He should also be sealing the seams. Thermoplastic is the way to go but make sure it is sealed. Thermo wouldn't have much off gassing if any after it is cool. Seamer down is a good tool also to use. The only off gassing is when the seam is constructed. If you are chemically sensitive I would have the installer set his iron outside as soon as he is done seaming as there will be some smoke from the seaming process. You may want to open a window and have a fan blowing toward the outside. Any thing you can do to get rid of the smells would be to your benefit. I've done jobs where I couldn't wear deodorant of after shave. Carpet will have an oder so there will be some off gassing there. Again have windows or the HVAC going to dissipate the oders or voc's. They should be gone in 24-48 hours. Daris

I prefer the koolglide iron. Main benefit is that it's not a one shot chance. With the koolglide, if there's a gap or overlap, it can be reheated and reset. Sealed seams with latex or thermo is fine.

You are asking a question that there is more then one tool and more the one way to do. Yes Kool Glide is a good seaming iron, but is it the only one No, Seamerdown is a great seam weight behind any seam iron, but there again is it the only way? No. do I use both of them, Yes. Does not mean I am better then someone who uses the regular iron and a normal seam weight. This is what I will tell you to ask. Does your installer follow the industry standard? This means that it is to be sealed, no cut fibers or loops, backing level and tight, no gaps, seam tape melted correctly. If all this is done you then will have a good long lasting seam.