Expert: Daniel Humphrey - 2/14/2009

Question
QUESTION: Hi Dan - we recently purchased a camp in Maine - post and beam concept - metal roof is properly vented and has foil lined rigid foam insulation is immediately above the ship lap ceiling boards. The camp is COLD (15-20 degrees) inside when we arrive and we heat with woodstove and more recently a rinnai propane heater. We run a dehumidifier set at 35% the whole time we are there, yet when we turn off the heat to leave, as the camp cools down, it starts to "rain" from between many areas of the shiplap. My husband is thinking about spraying the expandable foam insulation against the present shiplap and re-boarding over that...any advice or input??? Thanks!

ANSWER: Dear Barbara,

You've correctly identified the problem as condensation, with the water largely coming from the propane heater, which produces about nine gallons of water for every 44 pounds of propane burned. Your dehumidifier can take some of that water out, but the rest winds up condensing on any cold surface it reaches--usually the plywood roof decking or the roof metal itself, if there isn't plywood.

You need to prevent the water from reaching a cold surface. The shiplap is warm, and so is the foam, but it has cracks and possibly penetrations that allow moisture to pass through. You can place your moisture barrier any place before the cold surface, but not against it.

The proper way would be to install taped, primed, and painted sheetrock on the ceiling to stop most of the moisture, absorb a little, and give you some fire protection, then re-install the shiplap for appearance. Some builders would carefully install plastic as a vapor retardant between the sheetrock and the foam, but I consider it unnecessary if your sheetrock is airtight and painted with several coats of paint.

Foaming the joints would not be necessary if you do a good sheetrock job.


I hope this helps.

--Daniel







---------- FOLLOW-UP ----------

QUESTION: Thanks very much for the reply!

A quick follow up, the Propane Heater is completely vented, releasing 0 emissions into the inside air, also taking combustion air only from the outside.  So no vapor from that, as the condensation was present even before the heater was installed.  (It's not too much, less than 1/8 cup drips from each of several locations where the foil backed insulation is exposed.  The thought of foam insulation is that it is completely air tight and requires no taping, sealing or nailing/screwing, and also provides an extra layer of insulation to the open concept ceiling. The foam would then be covered with shiplap to give the room its original appeal.

ANSWER: That's good your propane heater is vented! So the moisture is coming from cooking and breathing, and bathing if you do that inside the cabin, and leaving the room along with the warm air.

If the foam you use is a 4x8 panel of extruded polystyrene, that would be pretty good, as long as you duct-taped the seams. If you are still considering spraying expanding foam, I would advise against its use except for filling in behind visible gaps. Foam is messy (particularly upside down), time consuming, and leaves bumps that must be leveled; and often removing the bumps pulls foam out of the seams inadvertently.

From experience, the most cost-efficient and effective method of stopping moisture from reaching the roof sheathing is to sheetrock and tape and paint, caulking the seams around the walls with latex (drywall compatible) caulk before taping to air seal those areas. Although sheetrock does not have any considerable insulative value, it is inert and provides thermal mass and a fire barrier. Sheetrock is also cheaper than foam and fairly easy to seal; the paint provides the main moisture retardant. In all cases such as yours, the room becomes easier to heat because no warm air escapes. It can be covered with shiplap for appearance, perhaps the same shiplap that is on the ceiling at present.

I take it the idea of sheetrock is not appealing. When you say your ceiling is "open concept", does that also mean that the rafters are exposed, and the shiplap is infill? If that is the case, I have other options for you.






---------- FOLLOW-UP ----------

QUESTION: Sorry, My husband wanted me to tell you that we're considering PROFESSIONALLY APPLIED spray foam insulation.  Yes, the rafters are exposed, which would allow for at least 2" of foam between them (thus, completely airtight) then re-applying the ship lap just for looks. The foam should prevent any moist air from ever reaching the metal backed foam insulation that's in the ceiling.  Since the floor is not yet insulated,  we'd probably use the same company that is going to apply foam to the bottom of the camp to then insulate the ceiling.  Mark feels that the foam insulation may also add a certain amount of structural reinforcement as well.  The building is only three years old, and again, the amount of moisture is really minimal, although drips on rugs, furniture and the wide pine wood floor would eventually add up to being more than just a nuisance.  We really appreciate the input!  I guess I should have been more specific about the spray foam being professionally applied, not the kind you'd do yourself from a can!

Answer
Well, that is a much better scenario. Spray-on foam done professionally will probably seal the air leaks and provide another vapor retarder. Foaming under the floor will definitely be a great help in keeping the cabin comfortable.

The  other way you could do this is remove the roof metal and insulate and air seal from above, essentially thickening the roof structure as much as an additional layer of foam or as little as a sheet of plywood, depending on what is already there.  A small cabin could be done very efficiently without damaging the interior or requiring any tedious interior infill and trim work between the rafters. I always build roofs this way with a timber frame home and on any building that has an open ceiling and needs additional insulation, since it is by far the cheapest, neatest, and gives the best results.

Since the condensation is coming from the warm interior air migrating to whatever roof sheathing you have--the metal or plywood surface exposed to the cold--you can seal the air flow, provide ventilation beneath the cold sheathing surface, and solve the problem.  Do you know what is above the foam? (By the way, the condensation is not coming from the foil facing the room, in case you wondered, since this would be room temperature.)

If you know what the roof structure is, I could recommend a method of doing it from the exterior.