You may get conflicting advice on this product, here, on this Forum. Personally, it's not high on my list as a product to use to keep radiant heat from entering in the house. My own, personal view is that the radiant heat has a 'right' to come in BUT I have the 'right' to vent-it-out. In other words, in your situation, I'd install soffit vents (if you don't have them now, then install soffit chutes, then install a ridge vent. This way, the heat can 'come it' and get 'pushed out' the ridge vent by rising air from the soffit vents. I don't know - for some reason, radiant heat is like snake oil.
If you want to read the threads on a recent post, go to white paint for the underside of the roof, reflects heat?. You'll find a lot of replies ON the subject as well as OFF the subject. BUT, it does 'address' radiant heat, et al., fairly well (from a DIY perspective.) Granted, none of 'us' are experts on this 'stuff'.
Well, anyways, if you have more specific questions on this topic, feel free to Post up again. My best to ya and hope this helps.
PS: I know I didn't, exactly, answer your question. That's because I can't exactly answer your question. (I've never bought or used the paint, nor do I personally know anyone that has in SE PA.)