How to install eave vents where there is sarking?

[mariuskoen]

Our home in Perth WA is around 12 years old and is extremely hot even on a 25 degree day.  I'm wanting to install some under eave vents https://www.bunnings.com.au/bradford-csr-poly-eave-vents-4-pack_p0816665 as I have a Bradford Airomatic Roof Vent Fan https://www.bunnings.com.au/csr-bradford-250mm-woodland-grey-airomatic-smart-roof-vent_p0811143 on my roof.  If I move some tiles around the eave area, I find the attached reflective foil vertically in most spots just above the wall cavity at the height of the eave.  It extends upwards to about 3cm from the roof tiles.  It seems to act as a reflective barrier between the eave area and the roof space.

Would it be ok to cut 3 or 4 openings around the perimeter of the house to allow air from these under eave vents to enter the roof space?  Or what exactly is the function of this foil material?  It is similar to the material car sunshades are made of.

[JacobZ]

Hi @mariuskoen,

Welcome to the Bunnings Workshop community, it is fantastic to have you with us.

The foil is known as sarking, and it is a type of reflective insulation often installed under tiled roofs to reflect radiant heat from the sun, reducing the amount of heat transferred into the roof cavity and ceiling space below. This helps keep the house cooler in summer and can slightly reduce heat loss in winter. The reflective surface faces the air gap beneath the tiles, bouncing radiant heat away before it can penetrate the roof structure.

Putting a few holes through the sarking around the perimeter of your house shouldn't have any adverse effect.

In saying this it is important not to simply remove large sections of the foil. Doing so could reduce its effectiveness and allow heat, dust, or moisture to enter the roof cavity. Instead, you can create small, controlled openings directly above each eave vent. About 100 millimetres in diameter should be sufficient. Make sure to seal the edges of the cut foil with Ametalin 48mm x 50m Insulation / Ducting Tape to prevent tearing. 

This will allow enough air movement into the roof space without compromising the reflective barrier.

Let me know what you think and if you have further questions, please don't hesitate to ask.

Jacob

[mariuskoen]

Jacob

The Subject of this post is wrong as it gives the wrong impression of what I'm after.  Maybe it should read: "Installing Eave Vents" or something similar.

Also the foil barrier doesn't sit horizontally under the tiles.  Rather it sits vertically like an extension of the wall cavity upright as if to increase the height of the wall into the eave space.  All I can think is that it's trying to keep air from the eaves out of the roofspace but this doesn't make any sense to me as effectively very little air will get into the roofspace for ventilation & cooling.  This is why I think the home is so warm on even a 25 degree day.

I just very hesitant to cut into or remove things if I don't know what their function is.  The other option is probably to install the eave vents but just leave this foil reflective material in place as there is a very small gap above the foil.  But I don't think it will achieve much in terms of ventilation then.

[JacobZ]

Hi @mariuskoen,

It doesn't make much sense to me either for it to be acting as an air barrier, as tiled roofs aren't airtight, so outside air would be able to get in through the air gaps in the tiles anyway. 

It could be acting as a moisture barrier inside the wall cavity.

Are the locations where you've seen the sarking over doors or windows?

It could potentially be there to prevent moisture from getting up and under the top of a window or door.

If you read through Part 5.7 Weatherproofing of masonry, particularly Part 5.7.4, does any of this sound like the situation where you've found this sarking?

Let me know what you think.

Jacob 

[mariuskoen]

Jacob

Not sure I understand. 

"It could be acting as a moisture barrier inside the wall cavity." - That means it should be in all wall cavities right?

"Are the locations where you've seen the sarking over doors or windows?

It could potentially be there to prevent moisture from getting up and under the top of a window or door."  - Yes I did find it above windows but I think it spans the whole roof.

What would it achieve though if it sits horizontally like a wall above the window?  Or is it's purpose to guide the air up towards the tiles and then through the slight opening above it for circulation?

Do you think it's ok to cut openings above the windows as I'm planning to put some eave vents above some windows?

[JacobZ]

Hi @mariuskoen,

I'm just trying to eliminate some possibilities so we can work out what purpose it is actually serving. It doesn't make any sense for it to just cover the small gap between the wall cavity and the roof tiles, as it is not airtight, so it wouldn't stop air transfer to the roof cavity anyway.

Sarking is oftentimes used to prevent moisture transfer, and points around windows usually call for waterproofing measures, so I thought it might be related to the windows in some way. If it extends the full way around the wall, not just the windows, it probably isn't directly related to the windows.
 

I think it likely extends down into the wall cavity and is sitting against the inside wall to prevent thermal transfer and moisture transfer to the inside wall. 

I don't think it is really doing anything between the top of the wall and the roof battens because air is going to be able to get around it through the gap above the battens anyway. I think its purpose is inside the wall cavity, not above it, and it has just been fixed there because it was an easy place to attach it.

Ultimately, I think it would be fine to cut some holes in it to allow for ventilation.

Jacob

[mariuskoen]

Thanks Jacob

[mariuskoen]

@JacobZ 

Update (just in case anyone's following this post).  I had another look.  The foil (moisture barrier) just spans from the eave height up to top of the wall.  It sits loosely against the wall but doesn't extent into the wall cavity.  So my conclusion is it must just act as a moisture barrier for the wall.

I found a tear in the spot where I wanted to install the eave vent.  So I removed that section and installed a piece 20cm lower in height as it was damaged anyways.  Then installed my eave vent.