from Lanny in Chippewa Falls, WI :
#232
I have a Wausau home. It was built in 1973. The ceilings are the cathedral beam open ceilings. Looking at the building prints there is only 3 1/2 inches of fiberglass insulation in the ceiling panels. I plan on putting a steel roof on the home in June of 2009. What can I do to insulate the ceiling panels? Someone told me a bit about dense packing the panels? How is this done and will there be any issues of moisture condensing in the panels, or in the rooms?
The panels can be insulated, but make sure it is done right.
I have to warn you this is going to be a long, but hopefully helpful, answer! It sounds as though the existing cathedral ceiling in your home is unvented (also known as a ‘hot roof’). Assuming this, the combination of a steel roof and such a small cavity in which to provide insulation could spell disaster because of the high likelihood of condensation forming on the underside of the panels. Here’s why: The roof faces the night sky and radiates tremendous amounts of heat energy to deep, black space (especially on clear nights) that can cause the surface temperature of the underside of the panels to become lower than the dew point temperature, causing a change of state (of moisture) from water in vapor form to become water in liquid form.
As an analogy to help understand the dew point temperature, think of cars having moisture forming on them when left outside overnight. Frost can form on cars when left outside overnight, even when the air temperature is above freezing. Deep space radiation of heat energy is also why dew forms on grass.
The use of air barriers and vapor retarders (usually polyethylene film) on the warm side (in our northern climate) is intended to reduce/eliminate the migration of water vapor to reach cold surfaces, but often there are many penetrations in the well-intentioned vapor retarder. These penetrations become pathways for water vapor (driven by vapor pressure and stack effect) to be transmitted, through the fibrous insulation, until it reaches a condensing surface (underside of roof or roof framing members) and becomes liquid water. This liquid water (condensation) can saturate the insulation, reducing its effectiveness. It can cause structural damage to the rafters and ceiling below, and be the moisture that molds need to proliferate. The use of a steel roof exacerbates the problem in terms of radiating heat energy to black space.
Since you are planning to replace the roof, consider exposing the new roof’s underside by removing the existing ceiling, existing fiberglass, and re-insulate using closed cell, spray foam applied directly to the metal roof’s underside. This will eliminate the underside of the metal roof being the first condensing surface as the foam will bond to the steel. The use of this type of insulation, which has a high R-value (indicates insulation's resistance to heat flow) per inch, will keep the inside surface temperature of the foam above the dew point temperature. The result is no condensation and a very tight thermal boundary (the area of the structure which is meant to keep warm air in during the winter, and cool air in during the summer), at the roof. This will help to make your home more comfortable and less expensive to heat/cool. The Building Science Corporation has a very informative Web site if you’d like to learn more about unvented roof assemblies.
An alternative would be to use closed cell, spray foam applied to the top side of the existing ceiling when you remove the old roof deck. With either choice, I would recommend the cavities be completely filled to take as much advantage of the 3 ½” as possible (approximately R-20 value). Please be advised that this method of insulating, however, will not guarantee the elimination of condensation on the underside of the metal roof, since no spray foam is applied to the underside surface.
Consider hiring a Home Performance with ENERGY STAR partnering consultant to come in and do an evaluation of the house when you start your project. Through performance testing with diagnostic equipment, a consultant can provide you with a written report or “roadmap” of energy saving improvements tailored to your home and lifestyle. To locate a consultant for your area (in Wisconsin), visit the Focus on Energy Web site or call 800.762.7077 to request a list of consultants.
Published Thursday, May 21, 2009