my flat roof on my lounge 15 years ago. Plaster board, fibreglass, OSB and felt. No sweating, no damp, no problems. Still as good as the day I did it. I can't help but think that a lot of this is snake oil and more money for the men in suits. Great explanation Roger, as always.

Top advice !

Warm or cold roof, always avoid moist getting in the construction from the warm side of the construction, Make a 100% damp sealing above the plaster board.

verry informative

I think I would have preferred looking at a flat roof rather than a dozen mushrooms poking out of a roof.

Roger, what would the position be if you filled with 225mm insulation so no air gap and then put in well sealed membrane and maybe for good measure the backed plasterboard so in effect an internal warm roof. I know the screws would perforate the membrane but they would be compressed and into the joists.

Apparently in the states they are specifying tissue based pir board insulation when used with a separate vapour barrier. All to do only allowing one vapour barrier. It's constantly changing.

Is it normal on the roof to fit 18mm plywood then osb on top before the grp . Or is this overkill and trying to rectify a problem with the plywood , thanks

Hi, thanks for the video. Very informative. I'm part way through the purchase of an edwardian money pit which the wife loves - so hard to say no. The attic above the main living space has already been converted to a bedroom but the attic above the back bedroom and bathroom hasn't been done. It is about 20 meters long and has a pitched roof. I was hoping to fit celotex between the rafters (leaving a 50mm airgap) and then fitting a layer of insulated board to the rafters to improve the thermal insulation (as 50mm celotex won't be that great). This would allow me to board the space out and have a nice bit of storage there. Question is: how to ensure airflow through the air gap behind the celotex on the pitched roof? Thanks.

food for thought. i have two flat roof extensions. both had no air space, both rotted. hopefully the roofer done it right this time. i have edpm roof covers on now.

Really interesting explanation. Jaw on the floor over the 120mm issue. One more brick course on the parapet should sort that out.

brilliant info - would be nice to see a video showing what it looks like

Good video Roger. I do a bit of GRP roofing works but our boss gets us to put 100mm on top and 50mm to the under side. When I asked about vapour barrier he said with this system you don't need one.

Really enjoy watching your videos and on a very steep learning curve as starting from pretty much zero. I am building a warm roof but feel like I need two layers of vapour barriers. My garden room will house tropical plants. It is heated and insulated with 50 to 100mm PIR depending on location. I need to protect the inside of the roof (OSB) from rising moisture and there will be quite a bit of it as I water plants in the room. What's the right solution? I have the Dupont Tyvek membrane and was thinking from inside to outside: Membrane over joists, OSB, PIR, OSB, membrane then finish with felt Hopes that makes sense and look fwd to your opinion. Warmest thanks Jerome

Question. How do you deal with Link Detached garages. It looks like Im going to do a cold roof but what should I look out for and avoid when taking my neighbours flat roof into account. Thank you.

When I've done cold roofs I always put plasterboard with a 25mm backing under the rafters to deal with cold bridging. Do you not think it's really necessary?

Ever thought about fitting cross battens running the opposite way to the joists,this creates a continuous air flow of 20mm. No need for mushroom vent. As long as you have.front and rear air soffit/ facia and abutment vents.

Great video. Can I ask if kingspan over the top floor ceiling will create moisture or will this be warm air on warm side of insulation? (this is a pitched roof loft so plenty of space above but we were wanting to put green loft boards over the kingspan also

I built a garage extension where I didn’t have the height for a warm roof or the joist depth to achieve the required R-Value with venting, so I used Closed Cell Spray Foam. It is allowable under the Canadian Building Code for use in unvented roofs because it acts as a vapour barrier. Pricey though.

Nice one...

Thank you : )

Great video thank you. We never got planning permission as the size was just shy of 40 sq metres which is allowed in our country. There was a warm roof in the plans, now we have discovered the engineer told the builders we need a cold roof due to height restrictions and no one even told us about this change which i am not happy about at all. The roof is nearly finished now. We live on a height where it gets very windy so I am concerned about whistling through the roof now (we had whistling in the eaves of existing dormer roof until I got foam insulation to block it out) and also cold air through the roof which means increased heating costs for the rest of our lives. Can someone please advise if we can now do anything about this? Seriously concerned :-(

I’m currently doing some simpler the building inspector wants us to put 100mm king ip the under side of the osb and then 25mm on the under side of the joists. Then 12mm and ply then plaster board do you you think this is right?

In my workshop which is basically a large shed I stapled foil bubble insulation under the roof cladding. Will this cause problems? It's tounge and groove panels only

Hi, Love your video, never watched others that actually put confidence in me. Can I ask a question? I am insulating (cold roof) my workshop but only have 65mm/2.5inch joists. I can either get 30mm or 40mm PIR leaving either a 25mm or 35mm gap for the ventilation. I have soffit vents all the way across and a open channel the other end for ait flow.. Would you go for the 30 or 40mm PIR? is 25mm air gap enough? thanks!

The GRP mushrooms cost the guts of 70 euro a piece here in ireland. We tend to use them sparingly.

I'm surprised the neighbour is even aware of the 'options'. This is what was granted permission, these are the plans and dimensions, and this is what is getting built!

In your opinion, do you prefer a hot Roof? we are doing a barn conversion they have recommended. We have a Zinc Roof but we cannot decide on hot or cold roof am I right in thinking a hot Roof is better. Does this also work in keeping the heat out in the summer? also am I right in thinking we would not need to add additional insulation in the roof . Or would you recommend adding extra? Just found your channel, loving the information 👏🏻👏🏻

If you use a closed cell foam and something like gappo-tape to seal the edges, would you still need the vapour barrier?

Nice video Skill Builder, well explained! It can be a real headache always thinking about ventilation. Looking forward to seeing the series 👍🏻

👍👌 Good video. Thanks, 👍

Where do you put the electrics/extractor tube if the space between the joists is filled by insulation? Is it better to get a warm roof if you don’t have problems with the neighbours?