Everyone has already hit on the note of removing to siding and adding ventilation, (and I just realized this was from a year ago) so I'll just make a couple of other suggestions for you or someone else looking. I'm in the Dallas area as well! Can definitely tell you Silkies in our extreme heat are going to struggle, so you'll need to be sure on focusing on a design meant to keep your chicks cool when temperatures soar. It's much easier to winterize later for our short cold snaps.
Youd also benefit from having a large shaded area by your coop, maybe even over it as well, to give them the ability to get out of the sun during the day. The sun and UV indexes get extreme and having shade can make those hot days 100 times more tolerable. You can use an outdoor shade mesh, tin, canopy, or tarp (just note that tarps may need to be replaced probably around once a year depending on quality and your set up. Sun exposure and our wonderful North Texas strong winds degrade and tear up tarps over time. Trust me, I've been using tarps for various reasons my whole life, spend a lot of ti.e repairing and replacing them. Outdoor shade mesh is less susceptible to wear and tear than tarp, as well as allowing more ventilation, and a good set up can last you a few years.)
My Leghorns also love to play water when it's hot. Not sure if Silkies are big water lovers, but a shallow container with a bit of water to run through might be worth trying this summer.
I'd also highly suggest you paint your coop something other than black. Other than the fact it will fade quick in our environment, black will also attract heat and sunlight like a magnet and make your little coop hotter.
Main things though are to keep your coop open and ventilated and have ready access to lots of shade.
Good luck with your Silkies!