I got asked this question again the other day that I get asked variations of fairly frequently, so I thought I'd share my answer here. The question is essentially, "how does a body absorb any benefit from the herbs in Thai hot herbal compresses (luk pra kob) through cloth?"
Steaming the herbs releases herbal alkaloids into the steamy watery herb mix - just like how if you steep a tea bag in hot water, the herbs infuse the water - you can then remove the tea bag, but your water now has herbal properties in it. So your herbal compress is kind of like a cup of tea that is packed with herbs - If that cup of tea is still hot, you can smell the herbs in the steam that drifts off of the cup. The reason you can smell the herbs, is because that steam is actually infused with tiny particulates of the herbs. This is why in Thai medicine we say that the sense of smell is related to earth element - because if there are no earth particulates, things don't have a smell. So we know that the steam that smells herby, actually contains bits of herbs that are too small to see or separate out.
Now, how those herbs get into the body of the person we are using the compresses on? The heat/steam from the compress is transferred to your client via convection, the method of heat transference in which heat moves to colder areas. Your client's body is colder than the compress and so it absorbs the heat and steam from the compress. Your clients should be able to feel that the compresses cause a softening in the skin and tissue as the moist heat opens pores and calms the tissue. The key word here with these compresses is "moist heat", as, in addition to the general relaxing properties of warmth, we have a transference of the herbally infused steam. Steam and heat travel easily through cloth to the skin. To help this out, most Thai herbal compresses have camphor in them, which is an herb that serves as a vehicle for transporting other herbs into the skin and on to the other layers of the body.
Many people use Thai hot herbal compresses wrapped in a face towel, and through the client's clothes. Despite these layers of cloth, the compresses still work; and it's not simply the effects of heat. If you do an easy experiment of applying a clay or gel hot pack to one side of your body and a hot herbal compress to other side, you will most likely be able to feel a difference. To my body, simple hot packs are wonderful, but the herbal compresses are noticeably better. This said, while the compresses do work though multiple layers of cloth, they work even better when applied straight to skin. But when applying straight to skin you must be extra careful of the potential to burn your client, and some would hold that these compresses should not be used on more than one client (since the compresses are steamed between uses, they are effectively sterilized, plus many of the herbs in the compresses have antibacterial properties, so the single use requirement is debatable). Increase the potency of the compresses by dipping them in clear alcohol (such as vodka) before applying to the client's skin.