69 Spray Foam Insulation – Why?

So now it’s time to put the spray foam insulation up in the roof. This stuff is sorta like the spray foam you buy in cans at Walmart to plug holes and keep out water, bugs and unwanted airflow. Why use spray foam at all, or stated differently, why not just use fiberglass insulation like everybody else does. Answer: Metal roofs sweat. That sweat is condensation which occurs when moist air comes in contact with a cold (metal) surface and creates water droplets (same principle as drops forming on the outside of a cold beer can in the summer). If enough water drops form, it rains in your attic, which soaks the fiberglass insulation and dramatically reduces it’s ability to insulate - the same principle as wet socks make for cold feet, even if the socks are wool (a good insulator). I have read that damp fiberglass insulation has it’s R-value cut in half over dry insulation.

It also explains (at least to me) why in most houses the roof has 10 – 16 inches of insulation, while the walls have only 3 to 6 inches and the concrete floor has none (under the concrete). Don’t laugh, in really cold climates they DO put insulation under the concrete. Television marketing has conditioned people to believe “heat rises”, so they need to add insulation in their attic to prevent this heat loss and save money on their utility bills. Actually, heat flows in all directions equally, it doesn’t “prefer” one direction over another. When you condition people to think heat rises, then it becomes an easy sell to add more insulation in their attics where it’s easier to install as opposed to more insulation in their walls, harder to install. Taken to the logical conclusion, your windows (even the really good ones) lose more heat than most any even poorly insulated wall or ceiling. Isn’t advertising great.

So, how does spray foam keep it from raining in the attic? The simple answer is by keeping the moist air away from the cold metal. A longer answer is by insuring that all the surfaces inside the roof stay above the dew point temperature (where condensation occurs). I’m not going to delve any deeper than that into how, why and where condensation occurs. If you want to go deeper, check out buildingscience.com and start reading or get out your college textbooks on physics and thermodynamics because that’s where you are going to end up anyway. And let the record be straight, I barely passed thermo… with a “D” and was proud of it.

This post has veered way off direction, so I’m going to stop.