Hi Rosellie,
You’ve probably read this already but post #89 here has some general information about the different types of mattress protectors that can help you decide on the pros and cons of each of them and the tradeoffs that are involved with each of the different types.
Hopefully some of the members here that have had some personal experience with more than one of them will see your post and share their comments about how they compare.
The Smartsilk is a mattress pad and would be a little thicker and less "stretchy than some other protectors which means that it could have more of an effect on the mattress than mattress protectors that are more stretchy or thinner.
A forum search on smartsilk (you can just click the link) will bring up the forum posts that mention it and some comments about silk in general and there are some brier comments about silk mattress pads in post #10 here as well but unfortunately none of the members that either purchased it or were considering it came back to share their experience.
There are also some comments about a different silk mattress pad in this topic that may also be helpful.
I didn’t realize that the smartsilk mattress pad had a semi breathable/water resistant membrane (it only says that it contains “Water repellent non-woven materials”) but if it does it would reduce the breathability of the mattress pad to some degree. I talked with Costco and they emailed the manufacturer to ask what the non woven material was and they will email me as soon as they get a reply.
Phoenix