Martin writes from Thornhill, Ontario: "I am installing hydronic radiant heated flooring and would like to install 1/2" Celfort 200 on the sub-floor, then fix the tubing to this and pour over-top with 1.5" Maxxon over-pour. It's an older house and my existing sub-floor is made of 1"x6" planks with 1/4" spacing between each plank. Because the Celfort 200 is combustible, would the gaps in between the 1"x6" planks be a problem? The top would be fine as it's covered by fire rated material, but would the underside be ok, or would this not be acceptable for fire issues? Would you have any recommendations?"

Answer: If the space below is a crawl space or non-living space, then foam can be left exposed. If the space below is a living space, small gaps between floor planks should not be a problem as it takes very high heat for foam to ignite and the fact of the matter is that the planks and the floor trusses would start burning before the foam would ignite in a fire. Finally, a local building inspector could help with the municipal bylaws and permissions. And it is always a safe practice to cover up lumber with fire rated gypsum boards.