Hey Connor, warm water isn't needed or even recommended to mix Sodium Hydroxide,..it creates enough heat on it's own. ..I think it can be mixed as much as 16 OZ per gallon.
How much per gallon are you trying to mix at a time that it won't all dissolve? 16 OZ per gallon would be REALLY hot,..can't imagine needing more than that to do a log home.
* I personally like a mix of Sodium Hydroxide and Sodium Percarbonate along with a soap to get the dwell time up and also for cling,..especially on a vertical surface like a log cabin.
Jeff