The Trouble With Cathedral Ceilings
Cathedral ceilings (sloped ceilings where the lining is attached directly to the underside of the roof rafters or battens) can be an attractive feature in any building and, especially in a very small home like the Greeny Flat, can help to create a sense of space by allowing for higher ceilings. However properly insulating and air-sealing cathedral ceilings can be a real problem. It can be very difficult to create an effective air-seal which can lead to substantial heat loss and, more importantly, to serious condensation problems within the roof cavity. Without a good air-seal warm moist air from inside can migrate up into the roof cavity and, unless the cavity is well ventilated (which can also be difficult in cathedral ceilings) the moisture can condense and create mould and rot problems. If the cavity is well-ventilated then the heat loss issue can be even worse.
The Sandwich Panel Solution
Sandwich (otherwise known as SIPS – Structural Insulated Panels System) panels are one possible solution to the problem. Generally SIPS panels have a polystyrene foam core which provides the insulation layer with two rigid layers glued to either side which provide structural strength and durability. These rigid layers can be made from a variety of materials including plywood, OSB, fiber-cement board, sheet metal, or corrugated metal.
The panels that we used for the roof of the Greeny Flat came from a company called ‘Versiclad’ based in Liverpool near Sydney. We used their ‘Corrolink’ 150mm panels which have flat, white Coloubonded steel on the underside, 150mm of foam in the middle, and corrugated Colourbonded steel on the top surface.
Photo showing the ‘Sandwich’ construction of the SIPS panels in the Greeny Flat roof.
Depending on your Wind Class zone these panels can span up to 7.5m so from ridge to gutter without the need for any other structural framing in the roof.
Photo taken during construction of the Greeny Flat showing how the panels span from the ridge to the exteior wall.
Below is a link to a tech sheet on the product.
The foam material is expanded polystyrene. The rated insulation value on the 150mm panel is R3.6 but it will actually perform MUCH better than a framed ceiling with R3.6 batts for two reasons. 1) There are no thermal breaks throughout the entire ceiling whereas a framed ceiling has a thermal break at every piece of wood, and 2) there is much less potential for air leakage with these panels. They also have the advantage of not having a ceiling cavity so there is nowhere for condensation to accumulate and no organic matter to harbour rot or mould. So for cathedral ceilings they can be an excellent solution.
The price of the panels themselves can be quite high (we paid about $10,500 for the panels and flashings for our 72m2 roof) but you have to consider that a) they install very easily and quickly (as long as you have a good even plane on which to lay them) which can save you a fortune in labour and b) once they are up, everything is finished (structure, insulation, ceiling, and roof) and shouldn’t require any maintenance. Trying to get the same sort of performance with any other method would require a LOT of time and messing around so the final cost comes out about the same.
There’s a part of me that hates to use polystyrene because it is a petroleum-based product and, when combined with the metal on both sides, these panels have a very high embodied energy and carbon footprint. But I think their use is justified, especially in a cathedral ceiling situation, because of the energy and carbon they will save over the life of the building through savings in heating and cooling as well as in durability and low maintenance.
There are other companies that make similar types of panels but these were the only ones that I found that had a flat ceiling with no ribs or corrugations. This not only looks great but it makes it much easier to air seal around the perimeter.
Photo showing the finished ceiling with the white metal underside of the SIPS panels and joints where the panels meet.
Since building the Greeny Flat I have discovered a company that makes entire little kit homes using these same panels. They are called ‘Shack In A Pack’
and their home kits are very reasonably priced. Their designs look very simple, practical and attractive so I went to view a display home at Unibuild
in Albion Park. Apart from the fact that the display home there is poorly put together I thought the overall ‘Shack in a Pack’ concept was a good one and would be particularly well suited to a little beach house or holiday cabin. It felt a bit like being inside a big esky but a lot of that was because there were no furniture or personal touches. With some nice interior design and a bit more attention to detail I think you could make a very nice, energy efficient, easy to build, comfortable, and affordable house using their system.