Help needed with roof venting

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kiwigander

231 posts

Master Geek

#3118868 22-Aug-2023 01:26

An old North American technique that you might consider:

Install large vents in both gable ends

Install a large axial fan in the attic (can be in the middle; ours was) drawing air from one gable end vent and pushing it towards the opposite end vent.

This will eliminate the effect of having a blanket of hot air over your living area.

If you've got an attic hatch, and you leave this open while the fan is running, then the Bernoulli effect of air flowing fast through the attic will draw air up out of the living area, too (so you want a window or two open on a lower floor).

You need a big fan to make this work (my experience was with a 900 mm fan), and any insulation up there has to be well fastened down.

A major advantage is, you're not disrupting the integrity of the roof in any way – you're not touching the roof at all.

MikeAqua

7785 posts

Uber Geek

#3118908 22-Aug-2023 09:01

kiwigander:

An old North American technique that you might consider:

Install large vents in both gable ends

Install a large axial fan in the attic (can be in the middle; ours was) drawing air from one gable end vent and pushing it towards the opposite end vent.

I install gable-end vents on a house I lived in near Blenheim (Wairau Valley). I didn't think of installing a fan. Being in Blenheim, there was never any shortage of wind. The house had 1.2m deep eaves, so good protection from the rain.

I installed ~300 x ~600 metal louvered vents, and made some wooden vents, with insect mesh, to overlay them. The wooden louvres are for aesthetics. If it was windy, you could feel the air moving through the roof space if you were up there.

Mike

tweake

2391 posts

Uber Geek

#3119038 22-Aug-2023 16:48

decibel:

My neighbour imported one of these - https://www.bunnings.com.au/products/building-hardware/building-construction/ventilation/roof-ventilation

You are meant to put ventilation grills under the eaves so that it does NOT pull air from within the house. Also, during winter, it can be closed.

the thing is it always will pull air from the house due to stack effect, ie its always trying to push air up though the ceiling. adding air flow through the roof space just gives it a helping hand. only way to stop that is to have an air tight ceiling.

i know of one place where the owners did that, but they never mentioned if it actually worked.

tweake

2391 posts

Uber Geek

#3119054 22-Aug-2023 16:53

kiwigander:

An old North American technique that you might consider:

Install large vents in both gable ends

Install a large axial fan in the attic (can be in the middle; ours was) drawing air from one gable end vent and pushing it towards the opposite end vent.

This will eliminate the effect of having a blanket of hot air over your living area.

If you've got an attic hatch, and you leave this open while the fan is running, then the Bernoulli effect of air flowing fast through the attic will draw air up out of the living area, too (so you want a window or two open on a lower floor).

You need a big fan to make this work (my experience was with a 900 mm fan), and any insulation up there has to be well fastened down.

A major advantage is, you're not disrupting the integrity of the roof in any way – you're not touching the roof at all.

that is/was done with timber commerical buildings, like cafe's. thats to handle the amount of moisture from the people in the building. i have seen it in a cafe converted to housing, and the fans where disabled. the owner put in double layer of ceiling insulation.

in north america its often a bandaid attempt to stop frosting, which is caused by moisture in the home coming up through the ceiling and freezing on the roof. in spring it thaws out making "attic rain". trouble is the fans pull even more moisture out of the home which often made the situation worse.

Hammerer

2476 posts

Uber Geek

Lifetime subscriber

#3120575 26-Aug-2023 13:13

olivernz:

I want to ventilate my roof space for some relief in summer. Anyone know someone in the Kapiti/Wellinton area that is passionate about the topic?

I was very passionate about the topic because we had the same overheating problem with a dark north-west facing coloursteel roof (15 degree slope for 150 square metres of roof) which replaced an asbestos roof. The air temperature in our ceiling space would cause me to sweat within seconds of putting my head up through the hatch. We had two small air vents in the roof which made almost no difference as solar gain was so high compared to the small air flow leaving. The heat radiating from our ceiling was really obvious on my bald patch.

We solved the problem by adding a thick 30cm polyester blanket above the existing pink batts and exposed framing.

If the insulation hadn’t worked then I would have installed venting exhaust fans in the roof gables.

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