i would buy an adheisive specific for foam board. Look at the labels on the adheisives in your local home improvement stores if they dont have a specific foam board product. Depending on what the foam is made out of, some products may react with the foam, eat it away and not provide any holding power at all. Id recomend caulk if your worried about the gaps, but i used plastic sheething tape. Its not cheap but provides a good barrier. My suggestion is build your chamber well with quality components and it will last you a long time and hold temps much better if your always looking for the cheapest option. Also you should make sure you pick up the water resistant foam, jsut in case you get water, condenstarion, excess blow off in your chamber. Many foams are not mositure tolerant.