Foil vs Bubble Foil or Foam Foil
Hi I’m insulating a shed-like structure on my property to create a comfortable living space. With regard to the tin roof I’ve been advised to use a layer of Bubble foil on the battens directly underneath the tin and then below that in the remaining ceiling space (above the plasterboard) Fibreglass batts. My question is: In this kind of context will bubble foil give an improved insulating performance over and above foil (sisilation) on it’s own directly under the tin? i.e. Is bubble foil worthy of the added cost??
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Bubble foil adds AT MOST R2 to the assembly compared to the performance of a more rugged aluminized fabric type radiant barrier or foil, and is not worth the additional cost.
None of the above are very effective without air spaces on both sides of the radiant barrier with the RB an inch or more away from the surface on the other side of the air space. A single layer of RB comes nowhere near code minimum performance for living space anywhere in the US. I'm not sure what the performance requirements are in Oz.
If it's on the exterior of the assembly with fiber insulation below it can add something to cooling season performance, but the wintertime vapor retardency may become an issue. Bubble pack is a true vapor barrier, as are most foil products. But perforated foils and perforated aluminized fabric manufactured with a grid of tiny pin-prick perforations have sufficient drying capacity for most applications.
Where are you located (for specific local climate data), and how thick is the fiberglass going to be?
You don't want foil-faced bubble wrap. You want real insulation -- insulation with an R-value. One option to consider is to include a layer of rigid foam in your assembly. If you go that route, you can choose to purchase foil-faced rigid foam -- either foil-faced EPS or foil-faced polyiso -- for the slight boost in R-value associated with the radiant barrier facing. Remember, though, than any radiant barrier is worthless unless it faces an air space.
Here are links to two relevant articles:
Stay Away from Foil-Faced Bubble Wrap
Radiant Barriers: A Solution in Search of a Problem
Hey Dana, Thanks for your reply..For some reason the posting system here doesn't seem to allow me to respond directly to your answer with a reply so i'm popping this up as though it an answer to my question rather than a reply to your response!!...Anyhoo....I live in NSW approximately 20ks inland from Byron Bay on the east coast of Australia just below Brisbane. The fibre glass is 70mm thick (High density 24kg/m3). There would be an air space of 35mm above the foil (or bubble foil!). i.e between it and the tin. The Fibreglass would be above the plasterboard ceiling and approx 20mm below the underside of the foil. So the system would be: Ceiling, 70mm fibregalass, 20mm gap, Foil, 35mm gap, tin Roof. I'm not clear on what you mean when you say "the wintertime vapor retardency may become an issue in this context." I'm guessing but can you please clarify. Thanks for your help. Much appreciated.
Hi Martin...Thank you very much...Will check out the links.
The year round temperature of Byron Bay is VERY temperate, averaging only about 10F cooler year round than absolute paradise (aka Honolulu):
That's pretty much a US zone 1 type climate, where R30 (~200mm of mid-density fiberglass) would be required to meet building codes for heated & air conditioned houses. At 70mm mid density fiberglass would be about R12, quite a bit shy of that specification, and the radiant barrier WOULD make a substantial difference in the peak cooling loads.
The thermal improvement of using bubblepack vs.of foil or aluminized fabric radiant barrier would be less than 10% of the peak head transfer, and an even smaller fraction of the average in that stackup.
From a vapor diffusion point of view the vapor permenace of the radiant barrier doesn't much matter in your climate, as long as the interior side finish ceiling isn't too vapor retardent. Standard latex ceiling paints on gypsum board would be fine.
If there is space for more even fiberglass, use it!
If you are going to shade that roof with north or west facing solar PV panels, (recommended, as Australian style electricity rates, even if you have to buy a smarter inverter and some controls for self-consumption, never exporting to the grid) you might just skip the radiant barrier altogether. Power grid politics and regulations are evolving very quickly in Australia, making self-consumption or even outright grid defection more economically rational there than in most of the developed world.
Hi Dana, Have integrated your advice into the job and went with thicker batts to improve insulation. Thanks a million!!
Is that a million AUD$, USD$, or a million Korean Won? :-)
(Which ever it is, you're welcome!)
Ha ha, thank you. :-)