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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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  1.  
    It's difficult to run ventilation ducts to the ground floor rooms of the house we are renovating, as the ceiling joists all run the wrong directions and there's no route through the upstairs rooms to the loft.

    A solution could be to run ventilation ducts under the suspended ground floor to a small MHRV unit in the hall cupboard. A completely separate system will serve the upstairs rooms.

    We are lifting the ground floor boards anyway, so could fit ducts between the suspended ground floor joists.

    Has anyone done this, did it work, any problems?
    • CommentAuthorowlman
    • CommentTimeJul 16th 2019
     
    I don't know if you've already investigated, but I looked at pre-insulated ducting for my own A.2 A. heat pump.

    I discounted it all because of cost and decided to insulate manually, it may be different for you. I found two pre-insulated types, One a oval, (low profile) rigid polystyrene insulated type, German I think, looked good but dear. The second was a rigid metal type inner and outer with a rockwool sandwich between.
  2.  
    Cheers! Did you run your ducts beneath a suspended ground floor?
    •  
      CommentAuthordjh
    • CommentTimeJul 16th 2019
     
    I don't see any fundamental problems with the idea.

    As owlman already hinted, you need to think about insulating the ducts if they run outside the thermal envelope, and wherever they run if they're going to be carrying heated or cooled air.

    You'll want to decide the exact routes for your ducts in advance, as well as the type of duct, so you can consider whether you need holes in sleeper walls or whatever. Also consider how the ducts will route near the MVHR unit; it can get very congested. I'm happy that we went with semi-rigid ducting and distribution boxes. It seems simpler than fiddling with rigid ducts and branches, attenuators etc.

    Note that the ducts to the external terminals need to be very well insulated and vapour-sealed on the outside to prevent condensation collecting.
  3.  
    The loose plan is to insulate below the suspended floor using two layers of mineral wool, suspended on netting dangling from the joists. The semirigid ducts could run between the two layers of wool. Another plan would be needed if/where they need to run perpendicular beneath the joists, maybe wrapping them with something.

    So the ducts would be surrounded with insulation, but passing through the airtight layer (new chipboard floor), and would become inaccessible after the floor is laid.

    Haven't landed yet on how to get to/from the MHRV unit to/from the external air or what to do with condensate. Anyone done this?
    • CommentAuthorowlman
    • CommentTimeJul 16th 2019 edited
     
    Posted By: WillInAberdeenCheers! Did you run your ducts beneath a suspended ground floor?


    No, but in an un-insulated loft space above the usual loft insulation. I wrapped the rigid spiral metal duct in a foil faced rockwool. The straight runs are fairly easy but the various elbows, and convoluted bends in semi rigid spiral wound aluminium, are a bugger to do.
    I opted for the rigid metal duct because of it's better airflow qualities and robustness over the super flexi spiral stuff. The sort that looks like a large helical spring sandwiched between two layer of foil. It's flexibility and ease is great but IMO it comes at a price, certainly for my intended use.
    •  
      CommentAuthordjh
    • CommentTimeJul 16th 2019
     
    Posted By: WillInAberdeenHaven't landed yet on how to get to/from the MHRV unit to/from the external air or what to do with condensate. Anyone done this?

    The general principle is to keep the runs to the external terminals as short as possible; i.e. mount the MVHR unit as near to an external wall as possible. The condensate drain can probably just be allowed to drain onto the ground, since there won't be much (no idea of rules, though), or otherwise routed into a drain via a dry trap. Don't forget you can get condensate, and small insects in the ducts to/from the external terminals; especially the exhaust. So best if the duct runs can be kept straight and sloping down to the outside.

    I opted for the rigid metal duct because of it's better airflow qualities and robustness over the super flexi spiral stuff.

    Yes, I wouldn't recommend that stuff. The one I used is D-shaped from Ubbink but there are also circular ones from them and others.
    • CommentAuthortony
    • CommentTimeJul 16th 2019
     
    Rigid ducts for me too, inside the heated envelope ie above and surrounded bu insulation.
    • CommentAuthorGreenPaddy
    • CommentTimeJul 16th 2019
     
    Flip your house upside down (mentally not physically) so running ducts under the floor is exactly like running ducts in the loft. I would use rigid rectangular plastic domus easi duct, fixed tight to the floor boards, then kingspan directly under between the joists where the ducts run. Then wool as elsewhere. Ducts need at least as much insul as walls, roof etc, as you have moving warm air which can release a lot of heat energy.
    I think it's a great solution for the ground floor, which you're lifting anyway. Crossing the joists, as you say, is the problem. You'll have to add loads of insulation to these low points, watching out for the 150mm min. distance to solum. Form a box around these hopefully few cross runs, with say 150mm kingspan, foamed together.
    Letting the condensate run outside the thermal envelope will cause it to freeze and block, like happened to the early condensing boilers, so try to get it to a drain, even if under the floor.

    Hope that helps a bit?
    •  
      CommentAuthordjh
    • CommentTimeJul 16th 2019
     
    MVHR ducts are at the same temperature as the inside of the house. They don't need insulation if they are run inside the thermal envelope, but do need a lot if run outside, such as in a loft. The ducts to the external air are an exception since the air in them is at external temperature, so they are exactly the opposite.

    Ducts from a heat pump, or a simple heater, need insulation everywhere, unless you have a very well insulated house and are prepared to dump the heat anywhere inside the envelope. Ducts used for cooled air from a heat pump need insulation, because of condensation risk.
    • CommentAuthorGreenPaddy
    • CommentTimeJul 17th 2019
     
    Referring to ducts needing insulation, I was of course talking about those under the floor boards where there is currently no insulation. My point was really WRT pre insulated ducts which generally don't have anything like enough insulation. There must be at least as much insulation between the duct and the outside world as would be in walls and roof. That doesn't have to be touching the ducts, so of course ducts run inside the thermal env don't need additional insulation (in take and exhaust excepted).

    Will in A is planning to form a thermal env under the floor which the ducts will be part of, so from the face of the ducts outward will need a full thermal envelope's worth of insulation.

    Since the duct will take some of the avail space between the joists, I suggested adding a layer of foamed in place rigid board, as there may be less depth of wool. It also is belt and braces to ensure no cold air infiltration around the duct, which would lead to more heat loss than areas with no duct.

    Likewise ducts that drop below the joists would need special care to insulate the ducts. All the same principles as when running ducts in a roof space.
    • CommentAuthordickster
    • CommentTimeJul 17th 2019
     
    Ran some of our MHVR ducting (rigid) under suspended wooden ground floor, but ensured crawl access when building. I worried that the ventilated cool space would create condensation in the ducting in what is, in effect a u bend moisture trap.

    Sorry to say that we simply haven't bothered using MHRV, (or our underfloor heating) as it proved to be unnecessary (only two people, 3 dogs, lots of windows).
    • CommentAuthortony
    • CommentTimeJul 17th 2019
     
    If it was mine, I would engineer in a deliberate low point and make a little drainage hole to let any water out, if worried about water hitting the oversize drill a hole in it, if worried about air leaking in then a HepVO type trap could be incorporated.
  4.  
    Thanks everyone for the helpful comments!

    Thinking about the insulation, I will change the plan so the ducts are above the insulation layer, instead of within it. I like Paddy's idea of a PIR 'shelf' between a pair of joists that the duct can sit on top of, packed round with wool. This could also be a good route for running central heating pipes which would also have pipe insulation.

    By keeping the duct within the thermal envelope, the extract air will stay close to room temperature, otherwise the MHRV can't recover heat. The air will be well above dewpoint so would quickly dry any condensation that occurred.
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