Roof Space Project

My solution to airflow through roof space, a 300mm fan, 70 watt so not too much energy and 4 poll to keep noise down. I am aiming for 12 cub metres/min with this arrangement.

So far have made exhaust duct, the pic shows it at about install angle (90 degrees to roof slope). It is planned to have about 80mm of the bottom lip of the opening proud of the tiles.
The duct throat is 220 x 500mm so it will fit between rafters with 2 tiles removed. The final discharge is 370 x 500mm and will have a light mesh screen.

I plan to make an adapter to go between horizontal fan and duct by carving a polystyrene block then fibre glass epoxy the outside, later removing polystyrene.


Anyway I need some ideas on flashing for the exhaust duct. I was going to secure the duct to the rafters where it passes through so flashing wont need to take load.

I have actually, they are attached.

This just represents the main area we use Kitchen / Family /Dine, there are a lot of variations to model but I used this to home in on main impacts.

In this scenario I need to drop roof temp by 3 degrees to cover drop in AC load by 210 watts (at 3:1 ratio thats 70watts electrical input).

As the exhaust will be toward top of roof cavity and air will mainly enter through eaves vent then natural pressure will also be in favor, but haven't factored this.

Oh yeah!

Foiling is finished and exhaust duct is in and open although fan not yet connected.

Hot today, ambient is 39C and roof cavity is 38C at noon.

Now the roof cavity is performing like it has thermal mass, lagging in temp behind ambient till about 2pm when it catches up then goes above (as ambient drops) till around 8pm. Yesterday was 38 max ambient and roof cavity got to 41. Max roof temp above ambient I've observed is 8C, very dependent on wind and cloud.

Geoff,

Nice project. Be interested to see what difference the vent makes.

Did you happen to measure the roof temps on those kind of days before the insulation went in?

woodbe.

They are really quite impressive results

Prior to the foil installation it was common for the roof space to exceed 50 degrees in summer and I would estimate it was 15 - 20 degrees above ambient at the peak in the day and probably above ambient till well after 10pm. Roof vent prior was 2 whirlybirds. I have left the whirlybirds but not penetrated the foil with them so they will effect only a small area around the whirlybird between tile and foil now.

When I point my temp gun at the underside of the foil it is typically 15 - 20 degrees below the temp of the back of a tile in the middle of the day.

After seeing the performance of this foil and the cost ~$2.50 - $3.00 per sq m it is amazing that people build a tile roof without it. Although it took me considerable time (maybe 120hrs) to staple this stuff up in the case of a new roof it would be insignificant to add it under battens.

I will get around to putting the data logger in to see if some conclusions can be drawn of the vent.

I'm planning on allowing for blocking the vent in the colder months to limit heat loss.

BTW its now 5.30pm, ambient is 33C and roof space is 36C.


PS Now 7.15pm ambient 29C and roof space 31.5C

Hi Geoff,
Any pics of what the vent looks like from the outside and is there anything you would do differently next time around?
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Pic of roof vent as seen from back yard, still need to put mesh in front.


So far havent come across any major issues

Planning on putting a drain in the duct underneath for any splash that may enter in severe storms.

Good work. Doesn't look too bad and I bet its effective.
I wish I was building again.
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