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  1. #1
    Neo
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    Roof vent airflow calculation?

    Is there a calculation used for the input/output airflow volume to create effective airflow.. ie input volume 90% of output volume.

    Does the size of the roof space also have to be factored in to the size of the vents.. ie cm2 vent area per m3 of roof space?

    Cheers
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  3. #3
    Neo
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    Cheers Hugh.. says there the ratio should be 2:1 to reduce heat loss.. but getting rid of heat is exactly what I'm looking for. I'm sure I'd read on a thread here that the intake should be less that the output to create a fast airflow in hot conditions.

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    See if this helps
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    You will see that the soffit inlet needs to be bigger than the outlet otherwise you will decrease airflow.
    Any more and I'll start the meter running.
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    ^Excellent and clear info.

  7. #7
    Neo
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    Yes thanks Hugh. Same topic, different question so I hope you're not going to charge for this one.

    If the underside of the roof and the gable end walls are lined with radiant barrier foil, should I install fibre insulation, radiant barrier or nothing at all on top of the ceiling panels?

    I'm inclined to not put anything on top of the ceiling panels to allow heat dissipation from the room into the roof space, assuming the roof space is cooled sufficiently through venting.

  8. #8
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    Installing fibreglass batts on the ceiling panels will cut heat transmission through the ceiling.
    The ceiling space temperature will decrease with ventilation but heat transmission through the ceiling panels will increase due to greater airflow across the ceiling which decreases the static air film insulating effect. Having both will greatly increase efficiency. Some batts discourage rodents etc as well and have fire retardent properties. If you have airconditioning batts should save you about 10% in energy costs or conversely you could install a 10% smaller system. Hope that makes sense.

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    Neo
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    Well the aim is to transfer heat upward, but I guess it's dependent on air changes per hour if that heat transfer is upward or downward. Kind of a suck it and see situation I feel not knowing exactly how efficient the venting will be until it's fixed and the hot season kicks in.

    Knowing that the radiant barrier under the roof and venting should reduce the heat I'm inclined to leave off the layer on the ceiling panels and fit retroactively if required.

    One issue is the main room absorbs a lot of heat from the long outside wall in direct sun most of the day.. my concern is that a insulation layer on top of the ceiling panels will trap that heat inside the room.

  10. #10
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    The best remedy for walls is to extend a verandah to keep the western wall in shade. or plant trees that will keep the wall in shade. You could also consider a retractable awning which can be pulled out to shade the wall and especially any windows during the afternoon and retract it under the eaves at night.Maybe wooden lovers over the windows.
    Aircond tip: Dont wait untill the house gets too hot before turning on the A/C It can be false economy as the load from the hot walls will need to be absorbed by the A/C unit increasing the load and power usage.
    Alternative:
    If you have access to cheaper off peak power you can install a mini chilled water system by making an off peak ice bank. This way you use cheaper off peak power and store an ice bank overnight and use the cooling energy stored during the day time. I will give more detail if anyone is interested.
    If you are a handyman much of it can be DIY.

  11. #11
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    Quote Originally Posted by Hugh Cow View Post
    mini chilled water system by making an off peak ice bank
    Tried that years ago with bought in blocks of ice (can't have moving air from a ventilator or aircon for my work), didn't do much but caused a lot of condensation along the tubes. Gave up on it - I am not an engineer.

  12. #12
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    Unfortunately you do need moving air to prevent stratification. In commercial applications sometimes it is better to use a false floor for supply air and return the air through the ceiling. This tends to mimick a more natural air draft by allowing cooler air to warm and naturally rise to the return air plenum.
    If you submit a plan of your system I maybe able to show you how to improve it. My system uses a refrigeration system in an insulated drum to form the ice and water is chilled by pump recirculation to and from the drum to a fan forced chilled water coil. Running expenses in the day are only from the fan and water circulation pump. Even that can be negated with solar panels.

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