How to use reflective foil insulation to help reduce heat transfer?

[sakura_pixie]

I’m looking to improve the energy efficiency of our older home, as it gets uncomfortably warm in summer and cold in winter. I’m considering retrofitting Ametalin reflective roof insulation (or something similar) under the roof timbers to reduce heat transfer.

I’m based in Perth and have concerns about the potential for creating a high-humidity environment around the roof timbers, which I’ve read could lead to premature decay. Our roof doesn’t have soffits and is quite open, so I’m hoping this design allows any moisture to dissipate or drip out easily. Would this setup be effective, or is there a better alternative for achieving improved energy efficiency?

For context, we’re also planning to remove the old insulation, vacuum the ceiling, and install new insulation, which should help with temperature regulation too!

[JacobZ]

Hi @sakura_pixie,

A warm welcome to the Bunnings Workshop community and thank you for your question.

Unfortunately, because sarking like the Ametalin 1350mm x 60m SilverSark HD Reflective Roof and Wall Insulation is supposed to be installed on top of the rafters or trusses, it is not a good option for retrofitting. I've asked representatives from Ametalin before and their answer about retrofitting sarking to the underside of the rafters is that they don't recommend it as they will be fully encapsulated which can create a pocket of air where moisture can build up and cause issues.

A better option for retrofitting insulation to rafters is Foilboard as it can be installed between the rafters using Flexi-fast Fasteners. You can check out the Roof Installation Guide for some guidance.

I think the main thing you can do to improve your energy efficiency is what you are already planning to do, remove the old insulation and replace it with higher R-value insulation installed so there are no gaps between. For ceiling insulation, an R value of 4-5 is good. Depending on the spacing of the ceiling beams, this Earthwool R5.0 210 x 580 x 1160mm 7.40m² Insulation Ceiling Batt should be a good option.

Allow me to tag @Nailbag, @TedBear and @Dave-1 to see if they have any thoughts.

Let me know if you have any further questions.

Jacob

[Dave-1]

Good Evening @sakura_pixie 

I have had the same idea you and came up against the same answer as @JacobZ has told you. 😕 Replacing the insulation is a good idea, fixing any leaks you have into the roof cavity before changing the insulation is prob a wise option (My house had an instance where water got inside and someone has done a rubbish job removing the old insulation when wet) My place is the same as yours with the roofing and insulation. I have got to the point of considering replacing my tiles with tin and installing sarking in the process.

I really like @JacobZ's foilboard suggestion to the point I might try that myself. Tho the "how to" link dosnt work atm  

Another thing I have considerd is a whirly vent fan to draw the heat out of the ceiling space. These seem to work a resonable amount. 

Dave

[sakura_pixie]

Thanks @JacobZ and @Dave-1!

We had our roof repaired last winter after dealing with a couple of leaks as well! From what I understand, our roof has decent ventilation, with open areas around all sides. In fact, one of our holiday projects is to install vermin mesh to address that! With this setup, I’m wondering if whirlybirds would still be beneficial for us.

The foilboard suggestion sounds promising... I actually started looking into it right after posting my question. I just want to make sure I'm investing in the right solution for our home before committing to purchasing all the materials

[Dave-1]

Good Evening @sakura_pixie 

Its nice to have a ventilated roof space, especially in summer. The Foilboard will go a long way in winter and summer I would think.

As to the whirlybird and if it would help, hard call Id say. However there are equations for working out how much air is excited via a whirly bird with such and such breeze. You do need the air to enter into the cavity to exit it and draw the heat out. Need to find someone who has had one installed and see how well it has worked.

Dave

[Nailbag]

Hi @sakura_pixie 

I am a huge fan of the foil boards @JacobZ recommended. I personally have used these and found them to be very effective.

Probably the most effective system in addition was to install 2 x thematically controlled roof vents. About 20yrs ago I lived on the Sunshine Coast in a traditional old style government high-set home. It was ridiculously hot to sleep in over summer. I invested in 2 x thematically controlled roof vents and a few internal ceiling grills. When the roof cavity reached 30deg the solar powered roof vents similar to these kicked in drawing heat out. Basically like whirlybirds without relying on wind. The strong suction pulled air through from the ceiling vents into the roof cavity and out in to the outside air. I had one grill installed at the end of a hallway central to 3 bedrooms. As your walked down the hall you could actually feel the temp drop, which was around 7-10deg. It was the best investment in that home I made.

Nailbag