Mix drying too fast

[Benno]

I've repotted my phals about 3 times in as many weeks trying to get the right mix lol. The first couple of times I had not very good store bought mixes that had very non uniform particle sizes and stayed too wet.

I have now got medium sized Kiwi bark (8-20mm, I'd say the average is about 10mm) which I've mixed 4:1 with simioar sized charcoal and then added a small amount of nz sphag.. the kiwi bark is excellent stuff - my pots drain very fast indeed, but now they are drying too fast! They are quite light after watering or soaking for 30 minutes. Today I had to water 24 hours after the previous water, with overnight temps of 19C and daily max of 28C (66-82F), humidity ranged from 91% to 50% over that temp range. The orchids are in clear plastic orchid pots and I have drilled a few 3mm holes around the lower circumference for slightly better ventilation.

What do you suggest I do to extend the watering period to 3 days or so for somewhat lower maintenance plants? It often gets to low 30C's here in summer. Add more sphag to the existing mix? Would perlite help or make no difference? I've just tried soaking some 3mm perlite to experiment and it didn't really seem to hold much water.

Thoughts welcome...