At our farm, we are trying to minimize costs in every way, and that includes electricity. Below the Mason-Dickson Line, our most energy intensive time is not winter, but summer. Temperatures during peak solar activity often reach 105 degrees and stay just a bit below that for days on end. Our winters, even in these last few years when solar activity declined, rarely have more than a weeks worth of days below freezing. This means for us, that the biggest enemy is heat.
In modern suburban home design, everything is about aesthetics – selling the house. The appearance of a home is the number one issue for contractors and real estate agents. A secondary is storage and overall square footage. Due to these two items, attic design is way down the list, yet it is the design of the attic and the roof material that largely determines a home’s ability to shed heat.
To maximize space under the roof, dormers are often used. This means very restricted attic space and often no attic space directly overhead in upper rooms or in cathedral ceilings. Even insulated heavily, one can still place a hand on the ceiling and feel heat radiating from the roof above during our summers.
The way to reduce heat is to reflect it or shed it – period. Thermal barrier insulation has helped tremendously in the IR reflectivity area, but it is difficult to refit in an existing house. In a large dormer or cathedral ceiling, it is nearly impossible, but roofing products exist if you are willing to pay for re-decking or specialty products.
Shedding the heat is another issue, and both cathedral ceilings and dormers are very poor at this. To shed the heat, you require a straight path and an entry and exit point for heat that are very linear. Any turn in the heat flow path makes a hot spot.
Also, most suburban roofs are composite shingles, and dark as well. HOA’s and cities restrict roofing options in favor of aesthetics – which means you cannot put a white or reflective roof on your home. It is often impossible legally to even change to color of your shingles more than a shade or two – due to regulations.
Moving to the country, where there are few of these restrictions, allowed us to make a roof that performs better than any builder roof I have lived under. In the height of August, the Dog Days, the underside of this roof (interior ceiling) is actually cooler than the ambient temperature by 2-3 degrees. The entire upper floor is cooled by a single 110VAC attic fan and is livable without air conditioning. The only time it is actually sweat-level hot is when the attic fan is off in summer. The entire upper floor is cooled with one 110VAC window unit designed for 500 square feet of space. The space in the upper floor is actually 750 square feet with the cathedral ceiling making an even larger cooled volume.
The upper floor ceiling is a 17-foot high cathedral – no coffers or other interior modifications – a straight cathedral design. It has partially exposed rafters, and the ceiling is being put up on weekends, and is constructed from waste pallet boards we find online for free.
Here is what we built.
The roof was constructed of ½” CDX plywood base covered by 15# roofing felt. 1X4 nailer strips were then nailed over the roofing felt with 18” in between the nailers. ¾” thermal barrier sheet insulation was then laid between the nailers as a simple tin roof was installed.
When installing the thermal barrier, 24” wide strips were cut that were 4’ in length. This allowed room to work on the roof installing the tin. With 4’ lengths, 2 sections of tin could be installed without crushing the thermal insulation. The tin and thermal barrier were installed from one end to the other in this fashion.
After installation, the tin could not be touched due to the August heat, but the underside of the roof (1/2” CDX) was at ambient air temperature. This is better than my suburban home, where the asphalt shingles and thermal barrier with ¾” CDX decking still make for 12-25 degrees hotter beneath the roof in the same sunshine. The asphalt has no reflectivity, and actually absorbs and retains heat.
The overall schematic for the roof is in the drawing below:
The construction of the rafter-run nailers and luan plywood sheathing is seen here:
The tin was run to within 4 inches of the apex of the roof. Screen was installed across the apex along with the roofing felt, to preclude insects (wasps and mud dobbers primarily) from entering the air space between the rafters. A simple ducted tin roof cap was installed during the roofing of the 2nd half of the roof. This allowed the ability to reach over the roof and secure the ducted cap as the roof 2nd half of the roof was secured.
Internally, we wanted exposed rafters. This led us to creating a vented air space between rafters. Dead air space is a good insulator, but vented air space can prevent heat buildup provided airflow is unrestricted.
To accomplish this, we added nailer strips on both sides of each rafter, approximately 3” from the internal edge. This allowed us to secure and seal 1/8” luan plywood to the nailers with finishing nails.
At this juncture, the boys are tearing apart old pallets, sanding and cutting planks to fit perpendicular, between each set of rafters. Aesthetically, it is interesting, which was wanted in a roof of this size within the apartment. We had discussed using old fence pickets to do the same thing, but the boys wanted to save those for a wall treatment within the apartment.
Out goals in building this were manifold, one of them being to recycle or re-purpose as much material as possible to lower costs and still have a novel looking interior. The others were to teach the boys proper roof construction, the importance of things being square and accurate prior to roofing and to reduce or potentially eliminate the need for all but minimal air conditioning.
With fall rapidly disappearing down here, testing the cooling was a bit late, but on 90+ temperature days, the building cooled tremendously well with simply the attic fan running.
Some notes about this design:
- sealing the luan plywood is very important, as the attic fan will draw hot air from beneath the roof if the vent space is not completely sealed from the interior of the house.
- Sealing at the juncture between the walls and the rafter runs is equally important for the same reason – the draw from the attic fan will pull in hot air
- In general, sealing every nook and cranny is desireable when using an attic fan – sealing leaks forces the air draw to come from the windows that are open rather than miniscule leaks within the interior room construction
- It is difficult to run the thermal barrier insulation any other way than concurrent with installation of the tin roof – we recommend cutting the thermal barrier to fit between the nailer strips and running it in 4’ lengths
- The roof itself has barriers of plywood, felt and tin along with the thermal barrier – it is light in weight per square foot compared to standard asphalt roofs
The tin reflects a lot of heat, and the thermal barrier does as well. By placing the felt layer below both, it serves to both seal heat out and keep air within the rafter spaces isolated. The vented space between the rafters allows cooler air to draw any residual roof heat out the top of the building. The sealed luan plywood sheathing between the rafters keeps warm outside air from being drawn into the room when the attic fan is on, or when the air conditioner is running.
When we tested the design, the door was left slightly ajar. Our door is made from 2×4 cedar lumber and weighs about 85 lbs. When the attic fan began to spin at its normal slow speed, the initial air draw pulled the door open, even with one window open. That night, the temperature was 89 degrees, yet both boys awoke with their covers pulled over their bodies – the airflow across them from the windows had both of them very comfy even in the late September heat.
Attic fans work very well here in the south, as long as your expectations are realistic and your home design complements their use. This means opening windows with good screens, the interior well sealed from attic and other external spaces, and having the airflow path open in as straight a line as possible to the attic fan intake.
I am sure there are myriad ways to do things like this, but for us, this was both rapid and less expensive than having a standard tin roof installed. It was a huge improvement in cooling over the asphalt/felt roofing of the other buildings on the property. It also improved the attic fan cooling tremendously, as well as the efficacy of the small window unit air conditioner. Our first 2 electric bills, with air conditioner running most of the day, were less than $50. The bill in November with attic fan only was $35.
It was a little more detailed work than a traditional roof/ceiling design, but we are very happy with the results. We will post some interior pictures when I get up to the farm this weekend – hopefully the boys will have made some progress on the ceiling.