Gordo

The US do it the opposite way around because it is warm out side and colder and air conditioned inside. So tenancy for moisture drive is reversed and VCL should on outside (VCL should always go on warm side of insulation). the problem we experienced was in cold snaps the cold humid air came in through panel joints, condensed in the joint and then dripped. This was happening in multiple locations and was a surprisingly expensive repair. The building was approximately 20 years old so don’t understand why it happened after this period

I can’t say I notice any significant rain or birds noises on composite roof cladding. Be very careful to make sure vertical over lap joints are well sealed and screwed to avoid condensation at joints. We had a problem with this in a sports hall and thought it was an intermittent leak which couldn’t be found. Turned out to be condensation but the amount of moisture was incredible so I was sceptical at first. Not so bad in an open building but not so good in a dwelling

Good for you. As long as you satisfy yourself

Cavity barriers should be fitted to top of cavity (quite important this as any fire would otherwise enter roof voids and may spread smoke and fire rapidly). No need for CB at floor level or bottom of cavity in a dwelling house. I’m not too sure intumescent CB are suitable for timber frame construction. I know this is popular in England but i believe they are not accepted in N. Ireland.

Intumescent cavity barriers should only be used horizontally. Vertical cavity barriers are usually only required at 10m centres for timber frame in (N. Ireland anyway). Ventilation is generally only recommended at bottom of cavity now.

The fixings are just to hold PVC roof membrane and insulation down against wind uplift. Therefore I’m pretty sure it is adequate to screw it just to plywood deck. It shouldn’t really be necessary to screw to solid grounds. Correct me if I am wrong. I read somewhere that the the fixings through VCL are considered self sealing. Sounded plausible enough.

I’d run a mile from a liquid waterproofed lol. I haven’t seen anyone actually use mineral wool in a warm deck but have seen it specified. Which type are they using? maybe of the slabs like drytherm. Just interested as I would have expected it to be way too compressible for even infrequent light traffic.

Cement board fully bonded to insulation would be my initial thoughts. I haven’t researched it much but seems to be a thing. To date I have always gone with pvc roofing mechanical fixed to insulation. But acknowledge risk of insulation being compressed if subject to ANY traffic even window cleaning. So if I am ever in that position I will then fully consider alternatives to plywood. Id welcome anyone’s thoughts

Condensation from interior sweats on underside of a cold impermeable layer. Some membranes are slightly breathable to an extent. Do as you wish. I have seen this practice also, doesn’t meet its right. I have also seen the cold deck plywood rot out. I wouldn’t do it

I see a big flaw in you calculations lol. No one walks flat footed, they walk heal to toe and the pressure would be quite extreme at point of impact from the heal. Warm deck flat roofs have often be the subject of concern where infrequent pedestrian traffic happen ie window cleaning. The insulation can often be seen to have compressed where trafficked but the 6mm ply will adequately spread the load so as not to deform insulation IMO.

I would be worried that the cold plywood deck between the weatherproof coating and the insulation will suffer from condensation and rot out.

I would suggest casting a 150mm reinforced concrete ring beam on top of rubble walls and then maybe light weight blockwork.

Why not just fit a Flexi liner with Spyder spacer things and vermiculite backfill to void. Would be a lot cheaper and should last a long time if you use the best quality/grade stainless. Or if you want to use twin wall then use a larger diameter pot to suit. How on earth are you going to provide brackets and support bracket to support the twin wall?

Yeah I agree rain ingress is the most likely source of problem. The criteria for severe exposure zones I don't know but just refer to maps of zones. Looks like you may be in a moderate to severe exposure zone at a guess. I'm no expert on renders but have shyed away from thin coat renders but see it is getting more popular for some reason (I guess cost savings of not having to paint). I am glad I don't have to wrestle with your problem and the best solution. I Would suggest a few trials of surface coatings to see if they solve it. But think breathability of surface coating is the problem. Possiblity a product problem if applied direct from the tub? Or it is applied incorrectly maybe. I guess there is a recommend coverage rate that is key

A large chunk of the west of Scotland is classified as very severe exposure rating!

I see the BBA certificate state is is only suitable for sheltered to moderate exposure without special precautions (basically manufacturers installer and advice ie do not do it). I guess this apples to you in Scotland. Very severe exposure is not covered! Alarm bells wringing. I also note there is little shelter from roof overhangs at eaves and verges to weather. The BBA certificate appears to confirm that surface condensation should not be a concern if well insulated. im pretty sure the problem lays with the breathability or lack thereof in the failure. but need to work out where moisture is coming from. Condensation risk analysis are often done using the most basic model. There are more advanced full models available that will show the moisture profile through the element and show the potential dew point possibly in the external render. Just a thought if someone can do this for you, maybe a member of this forum, it would help rule it out. I think this thin render system may be inappropriate for your building in this location and it may be best to consider alternative options if you have to foot the bill

Wow that is awful ? I would put money on the problem being moisture trapped behind a waterproof thin coat render that isn’t breathing. Question is where is the moisture coming from and how to stop it. Could it be surface condensation in super cold periods maybe? A condensation risk analysis at 0 degrees and below outside would show if the risk is happening within the element or surface render. Check relative humidity indoors is it below 60%? I think this is the RH% assumed in condensation analysis and a higher figure would exaggerate problem. sorry just a thought on another avenue you wish to consider.

Bump up thickness of PUR should sort this if just marginal a risk.

The sf40 R value of 3.47 equates to equivalent to 75mm PUR insulation or 115mm mineral wool the YBS equivalent to 55mm PUR insulation. the sealed air gap, reflective surface and additional surface resistances will also contribute a little more to overall U value you don’t quote the cost per m2 so don’t know if it’s good value these foil insulation’s were the subject of protracted legal cases over false advertising. They were unable to prove that they performed as advertised (using the recognised BS hot box test) therefore they should quote R values for comparisons. But this is often hidden in small print. And although they may show BBA logo on advertising you will find it difficult to get hands on actual BBA certificate for their independent information. although I’d usually advise against a hybrid roof make up I doubt the lowish performance of the foil will be a concern with interstitial condensation but best do a condensation risk analysis

Most I see are around the 4 mark. But I lost all faith in air testing when I observed houses being tested where imperfections were sealed up with sticky tape and even internal room that must have been draughty where the door was sealed up with tape. It just managed to get over 4. The air testers are obviously keeping the contractors right for repeat business. the accreditation system is worthless. The certifying body would have to be spot testing some % of the houses after test to restore my faith

That sound a tad harsh. Did he not have his fan on the front door anyway. Guess he wanted a second payment? A second test would be just for kudos in EPC or if needed for eco certification.

I don’t see this as a wise change mid build. Flat roof is probably designed for a dead load of about 0.5 Kn/m2 dead load and you are thinking of adding an extra say 0.6 - 2 Kn /m2 ( probably closer to 1 Kn/m2 in my estimation for thin 75mm deep soil for sedums). Sorry but there is no way it was over engineered that much. You gonna have to do a lot of reinforcing works. Possible but costly so depends how big your pockets are for a whim here’s a guide on subject. I have a green roof on my log shed and I do love it but was done on cheap (because of what it is) but needs designed for at start.

Exactly. Don’t treat the symptoms of your problem before you treat the cause. aa said above the underlay is not a cladding it is a backup to water ingress and to stop a storm ripping all your slates off, should just a few come off and allow wind penetrates. I think this is the reason Scotland favours timber sarking. ”roofshield” is a good product which doesn’t leak. It’s the way it’s fitted that needs looked at. The damp insulation is undesirable but will eventually dry out, as the wall element should be breathable. But 20 years down the line, if the slates fail and allow some water to penetrate maybe intermittently, in a storm or gusty period, the underlay will not save you from water ingress to the fabric / interior. Then it will be a problem you wish you had sorted. Get it sorted now before allowing slating to commence

Definitely. But what I was getting at is the underlay shouldn’t be leaking, just before slating. That would concern me unless a spot repair was needed.