THL versus TPP
Both THL and TPP tests are conducted on all 3 layers of the garment together. You can't take the values of one layer and add it to the values of another layer to get the result.
Let's review and simplify:
THL (Total Heat Loss) - In short, breathability
Lighter/Thinner garment = Higher THL (Comfort).
Thicker/Heavier = Higher TPP (Protection).
There is, in most systems, an important inverse relationship between TPP and THL. The optimum values for each are needed to achieve both comfort and protection. But generally as TPP goes up THL comes down, and as THL goes up TPP comes down, so the goal is to balance the two values.
A lightweight, thin garment with high THL values may be comfortable but won’t offer you as much thermal protection. A thicker, heavier garment with high TPP values may provide more thermal protection but the heat stress and added weight can be dangerous. Hence the balancing act: the need to determine the level of protection required while maintaining a workable system. Keep in mind that the moisture barrier plays a crucial role in the breathability and THL value of the garment.