Last year I had the adventure of building a new home for my family. I used Wayne Atkins of SterlingBrooks Custom Homes. Wayne and I have been friends for a long time and he “gets it” when it comes to building a high performance, extremely well built home.
We decided to build a full foam encapsulated home with a non-vented attic. We used 6” of open cell foam on the bottom of the roof deck, sealed the attic and spray foamed all the exterior walls. Basically we built a giant Styrofoam box that was practically airtight. However, what we really wanted was a giant Styrofoam box wrapped with foil. Foam is great for slowing conductive heat, but in a hot Texas summer we really needed to put the home in the shade (shade from the radiant heat, not the light) – we NEEDED a RADIANT BARRIER!
Since the foil radiant barrier is the first line of defense against radiant heat and the foam is the 2nd line of defense against conductive heat, combining the two would result in the ultimate reduction of heat flow.
The challenge with a spray-foam enclosed attic/roof is that you cannot typically have a radiant barrier because of the lack of a proper air gap. The shingles are connected to the roof deck, and the deck is connected to foam. Some heat will still flow by conduction through the roof/foam assembly. With this type of assembly there is no air space for the heat energy to jump and be converted to radiant heat. You CANNOT just put foil under shingles on a typical roof and have a radiant barrier. You MUST have an air space on ONE side of the foil. Radiant heat by definition is heat transfer by non-contact across an air space. Without an air space you cannot have radiant heat and without radiant heat you cannot have (or install) a radiant barrier. Once you can convert heat to its radiant form you OWN IT! Conductive heat can only be slowed down, but radiant heat can be REFLECTED.
So how can you incorporate an air space and a layer of foil into a roofing system?
The options are limited. Metal, tile, concrete or other raised- type roofs will work and you can install a radiant barrier on top of the roof deck below the raised roof. The problem is that these type roof systems typically run $4-$5 per square ft. where a traditional shingle roof will run about $2-$3 per square ft.
I decided to get creative and build a double-deck roofing system. This system provides the best of both worlds: we get the radiant barrier and we get it at a reasonable cost.
This is how we did it: first we installed the first (bottom) deck. We stopped it about 10” from the fascia board. This gap allows air to come from the continuous soffit vents and from behind the brick into the soffit cavity and then flow up between the two roof decks. Then, we installed AtticFoil® Radiant Barrier Foil directly over the roof deck. You can use either the double-sided radiant barrier 48” product or a single-sided 60” radiant barrier product (the foil side MUST be facing up, toward the sky).
NOTE: We used a felt marker to mark where the rafters were on the foil. Without the marks on the foil it would have been hard to find each rafter for the next step.
Next we installed furring strips. We used 2×2’s, but you could use 1×2’s if you want. You will want at least a ¾” gap or better between the two decks. We were careful to leave gaps at the tops and bottoms of all hips to allow air to flow freely from the soffits to the peaks.
Then we then installed a foil-backed roofing deck material called TechShield on top of the furring strips. This was our “normal” deck that we tied into the fascia boards and then installed traditional roofing felt and shingles.
Ventilation note: Since the airspace between the two roof decks was only 1 ½” we did not need a whole bunch of ventilation. For every 1,000 square feet of roof deck, there is only about 125 cubic feet of space between the two roof decks. The roof was a hip style roof with a limited amount of ridge, so a ridge vent was not an option. I like 14” wind turbines. They work well, are cost effective and easy to install. When you combine the draw caused by the Bernoulli effect of the wind turbines with the natural stack effect of the air rising (or being pushed up by the cool air) the air between the two roof decks is easily being changed several times per minute. This airflow has a cooling and drying effect on both decks.
Nothing short of amazing!! We were under construction during the brutal heat wave of the summer of 2011. Example: On one hot-sunny day (about 100ºF) the top surface temperature of the shingles was close to 180ºF (using an IR thermometer to measure). The BOTTOM surface temperature of the lower deck was 110ºF. ONLY a 10º INCREASE!! Virtually NO heat was making it through the system. In fact, the construction workers would usually eat lunch INSIDE the home since it was relatively so comfortable.
By forcing the heat to JUMP the airspace between the two decks we OWNED the radiant heat. The top deck with the foil on the bottom worked off the EMISSIVITY quality of foil radiant barrier (the ability NOT to convert energy to radiant heat) and the AtticFoil® installed below the airspace on top of the bottom decked worked off the REFLECTIVITY quality to reflect the radiant heat.
I would speculate that the nominal 10º temperature increase in the bottom deck was mostly caused by thermal bypass through the furring strips. Even with the double-deck system, you still will have some conduction though the assembly where there is no air gap.
If you are building a home with a large roof surface area, or want to use dark shingles, please consider this system to incorporate a radiant barrier with a foam encapsulated attic. The upfront cost is a little more, but you will recoup the cost in additional energy savings and in being able to further downsize the size of the heating/cooling system during construction.