Coyne

Thank you for this link - I spoke to the specialist who made the video and he's helped with my hybrid roof build up.

I am building a bungalow with a skillion, low-pitched roof (just 11.5 degrees, so borderline a flat roof by definition). It will be a hybrid roof with insulation between and above the rafters (more above than between), with decking between the 2 layers of insulation. When looking at hybrid roof advice online, it seems the recommendation is for the VCL to go above the decking, as it would in a warm roof. So the VCL is essentially sandwiched between the 2 layers of insulation. Is that correct? I can't understand why with a hybrid roof, the VCL would not be below the rafters to prevent vapour entering all of the roof build-up? Does anyone see any problems with the following build-up: Vapour-control backed plasterboard directly below rafters > rockwool insulation between rafters > structural ply decking above rafters > then 70mm PIR boards above decking > then OSB sheathing on top of insulation boards > and finally EPDM roof covering I am looking at using rockwool insulation between the rafters, and PIR above the decking, to help with interstitial condensation risk where the VCL fails.

Can I ask a follow-on question to this (very informative) thread: When looking at hybrid roof build ups online - where there is insulation both between the rafters and above the roof decking - it seems the recommendation is for the VCL to go above the decking, as it would in a warm roof. So the VCL is essentially sandwiched between the 2 layers of insulation, with more above than below. Is that correct? I can't understand why with a hybrid roof, the VCL would not be below the rafters at ceiling level? I am looking at using rockwool insulation between the rafters, as it is breathable, and PIR above the decking. Thanks

That's great news you've had so much experience with steel! Thanks so much for your advice on this. I've attached a photo - the steels at the front of the building (that will receive the serious coastal wind) have widened ground pads in the concrete slab of 750 squared, so the distance from the steel to the slab edge is much wider here (up to 350mm) shown in this photo. The steels at the back of the building are almost flush with the edge of the slab. Ideally I'd like to fill the inside of the void with spray foam to insulate the steel - but I'm not sure how to weatherproof the outside. My finished external wall will be another 80mm wider than this photo once external wall insulation and cladding are fixed. The thermal break base plates I saw online (but didn't use) are these: https://farrat.com/structural-thermal-break-connections I'm relieved to here they aren't common practice! Thanks again for your help

Hi, Thanks for fast reply. Yes, I have built as far as OSB sheathing on the frame. The wall build up from here will be external wall insulation boards, membrane, batons and cladding. (I will take photos and add later today) So the steel frame sits inside the insulation layer. It is packed with insulation (as is the timber frame) and there is another layer wrapping the entire frame and OSB externally. I did miss adding a load-baring thermal break between the steels and the foundation slab (the steel company I used hadn't heard of them and it's only since the frame went up that I found them online - so I'm aware of the cold bridge I've created with the foundation slab), but otherwise the steels should be fully enclosed and I'd like to prevent there being a thermal bridge on the upper face of the base plate. I assumed spray foam was the the better option for filling the void for this reason?

Hi, I am halfway through construction of a small coastal chalet - I am building it myself and it’s the first time I’ve done this so I’m hoping this forum can help with gaps in my knowledge as I move from the “design on a page” to the reality of building it… The construction is timber frame with a steel skeleton frame. The steel columns in the corners are fixed to the concrete slab foundation; the timber frame inbetween the steels is built on 2x concrete blocks to keep DPC >150mm to ground level. The problem I have is where the steel base plate's wider than the steel column, and the concrete blocks are outside the base plate - leaving a void at the bottom of the I-beam under the timber sole plate in all of the corners - which is currently letting in the rain, and I fear in future will be a route in for rodents. How should I fill this void? Should the timber frame OSB sheathing and external wall insulation extend down to the slab and I fill the void from the inside with insulating foam? That solution doesn’t seem rain proof… Or should I create a concrete post base around the steels, up to the height of the concrete blocks? This seems a much better way to weather proof the corners and prevent driving rain coming in but seems tricky in reality to do without getting in the way of the external wall insulation reaching down to the slab. And a second question: On some of my walls, the OSB sheathing doesn’t sit up tight to the steel column - there is, on some walls, a gap of ~10mm because the timber frame stud work is wider than the steel I-beam. Outside the OSB will be external wall insulation boards, breathable membrane, cladding batons then timber cladding. Do I need to fill in these gaps on the inside of the OSB sheathing with insulating foam? The external wall insulation is to keep the steel within the warm side of the building - so I fear that quite large air gaps next to the steel will prevent the EWI from doing its job? Thanks so much for any help!