Stu84

Useful info thank you all. Sorry it's taken a while to post back. I've attached a section showing the roof build up: Renown concrete tiles battens underlay (HR or LR?) draped over counter battens but with a clear gap above the PIR for airflow. (Underlay not really visible on drawing attached) 40mm counter battens 130mm PIR 125mm C24 battens Regular plasterboard on underside of rafters (not shown) Guess any VCL would have to sit either immediately below the PIR or perhaps above the plasterboard... That said many reckon foil-backed and taped PIR basically is a VCL. Full plans are being drawn as we speak so excuse the total lack of detail shown at the eaves. Indeed, I'm not even sure if I can possibly set up a good air flow with the way its currently drawn, what with the air needing to get into the low>high channels formed by the counter battens, this would be difficult given the vertical distance from the air entry point (tiny 10mm gap between rear of fascia and wall) up to the counter battens. I'm starting to think a VCL with no low>high ventilation may be the easiest way to go. I'm just saying that the roof structures of older houses (80s for example) I've been in where there is air flow seem OK. And they haven't had VCLs installed nor LR underlays so air ventilation seems effective enough in many instances to solve rising vapor. But I appreciate that we are now trying to retain more heat etc. Worrying but no doubt correct thoughts regarding foil-faced PIR. This is why I'll probably end up going with a high-valued VCL as worried the PIR won't allow movement of any vapor through it should some get in. Mind you... I did see a PIR product by Unilin that claimed to be vapor-open. Perhaps that would be lower risk? Thanks all Stuart RN22TOP1 - Top Edge Abutment (Ventilated).pdf

Pitch is 18 degrees. So don't think that counts as a flat roof? Not against eave and ridge ventilation I just don't have the horizontal space (yes really) to have 25,000mm/m^2 grills and therefore can't meet building regs but can fit 10,000mm/m^2 which may be better than nothing. Think main question remaining is will PIR allow any vapor/moisture to move through? If not then little point in ventilation as moisture won't even get past in the first place. If having no ventilation and a high-rated VCL I'd go with an LR underlay to allow small amounts of vapor that may get through to escape.

Again everyone thank you for replies! Just a friendly reminder that this concerns warm pitched roofs (sorry should have stated in subject title). Thanks. Stuart

Massive thanks for the replies. > Is some vapor movement into a roof structure really an issue if there is some ventilation? I would go further and say perhaps it is beneficial for a certain amount of vapor to escape into the roof provided there is some low>high air flow through the roof? > If you have a completely airtight structure how does fresh air enter the building when windows etc are closed? dMEV is a great idea, but it seems like a fix for a problem that shouldn't exist in the first place if house designed well. I'm engaging with a designer. He initially said a VCL is a must and that I shouldn't have ventilation with a warm roof. I think the problem is that warm roofs are a modern concept and have primarily been spec'd for flat roofs which don't normally have ventilation. Much of the 'warm roof' documentation out there relates to flat roofs and people are applying that method to all designs. Call me old school and I could be wrong, but in my scenario (an 18deg mono-pitch extension) I see only positives from having some low>high roof ventilation. I can't see why that's a bad thing ever. It might not technically need ventilation if you used a VCL with a high Sd value. But why chance it all on a membrane? And it seems as though (at least as far as the average homeowner is concerned) trapped vapor / stale air that could result from VCL is a bit of an after thought and maybe a problem that could arise in 15 years time when these things have been truly tested in various scenarios. NHBC chapter 7.2 deals with ventilation for pitched roofs. Table 14 (see attached) suggests that where a VCL cannot be used then ventilation is acceptable. Admittedly it seems worded in favor of a VCL but doesn't seem absolutely necessary if have ventilation. I'm wondering if the ultimate solution is to have a VCL with a low Sd value (i.e. it's not a barrier but allows some vapor to escape into the roof) and combine this with some low > high roof ventilation. In my scenario space is very restricted, but I can have 10,000mm^2/m at the eaves and 5,000mm^2/m where the roof abuts. I'm hoping that this combination will allow the escape of some condensation/vapor through the VCL, but not enough to cause damage as the air flow will help dry. I see little wrong with the above until..... enter PIR! I have to use PIR due to required 0.15 U-value and restricted vertical height. But (please correct me if wrong!) I don't think PIR will allow for vapor transfer very easily, so there may be a risk that using a low-valued VCL causes condensation to rise and hang around/within the PIR layer damaging it.... Ah tis a challenge! Thanks for knowledge please keep sharing. Stuart

Morning all Mono-pitch extension with knock through to make open-plan kitchen/diner. Working on design for warm roof. It's a proper warm roof with insulation (130mm) PIR entirely above the rafters. I understand that if I incorporate a VCL below the insulation then I don't need ventilation at eaves/ridge. The thing is I'm not really sold on VCLs. If I install a VCL in a knock-through kitchen isn't there a chance that vapor/moisture gets into the existing dining room and potentially cause trouble there? Where does all the moisture go? The roof will be vaulted. My plan was to install some plasterboard beneath the rafters that would act as a partial moisture control. Then have ventilation at the soffit and where the new roof abuts the wall. And hope that this air flow will draw moisture up through the insulation and on its merry way to the outside world. Problem is most PIR blocks passage of vapor so perhaps that's where the plan fails? I suppose I could not tape the PIR joins and this would provide a pathway through. Thanks for your thoughts. Stuart

I'll take that. Thank you. Yea I didn't really know what to call them. They are facing bricks, made of clay, with holes but perhaps not quite as much air as the EU bricks!

Pic of bricks

Just raked out part of a bed joint. Seems I have the bricks with ten holes (2 rows of 5). Going to head to merchant see if I can find the same bricks and then will order some resin and experiment. Any brick ID pros? It's a rustic finish, 10 holes, and there are multiple thin scribe lines on the back side (cavity side) of the brick, perhaps to speed up breaking them to length? Or maybe to encourage rain that has got through to dribble down the outer leaf? Just guessing. House built '92. Thanks

Fair comment. I should imagine it ain't going anywhere.

Thank you for info. Yes, it seems common practice in EU. Tricky thing is their brick size/patterns don't match ours. The Fischer strength tests are with these European bricks, so it's kind of guess work to assume the same strengths can be achieved with UK perforated bricks. Just had a quick chat with friendly local building control. He was surprised that my house had bricks with holes in them, said vast majority of houses just had the frogs. He was indifferent about chemical anchors. His main suggestion was to get in contact with the starter kit manufacturers and get their advise regarding perforated bricks. In relation to fixing a ledger board, think I'd better consult a SE. Is there a favourite starter kit manufacturer out there?

Ideal. Think best thing I can do is stick one in the wall and give it a pull! Or trying to find a matching brick and experiment. Thank you.

That's what I was hoping to hear markc. I'm probably overthinking it but the Fischer loadings (they have lots of useful documents) assume a certain compressive strength of brick. I've read on a different post that if you can't positively identify your brick type then you have to assume a very low compressive strength... lower than what Fischer used for their experiments. Any idea whether all bricks of a certain age (i.e. post 1970s for example) can just be assumed to have have a certain minimum compressive strength? Thanks for replies

That's a thought. Would building control be happy? Do the instructions that come with the kits say bed joints OK? Thank you

Hi everyone. First post here. Great to be part of this friendly forum, lots of amazing info. I'm designing my own small side extension. A lean-to 4x3m. Looking to use wall starter kits and a ledger board to hold the rafters. Only thing is my bricks are vertically perforated, not solid right through. It's a 90s house so they aren't soft, but I don't know the brick type nor their compressive strength (more compressive strength = better resistance to anchor breaking through the side of the brick under load) The chemical anchors by Fischer seem on paper like a good option. This involves inserting a plastic sleeve with holes. You fill with resin which expands through the holes of the sleeve and push threaded bar in. I guess the resin bonds to bits of brick/mortar and also increases pull-out surface area. Anybody have any experience of this type of anchor in perforated facing bricks? Other ideas welcome. I'm trying to avoid tooth brickwork. I suppose I could replace a number of bricks with solid ones, but it would be quite a number all said and told. I'm hoping the wall starters don't take much load. Aren't they mainly to resist wind loading? It's the ledger board and roof weight I'm more concerned about. Thanks all Stuart