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.

Thanks very much for taking the time to reply everyone. I'll digest everything over the coming days. 👍

Thanks @crispy_wafer and @Susiefor the feedback. Looks like the Vent-Axia Advance S you mention use 125mm ducts instead of the bigger 160mm ducts that the Zehnder unit uses. Cool, hadn't looked at that unit. So did you simply get simple layout plans and equipment lists? Then just get on with it? Feel like I just want to do that rather than keep worrying about all the exacting design theory. I'm pretty good at following best practice and my work is very neat. I just wish someone would grab me by the scruff and say get on with it! I've also started reading about the filters, which seem to make a big difference. Seems tests can produce much better (manipulated) results if coarse G4 filters are used, rather than the much better F filters. I guess that's where the (more expensive) Passive Zehnder type units come into their own. Still nice and quiet when using better filters. This stuff is never easy! Edit: It's also ridiculous that Vent Axia units come with 5 year warranty and Zehnder units just a 2 year guarantee (talk about a put off). I take that back, Zehnder seems to have an extendable warranty as long as you replace filters as per their terms.

Progress update - After Zehnder tech guy strongly advising not installing the MVHR in our cold loft, we are now going to re-design it into the ground floor utility room. We've had two supply quotes (new build 127m2, other details about the property at the top of this thread). Installing ourselves. Quote 1 - BPC (free design) using a Vent-Axia Sentinel Kinetic BH system (up to 150m), with 75mm semi-rigid ducting. Quote 2 - Green Building Store (paid design) using a Zehnder ComfoAir Q350 Pre-Heater, with 90mm semi-rigid ducting. Our experience so far with both companies has been great. Green Building Store is twice the price, due to the design fee and of course much more expensive Zehnder unit. I'd like your opinions as to whether the paid design and bigger Zehnder passive unit are worth it - what would you do if this was your house? -- Is the Zehnder unit overkill for a 127m2 3-bed property like this? -- How would the paid design help me? I pretty much know where the ducts must run (not many options), noise from so called pressure drops etc may be unavoidable (my understanding is pressure drops = noise)? -- Fellow self-builders on here have built their own acoustic enclosures (I could also do that). -- Did you just install the damn thing with a design and accept the final outcome after commissioning? How much science is behind the design? Thanks everyone. So many decisions to make on your own, friendly opinions would really help.

It was the MVHR unit, cold will affect performance in winter, hot will affect performance in summer. Edit - Sorry think I already answered this above. Been a long day.

Hi @ruggersI've spoken with a tech guy at Zehnder and he strongly advised not installing the MVHR in a cold loft (mine is a cold loft), as it will vastly reduce performance of the unit. Both in winter (cold) and summer (hot). I explained I was trying to do what you're doing (insulated room around it), and he still said the same. He said to try and locate it within the building envelope. So I'm now burning time trying to find a way to do that.

Hi @TerryE Unfortunately loft space is not high enough for warm loft, in plans it needed to be subservient to neighbouring property. Long story. False ceilings are not an option, 2.4m room heights as it is and upstairs ceiling plasterboards already up. Happy to go into loft with ducting if needed as 400mm cross ways insulation going up there so ducting can fit well inbetween. But would love opinions on my last question at this stage then will move onto others - If I were to run my radial ducts for upstairs rooms within the stud walls and locate the room valves high up in those stud walls, rather than in the ceilings (this could avoid ducts going up into the loft altogether), would that reduce performance? Especially in terms of extract valves in the bathroom and en-suite? Again thanks in advance.

Hi Everyone, Had to finish another job in the house over Xmas but now full steam ahead on my MVHR stuff. I had a good chat with a tech guy at Zehnder. He said locating the MVHR unit in the cold loft (which was my original plan at the top of this post) will vastly reduce MVHR performance, both in winter and summer. So, I’m trying to re-design it into the building envelope. Haven't started first fix yet so can do this. Looking at different options. Hopefully a simple-ish question to start. Question - If I were to run my radial ducts for upstairs rooms within the stud walls and locate the room valves high up in those stud walls, rather than in the ceilings (this could avoid ducts going up into the loft altogether), would that reduce performance? Especially in terms of extract valves in the bathroom and en-suite? If the answer is as simple as all extract valves should be in ceilings then I'll ditch this idea and move on? Thanks in advance for any valued opinions.

Hi @TerryE , thanks for comment. The house is actually up and watertight. Airtightness was planned in from the start. For example regarding the mid floor, we took the walls straight to the top and put the roof on (7n blocks both skins) then windows went in (used a temporary scaffold birdcage mid floor on the inside to do this). Then parge coated the inside blockwork, then installed 9by2 ledger board / joist hangers / posi joists mid-floor (all specified with structural engineer). Doing most of the overall work myself, and every bit of the airtightness. Project is taking ages but standard is very high. I'm changing my mind on all my MVHR stuff, it's the next big to-do as I'm starting first fix now.

Mixture of parge coat and liquid membrane paint on the walls. Intello+ airtight membrane with Tescon Vana tape and Tescon Naidec tape to ceilings. Mixture of Orcon F, Airtight foam and Tescon Vana tape to all other openings e.g. around windows. I’m sealing everything.

Thanks very much @Conor and @JohnMo for the replies and info, which I'll digest over the next few days, and no doubt post further questions.