Thanks everyone
I grow these in a greenhouse, in fact I bought the greenhouse just to house my first Masdie
It is heated just to avoid frost not much more in the winter. Basically I aim to not let it fall below 5C, it occasionally has done, but generally it is between 5C and 10C during the winter.
In the summer the door is open non-stop and the window opens as needed (it's only a small 6'x6' greenhouse).
However the way I keep them cool in the summer is by keeping the roots cool. They grow in moss in porous clay pots, and each pot sits on an upside down porous clay dish sitting in a slightly larger right way up porous clay dish. For good luck I add lecca to the space in the larger dish. Water is in the bottom dish so it just reaches the bottom of the pot. All the clay is in thermal/water absorbing contact so the water in the bottom keeps the clay always moist while not water logging the moss. The evaporation from the clay means the clay is always cold to the touch, the hotter the day the colder the clay is. As long as they don't dry out they stay cool, they can get hot quickly if the water dries up.
We don't have massively hot summers in the UK, but I based my setup on discussions of 'coolpots' in hotter countries. I had read that as long as the roots are kept cool they cope fine with heat and that matches what I've found.
Below is a picture of one of my other orchids in-situ. You might be able to see the setup in the pots of the orchids around it. They key is to keep thermal/water absorbing contact around all the clay, but without the moss sitting in water all the time. The moss stays damp while the clay is damp, but not wet.
The roots love it, I get massive root growth on all my masdies and the roots are obviously attracted to growing next to the clay.