I am not sure of a product that would go under roof decking, but it's worth noting that formulations of OSB and other wood panel products used for roof decking have dramatically reduced emissions or offgassing in recent generations.
- For your project, you may wish to ask for a brand that has especially good performance in this area.
- Also, your attic is the one place in the home that you actually want to be drafty, so that moisture and stale air can escape.
When you're having your roof replaced, that's the time to get a high performance ridge vent installed, and be sure ventilation is adequate (and matched) at both the ridge and under the eaves, and that the vents at the eaves are not blocked by insulation.
GAF has a line of Cobra ridge vents that offer very high air flow, as well as a breathable roofing underlayment called Deck Armor.
Also, if your home has a hip roof or other design with a limited ridge line, you can add solar powered attic vents.
For more information:
Read "I'm concerned about toxic offgassing from OSB subflooring and roofing. Are safer products available?", a Q&A answered by Ian MacLeod.
Read "How effective are the different attic ventilation methods? I read mixed reviews, and many have a commercial bias.", a Q&A answered by Danny Kelly.