I am kinda of curious on why they are claiming the clay tiles are defective...
For tiles, I will say I am a big fan of Monier Roof tiles from working with them in Arizona. Most of the components are the same the world over though you can buy specially made venting pieces that slide under the tiles, different versions of underlayment, etc...
Tile roofs are generally well known for shading the existing roof which helps keep the cooling costs down. The older barrel style had more than adequate venting.
With that the best way to keep radiant heat out of your attic (which is where I assume your ducts are located) is to look into "cool roofing" tiles.
- If that doesn't fly, I know some roofers install a radiant barrier on top of the underlayment which in theory sounds nice, but I haven't seen any data to show if that really helps or not.
- The next big issue is venting, as most simply install 1x's to hang the tile off of flat on the roof. In places like Phoenix or Las Vegas without a bunch of rain the 1" gap they leave between the boards is enough to allow any moisture or rain to dry or run out but severely limits ventilation.
- There are other products with channels built in that help with ventilation & eliminating water.
- Another option is to install furring strips vertically and then horizontally leaving a nice area for ventilation.
Heading inside your attic is another great area for cutting down on your costs. Attics are going to be hot, so make sure everything in there is air-sealed properly and you have adequate insulation -- at least R30 though bumping that up can help.
Your ducts and air handler need to also be sealed up so you are not blowing conditioned air into the attic or sucking in that nice hot air and dust into your unit. If the ducts are not insulated they should be.
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Read "Do you recommend solar shingles? Is the technology advanced enough for durability and problem-free installation?" a Q&A answered by Randy Potter.