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 rafters are 2x10 pine, on 18-inch centers, and are braced with bolted collar ties due to the roof span of 32 feet.

The rafters are 2×10 pine, on 18-inch centers, and are braced with bolted collar ties due to the roof span of 32 feet.

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.

installing 1/2" CDX plywood deck with 15# roofing

installing 1/2″ CDX plywood deck with 15# roofing felt

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.

installing thermal barrier insulation and tin simultaneously - note 1x4 nailer to support roof and spacing for thermal barrier

installing thermal barrier insulation and tin simultaneously – note 1×4 nailers to support roof and spacing for thermal barrier

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:

Maxi-Cool Roof

The construction of the rafter-run nailers and luan plywood sheathing is seen here:

Internl Rafter

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.

freshly completed roof

freshly completed roof

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.

Oilman2


Comments

A Southern Roof System — 18 Comments

  1. Oilman, I love your stories and points you always make. I have a lot of the same bothers as you (dealing with crappy designs). This subject here is one that drives me nuts as it costs like heck to have the AC on in the summer to cool the house.

    I hate to say this, but I cant grasp you design. I am a skilled wood worker, but I am not familiar with roof design, so the article is a little above my head. Do you have actual close up pictures of the roof between each step? I see the drawing, I get what your stating in the article, but I can’t visualize how your doing it. Part of it may be a lack of vocabulary. I will google nailers and nail strips and re-read.

    Thanks again for taking the time to right these articles, I read every one. ~ my wife and the toilet set that continually loosens ~ dont get me started!!

    Kelly

    • I will try and send you a photo. But in the basics we:
      a) ran rafters
      b) decked roof with 1/2″ CDX
      c) applied 15# roofing felt
      d) ran 1×4 nailing strips
      e) put IR reflective panels BETWEEN the nailing strips
      f) put on the tin roof

      Inside, we nailed strips to the sides of each rafter, 2 inches back from their interior edge.
      We nailed and sealed 1/8″ luan plywood to make the area between rafters into a conduit for hot air to rise. It also left 2 inches of rafter exposed in the room.

      The ceiling boards we nailed into the same nailer strips right on top of the luan plywood.

      Each sealed space between rafter empties into the peak of the roof cap, to let heat out. Each space between rafters has inlet duct under the eaves to let cooler air in.

      I will try for some pictures, but I am getting hip replaced soon and things are unsettled in the oil patch too. Time, time, time…

      Oilman2

  2. Oilman,
    Curious as to why you didn’t choose a solar powered fan, like the Solar Royal on this website: energyefficientsolutions.com. That’s what I did it comes with a lifetime warranty if you register it and out of the box a 15 year warranty moves somewhere around 1500 cfm of air and no electric bill, which can run around $150 a year. Just wondering?
    Jack

    • Without battery bank, it will only work during daylight. We haven’t sprung for our forklift battery yet, so this was a more equitable option. We are likely to run a parallel DC system in the house, and will likely use radiator cooling fans instead of AC anyway. Greenhouses similar – more battery required for the swap to AC when DC components will work more reliably and effectively.

      And, this window unit cost was $0 – it was picked from trash, sanded, painted and restored. We try to re-use, repurpose or rebuild older equipment as much as possible.

  3. Nice job on building a convection-channel between the two ‘roof’ layers.

    Have you measured the air temperature at the soffit entry vs the roof-line exit? Would be interesting to see how much heat is picked up between the two points.

  4. I did some research on saving re-roofing costs for a 50+ year old retirement in Southern California. The planned re-roofing involved removing the 25 year old composite shingles, adding an insullation layer of felt type material, and then a composite shingle outer layer.

    I found that the Rino company has a new roofing solution for industrial applications that could be applied to homes. They spray a 3 to 6 inch layer of closed cel foam over the existing roof, without removing the old roofing material It forms a seal over the roof, without cracks or seams. They then spray a layer of the same type material they use for truck beds. That material is available in a wide range of colors, and is very tough. It protects the foal layer from damage or solar degradation. Depending on thickness of the outer layer, it will need to be re-coated due to that layer’s slow solar degradation. The lowest cost and thinner outer layer would be good for 10 to 12 years between recommended re-coating.

    The quote I received had costs comparable to the conventional roofing replacement costs, including a re-coating 12 years after the initial installation.

    The benefits of this process included:
    A new roof that is a sealed structure, without voids or seams.
    A 50% reduction in installation time.
    Comparable replacement costs and warranty.
    A significantly increased roofing thermal insulation, with a proportional reduction in utility costs..

  5. Oilman, Ever used a thermos? The way we insulated roofs was to get 1 1/2 in. foil backed duct insulation. Ran the rafters length ways. 4′ x 100′ rolls. Tin was called High cor 2, 26 ga.. After roof was installed. inside was done the same as the outside insulation. Void was left in between space of rafters. Foil tape was used to seal seams. Then strip the underside and install your material of choice. Same for walls. Then cooled the whole house not letting heat in to the house at all. Required at least a 2 ft. under house to draw air. More if you like. but this is the basics.

    • I am trying to build for the next generation or two to be able to have what I make. So I am trying to stay away from things, like tape, that can degrade. If I could find old-time painters caulk that wasn’t silicone based, I would go for that even…

  6. If I follow your diagrams correctly: you have multiple 18″ wide (minus the width of the 2×10 rafters) independent ducts that pull air from the soffit inlet vent under the eaves into the peak of the roof cap. It seems to me that you will get air movement in each duct just from the fact that hot air rises. I don’t see how a fan would improve on this unless you had one installed between each rafter on each side of the roof. So where is your fan placed?

    I achieved similar results by putting a fan in one end of a conventional attic blowing air out through a vent in the end near the peak, but that fan drew air from the whole attic area.

    • The rafter vents are sealed – they only draw from under the eaves and exit at the roof vent. We have vent holes drilled in each rafter space under the eaves, and can close them in winter. The fan draws from 2 windows at one end of the house, and exits the other end. Sealing the rafter vents was done with 1/8″ luan plywood and lots of ladder time, but it does work.

  7. I have done the same thing to my houses for the last 41 years. I used widow screen and covered it with 1/4″ hardware cloth because I worry about the window screen rotting away. What kind of screen are you using?

  8. I have been using this ventilation system for 41 years. I am concerned that the window screen will rot away and I have covered it with 1/4″ hardware cloth. What screen are you using?

  9. Nice looking roof system. There are a few things I would do different but then again there always is right. One thing I noticed is the placement of the collar ties. The trade school rule of thumb was always 1/3 the height of the roof system down. Might I suggest you keep an eye on the walls at the Centre of the roof run and make sure they don’t start bowing outward with time. I live in snow country so we worry about things like that not much concern there. Wind load can do some of the same thing though. Nice design, efficiency is always the best answer.
    Best regards, David Baillie

  10. You state an attic fan, but I don’t understand where the attic is.
    I see the roof with channels, and the luan cathedral ceiling, but no attic ?
    Does the fan vent the room with the AC, or is it tied into the rafter channels?

    • It sits above the bathroom and closet at the end of the room, opening out under the eaves. Technically, there isn’t an attic, but there is a tiny loft at one end – which is where we sat the fan.

  11. Hey, Oilman. Nice projects you done! I have some questions, what is the best material to insulate my roof myself, If in my country winter air would be -20 C. I think it’s not the same as your project in the photo?
    Cheers, Julius.

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