Part Twenty Two – Roofing And Mvhr (And A Quick Video)

This twenty second entry was published originally by JSHarris on the 29th November 2013 and received 1,750 views on the closed forum

This week marked the start of the roofing work and also the start of me working full time (more or less) on site. Having been retired for a couple of years, working all day has been taking its toll.

The roofers arrived and had to get to grips with the IkoSlate system. The first crew on site seemed to struggle a bit, but the guys we’ve had this week have done a cracking job. Meanwhile, I’ve been getting to grips with fitting the HB+ ducting system for the MVHR. This uses semi-flexible ducting that can (fairly) easily be fed through the PosiJoists to the terminal locations. The design was fairly easy. The idea is to get diagonal airflow across each room, with fresh heated air feeds in the furthest corner on each fresh air fed room and extracts from the bathrooms. This is what the ducting and a ceiling terminal looks like:

The blue ducts all come together close to the plenum chamber manifold for each (extract and fresh air feed) and go up through the floor like this:

On the floor in the service area above these pipes all feed into a plenum chamber like this, which is close to the MVHR unit:

The extract plenum chamber is mounted on the wall in the service area, and takes feeds from the two bathrooms, the kitchen, the downstairs WC and the utility room:

For a bit of fun, here’s a video of our build. The frame rate is time lapse at 15 minute per frame, so you can time some of the events, The boring black bits are night time, although the camera was set to turn off at 7pm and back on again at 6am. You can see that the MBC guys started before dawn on some days.….h?v=B5epVVl4tro

Finally here’s some shots of the roof slates:




 ProDave 29 Nov 2013 10:14 PM :

 I’m interested in the MVHR ducting (perhaps subject for a thread on the forum)

 Every system I have seen uses rigid galvanised steel ducting piped as a branch system and looks a pig to install. your plastic system looks much easier.

 And every system I have seen pipes air IN and OUT of each room. you seem to be implying you only pump air in to a bedroom, and only extract out of a bathroom, thus there must be flow under the doors etc between rooms.

 I’m interested in more details.


 jsharris 29 Nov 2013 10:29 PM :

 The design works by having small (~10mm or slightly less) gaps under some doors. For example fresh air is fed into the bedroom diagonally opposite the en-suite bathroom, and warm wet air is extracted from the bathroom. Downstairs the fresh air is fed into the two studies and the living room and flows under the doors, into the hall way and then to the kitchen/diner where there is an extract, and via another door to the utility with another extract, and so to the downstairs WC with the final extract.

 This means the flow of air is always from the main habitable rooms to the warm/wet or smelly rooms, which as well as providing adequate ventilation should also control odour spread and ensure maximum hear recovery.

 The key to the way the HB+ small bore plastic system works is because it’s radial, a bit like a microbore heating system. Each duct only has to flow enough air for a single room feed or extract, the manifolds take care of pressure equalisation. Balancing is carried out by fitting restrictor rings in the manifold connectors.. These come in a variety of sizes and are fitted as required during commissioning to get the right airflow in each duct.


 notnickclegg 30 Nov 2013 01:42 PM :

 Maybe it’s a passivhaus thing, but all the systems I’ve seen (not that there’ve been many!) have used the approach you’ve used Jeremy.

 Edited to add: Jeremy, it’s all looking like a real house! You must be stoked!




30 Nov 2013 01:42 PM

We found out a while back that BC allow a 15mm under-door gap (even) in fire doors.

 That came as a surprise both to me and to all of the fire service guys (my son is one such) I mentioned it to.


 jsharris 30 Nov 2013 02:07 PM :

 It does seem to be the standard way of doing MVHR. The air flow rates are pretty low most of the time. For example to change the air in the main bedroom once every couple of hours (which is a bit more ventilation than we’d normally run at) requires a flow rate from the bedroom inlet duct of about 30m³/hour. Flowing through a 10mm gap under the bathroom door to the extract duct would give a velocity under the door of around 1 m/S, or about 2.24 mph. not really a howling gale. When the ventilation is just trickling (as it would be at night) then the flow rates would be less than 1/4 of this, probably barely detectable even under the gap under the door.

 I’ve not had time to be really well-pleased yet, Jack, as there are still a long string of odd jobs I have to do and get right before the plasterers arrive. The last thing I want is to find I’ve missed putting in something absolutely vital at first fix!


 coopers 01 Dec 2013 09:19 AM :

 “the IkoSlate system. The first crew on site seemed to struggle a bit”

 Hi J, The slates look better than I thought they would. They look darker than the Tapco sample I have, so I must get a sample of the Iko. What aspect did the first roofers struggle with?


 jsharris 01 Dec 2013 09:36 AM :

 I’m really pleased with the very dark colour, it matches up against the black in-roof PV flashing nicely. The problem the first two guys had was that they’d not read the installation instructions and not seen IkoSlates before. They also didn’t seem to be overly enthusiastic about learning to use a new product.

 The second two guys (from the same company) had also never seen the stuff before, but they were keen to get it to work and it didn’t take them long to get to grips with the stuff. The main issues were cutting them to size, they had a diamond slate cutting disc in a mini-grinder that was hopeless, they resorted to a hand saw until I lent them my circular saw with a fine tooth TCT blade (pretty much the perfect tool for cutting them). They also took a while to work out how to fit the aluminium Kytun dry verge I’d bought. I think this makes a really neat edge to the verges and hides the edges of any cut slates. I lent them left and right handed pairs of aviation snips to cut that stuff cleanly, as they didn’t have the right tools to cut thin aluminium.

 Most of the fiddly stuff around the panels and gable is now done, so tomorrow they will probably make a start of the rear, which should be easy as it’s just a straight section with no cut outs or gables. The black fascias and guttering should go on later next week (with luck).


 coopers 01 Dec 2013 10:19 AM :

 Thanks J – that’s good info for us, if we use the same stuff.

 Are you having UPVC fascias?


 jsharris 01 Dec 2013 10:36 AM :

 I’m afraid we are………. It goes against the grain a bit to use UPVC, but the alternative near-zero maintenance solution up there was black aluminium, which is ludicrously expensive. Even the UPVC stuff is expensive enough.

 I did think of using larch boards up there, which would have been far and away the cheapest solution, but having played about with some sketches I felt the fascias needed to be differentiated in colour from the cladding. I could have stained the larch, but that would create a long term maintenance issue with needing to get up there every few years to re-coat them. Black UPVC seemed the best compromise, although we’ve opted for the slightly matt finish, so it won’t look too glossy and nasty.


 coopers 01 Dec 2013 05:18 PM :

 Sacre bleu! Le plastique !!!!

 I can’t speak French but that sounded good.


 coopers 02 Dec 2013 04:41 PM :

 actually, the coloured UPVC looks pretty good at high level anyway, and I imagine the matt finish even better – did you look at various brands, or are they all much the same, J?


 jsharris 02 Dec 2013 05:01 PM :

 They all seem much the same, as far as I could tell. There seem to be three or four manufacturers with a lot of their stuff being rebranded by retailers.

 I’ve decided to also use a bit of wide black UPVC fascia along the the bottom of the larcj cladding, to keep the timber up away from the rain splash zone. Seems a fairly easy way to get a maintenance free and weatherproof finish along the lower edge, and should help keep mice out of all that nice insulation under the house!


 coopers 02 Dec 2013 05:28 PM :

 Yes, I saw your discussions on that before – good idea – we will have the same issue with whole-house cladding.


 Nickfromwales 07 Dec 2013 09:08 PM :

 Sorry to be a late bloomer in this, but I must ask one question.

When working on ships, the ventilation systems had dampers ( electrically closed gates ) which shut in the event of a fire. This was primarily to stop the influx of oxygen rich air, and secondly to stop the transition of smoke to areas which weren’t immediately/currently affected by fire / smoke.

I was wondering if the mvhr system would affect a building in the event if a fire, or if this was already encompassed in the system design and I’m just not aware of this.

 ( will your reply show in notifications, or do I have to revisit here? )

Regards, nick.




4 thoughts on “Part Twenty Two – Roofing And Mvhr (And A Quick Video)”

  1. Dear Jeremy,
    We have followed your build & excellent advice with great interest for a couple of years now.
    At last our house is underway.
    We are now at the stage of having to decide on the roof covering.
    We love the look of your roof and have obtained a sample of Ikoslate.
    We are very impressed & would like to use it.
    However, we are a little worried that Iko do not offer any guarantee with their product.
    We know that you are meticulous in your research when it comes to materials.
    What gave you the confidence to use Ikoslate?
    We have not been able to find much information from independant sources.
    Best Wishes
    Moira & Henry

    1. Hi Moira,

      Iko import these slates from Canada, where they have been used for many years. I looked around at several similar plastic slates, and concluded that the Iko Slates were OK. They are accredited, and the main worry was whether they would fade. They’ve undergone an accelerated ageing test, and they reckon that they don’t fade after 20 years of normal exposure (IIRC). We’ve had no problems, and no sign of any degradation of fading, so I’m confident they will last at least as long as I do.

      One observation is that on a relatively steep pitch (our roof is at 45 degrees) the water does run off very quickly, because of the “slippery” surface. This has caused the standard flow gutters to over flow in very heavy rain, so we are going to replace them with deep flow gutters, to fix this.

      Hope this helps,


  2. Marley Deepflow are excellent but limited in choice of styling; in our village housing is very traditional, so had to miss on this build. Have you tried the brush gutter fillers against blockage by leaves?

    1. Hi Kev,

      Yes, I fitted the hedgehog brushes in the gutters, they do a good job, as I also fitted them to our old house. The problem is really a combination of a steep roof pitch, the slippery nature of the IkoSlates (and the big solar panel array at the front) and very heavy rain.

      The water comes off the roof so fast that the standard flow gutters just can’t cope, so over-flow. An acquaintance had a similar problem with a steep pitch and the gutters overloading and fixed it using the deep flow gutters, so I’m hoping that changing ours (when I get around to it!) will fix it. It only happens once or twice a year, when we get torrential rain, but it does stain the larch cladding a bit, so I really need to get around to fixing it soon.

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