I am an avid reader of your column and recently there have been several articles about proper home insulation. I have a specific question concerning the installation of insulation in the roof of my 20 x 24 ft. detached garage. I wish to insulate it to make it a comfortable wood working shop in the winter months. I will remove my vehicles before using electric heaters to warm it sufficiently as to make it comfortable to work in.
The walls are easy; fibreglass batting behind vapour barrier and drywall. The ceiling is where my problem lies. I wish to leave the 2 x 4 truss system open for storage and access to that storage. My thoughts are to cover inside of the roof with 2 to 4 inches of polystyrene and then seal with polyethylene vapour barrier. My thoughts are that as long as I am going to be cooling down the garage, after 4 to 6 hours of heated use by opening overhead garage doors, I do not need any air movement (soffit & peak vents) above the polystyrene roof insulation. What about air movement in the summer months? What are your thoughts?
Insulation and ventilation installation in garages, even for periodic heating for use as a shop, will have similar considerations to that of insulating a home. Proper insulation and air-vapour barrier installation and adequate attic ventilation is critical. If these issues are not done correctly, moisture damage to the roof structure and deterioration to the shingles is likely.
You are correct in your assertion that the walls are straight forward, but particular attention must be paid to wall protrusions such as electrical boxes and the wall to ceiling connection. Electrical boxes, which may currently exist, must have polyethylene air-vapour barrier installed behind them, to accommodate sealing to the poly applied to the inside of the newly insulated exterior walls and ceiling. The outlets and switches may have to be temporarily disconnected and the boxes removed to allow proper installation of poly or specially designed plastic vapour barrier boxes behind them. This vapour barrier can then be pulled through and caulked to the surface of new poly with acoustical sealant to provide a continuous membrane.
Insulating the ceiling or roof does pose different challenges. The simplest way to do this is to insulate between and over the bottom webs of the roof trusses. This can be done with friction-fit fibreglass batts, or loose-fill insulation installed after the vapour barrier and ceiling covering are installed. This method will limit the height of the ceiling, but will be the simplest to insulate and vent. Storage above the ceiling is still possible with plywood sheathing installed or laid down over the insulation and a removable insulated and weatherstripped attic hatch. This area should only be used for items that are not overly heavy, to avoid overloading and damaging the trusses. This storage will only be for items that are not subject to damage from freezing, but from your comments on periodic heating of the garage, I expect that was your intent.
If you are dead set on insulating the underside of the roof itself, this may be possible, but will be much more difficult to accomplish properly. Your suggestion of placing the rigid foam insulation directly under the roof sheathing, covered by poly, may lead to major condensation and damage to the roof sheathing and the trusses. This will most certainly cause premature deterioration of the roofing and limit the life expectancy of the shingles. This method may be employed to give an insulated cathedral-style roof, but adequate ventilation is critical.
For this method, the rigid insulation should be installed on the underside of the top webs of the trusses, allowing the cavity between the trusses to act as a ventilated space. This method will make installation of the poly air-vapour barrier very difficult, as it will have to be sealed around all the webs of the trusses that stick out below the top cord. You will also have to ensure that the bottom ends of all the vented spaces between the trusses are open and that continuous vents are installed at the soffits. A ridge vent will have to be installed at the peak of the roof that will allow ventilation of each and every one of the cavities between the trusses. Also, the vapour barrier will have to be covered with drywall or other sheathing to ensure fire safety.
Even when properly installed and vented, there is much more chance or air and moisture leakage into the vaulted ceiling style you desire, due to the many joints and protrusions in the air-vapour barrier. If the garage is heated periodically, there may be less chance of moisture damage, but it still exists. It may be a better use of your resources and creativity to find a clever way of installing an access hatch for storage, perhaps on the exterior gable end of the garage. This, combined with the traditionally insulated and vented ceiling, will be an improvement in performance and will eliminate the condensation concerns of your initial desire to insulate the underside of the roof. It will also be considerably less difficult to construct and may be less costly in materials.