Johnny Jekyll

@ruggers thanks for the reply. Yes I've decided to ditch the PIR between the battens. I've since found out that the Soudal Soudatight LQ (airtight paint) that I have on my walls is also vapour retardant so I'm not going to add insulation behind that. I'm painting some more of that on, then I'm leaving the service void between my battens empty. I've started plaster boarding and just need to move onto the next part of the project. I'll monitor the humidity in the house and make sure the Zehnder MVHR does its job once moved in. Overall it's now more than four years, and still a long way to go. 😩

@SteamyTea Ok so may I ask: 1. Would the PIR boards foil sides act as a VCL? If so, presume I'd only need to foil tape over the battens to make it continuous? 2. Do I need to foam seal the gaps to the mid floor area? I have plenty of Soudal Window Door SWS Airtight Expanding Foam on site. 3. Do I need to carry the VCL through the mid floor area where the posi joists are (they are installed using joist hangers to a 9by2 wall plate which is resin bolted to the wall as per structural engineer design)?

@SteamyTea Thanks for the advice. Let's say I don't fit the 25mm PIR between the battens, and just leave the service void between the battens empty. Considering it's masonry construction and the walls are airtight (combination of parge and airtight paint), am I correct in thinking I don't need a VCL? I've already fitted Intello to the ceiling (cold loft) and the dormer area. I don't want to install a VCL to the masonry walls or mid floor area, I'm way too far into fist fix for that. Your opinion is much appreciated thanks?

Thanks guys, I agree I can assure you. But I still need to make the decision, do I add PIR in between the battens? I have it on site and would like to, just don't want to create a problem by doing it? P.S. The gap in the cavity is behind the insulation, between the insulation and the inner leaf, so will create a bit of a thermal loop.

Thanks for replying @Redbeard . Sorry the cavity is supposed to be 100mm, although it's more like 110mm. The full fill PIR is Cavity Therm and there's a little gap behind in many places (see photo below). The PIR is rigid in place and the house is rendered and painted so no access to the cavity anymore. I simply worry that cold air may get behind the PIR in places so thinking 25mm of PIR between the battens can only help, if just a little. Considering I have an excellent MVHR system going in, and if you guys don't think there should be any mould or other condensation issues, why not add the PIR between the battens (I already have it on site)? If adding the PIR: 1. Should it fit flush against the wall or flush to the front of the battens (I had to space the battens off the wall to get them vertical in places, so there would be a gap either in front or behind)? 2. Presume VCL and taping the battens isn't really needed considering it's a masonry construction? Thanks again for any advice.

I'm driving myself mad, please help. Our new build is up and watertight, currently in first fix. As in the picture the walls are made up from out to in - traditional sand cement lime render with Emperor paint, 100mm medium dense outer leaf, full fill PIR, 100mm medium dense inner leaf. It's passed SAP under 2019 regs when we started (yes it's taking years to build this thing). Air tightness is excellent through a mixture of intello, sand cement lime parge, tescon vana tape, and airtight paint. A Zehnder MVHR system is going in. The floor has 300mm of PIR insulation below the screed. The cold loft has 500mm of loft roll insulation as ceiling level. I have created a service void and straightened up the walls with 25mm battens ready to plasterboard. Here's what's driving me mad. The main cavity wall insulation did not go in too well (I won't bore you with the reasons why). To help, I have purchased 25mm PIR boards to add between the battens internally. But now I'm worried will this affect dew points and cause any condensation / mould problems to or behind the plasterboard / plaster. Or will the MVHR solve all that for me? Am I worrying about nothing or making an error? Any advice greatly appreciated as I have 5 days off work now and want to get cracking (the wife is going mad). Thanks.

Thanks @saveasteading , so what would the ideal mix be when using sharp sand? 3 sharp to 1 cement? Any building sand?

Building a manhole. Can't source any Grano Dust locally for the benching. Is it ok to use Sharp Sand instead for the benching? If so what's the ideal mix? Presume adding a bit of Febmix helps (I'm not quick enough to work with SBR)? Thanks guys.

Design one with two ducts per valve in larger (or multiple occupancy) areas seems to be common with 75mm ducting. Design two which I’m presuming will be your approach because you have purchased 90mm ducting seems good. I’ve read that 90mm ducting enables a lot more airflow, which is why only one duct is needed to most valves. The valves are on the opposite sides of the rooms to the doors (near to windows), which is great. The only question I would ask BPC is why are valves being used in the hallways? I was under the impression valves aren’t needed in hallways because they are not habitable rooms? Maybe a Build Hub colleague with more knowledge can jump in and explain? Also I will follow with interest (to try and learn and bit) what others on here will say about the PAUL calcs.

@Ola did you reach a decision between the Vent Axia Advance S on 75mm ducting versus the Zehnder Comfoair Q350 on 90mm ducting? I really want to go for the Zehnder but at twice the price I'm struggling to press the button on it. The Zehnder benefits include: - pre-heater (not sure I need that in London) - 160mm and 90mm ducts (which is of course better airflow) - it can take the F7 ePM1.0 filters which are also big in physical size (the Advance S can take F7 ePM2.5 filters but I believe the actual size of their filters is smaller) The Zehnder will be quieter and overall better, but is it worth twice the price? Others feel free to jump in and bash me over the head with something!

Thanks @JamesP and @Happy Valley for the replies. 👍

Last question @Happy Valley - did BPC provide you with airflow calculations etc for building control? I know the commissioning with report come after install, just wondering if BPC provide airflow calcs as part of their service? Thanks, last question on this.

Thanks very much @Happy Valley , may I ask what the total square meterage size of the house is? Mine is 130 square metres (65 downstairs 65 upstairs) so that unit would be easily big enough.

@gavztheouch and @Happy Valley just seen this thread with you discussing the Vent-Axia Sentinel Kinetic Advance S. Looking at this unit myself. May I ask did you install it? If so, what do you think of it when running? As quiet as you would have hoped? Normal vs boost? Did you use an ePM2.5 filter? Any opinions much appreciated thank very much.

Our plan is to connect to a 100 year old common sewer on private land (our own land) for our self-build. The existing common sewer is 150mm salt glazed, in collar sections, submerged in concrete (see photo). We have permission to connect via either a manhole or preformed 'Y' (oblique) junction, built to Sewer for Adoption 7th Edition standards. The choice is ours. We will be installing ourselves. Preformed Y junction would prove a nightmare with all the concrete surrounding the salt glazed pipe, trying to shave it clean enough to accept the shear banded couplers on either end without cracking it. As a result, manhole may be preferable, except am I dealing with the same problem? To explain... Sewers for Adoption states that you should allow for any differential settlement between the manhole and pipeline, with short lengths of either spigot/socket butt pipes being built into the manhole wall. So, considering the existing common sewer is submerged in concrete, would I still need to shave back and add rocker pipes or shear banded couplers (flexible joints) to the outside of the manhole to allow for the differential settlement? I’ve spoken to two friends with knowledge / experience in sewer works and they said I shouldn't need the flexible joints because it's already submerged in concrete. I thought I’d get your opinions too before I bring this to the attention of the water utility. What do you think? Thanks in advance.