I have a 9 foot square room at the back of my house we have always called the summer kitchen. It's uninsulated and is at ground level, attached to my back door. Consequently, we can only use it in the summer. It would like to use it all year round and want to know the best way to insulate it. Should I get blown cellulose in the walls, ceiling and under the floor, or what? There is a little crawl space under part of the floor (about 2 feet decreasing to 12 inches as the ground slopes). Years ago my husband put some fibreglass batts under the floor, but there's no vapour barrier etc. The room is stuccoed on the outside, like the rest of my house.
You have a common desire, like many people who live in older homes with front or rear unheated porches. I see many of these structures, which are often minimally or improperly insulated before extending to year-round use. If not done properly, this can cause various problems, which can damage the structure over several years of use. Caution must be exercised before attempting this retrofit to prevent costly damage and repairs.
One major item must be evaluated before the decision to heat your “summer kitchen” is made. The design and structure of the roof is the key to this decision. Quite often these rear porches were built with a low-slope shed roof, which was nailed or secured to the exterior wall of the house. If this looks familiar, insulating and heating this area may be very difficult. There is normally a lack of sufficient room in this style of roof or attic to allow for installation of adequate thickness of insulation or proper ventilation.
These shed-roofed porches are the ones that have most of the problems. Too often these roofs have a few inches of insulation stuffed in between the rafters and covered with drywall or panelling on the underside. The roofing may be standard shingles, which makes the situation even more problematic. If these rooms are used as 3-season, unheated spaces, this design does not normally create any excess moisture problems. As soon as heat is introduced in the winter, the trouble begins.
Heating the living space below an unvented and poorly insulated roof structure will cause heat and moisture to escape the roof causing premature damage and deterioration to the roofing. Once a significant layer of snow builds up in the winter, the escaping heat will cause melting and ultimately ice damming and leakage. This will cause moisture damage not only to the roof system but may also leak into the exterior walls and older windows, causing major rot. The only way to avoid this is to install costly blown-in foam insulation between the rafters with extruded foam insulation underneath, to provide a tight air-vapour barrier. This is rarely done due to the limited ceiling height and the expense and difficulty in proper installation.
If your room has a decently peaked and pitched gable end or hip roof, then heating and proper insulation may be a possibility. If there is room to install a polyethylene air-vapour barrier and insulate the ceiling and provide adequate roof and soffit venting, heating could be a reality. An attic access hatch should be installed after completion of this process to allow for inspection and repairs as required. Once you decide if this is a feasible design possibility, then the floor and walls can be addressed.
Heating an older building with minimal insulation in the walls and only in the floor above a shallow crawlspace is possible with some modifications, particularly in the floor system. Insulation of the exterior walls should be the last item of concern, and may not be essential in making your decision. An access hatch should be installed in the exterior skirting of the crawlspace to allow for entry and installation of proper insulation, vapour barrier and heat. The fibreglass batts should be removed from the floor, as their function will be all but useless. Because heat rises, and the floor in not well sealed, the floor insulation will not prevent much heat loss or cold air infiltration. The proper location for insulation is in the skirting around the perimeter of the heated crawlspace.
Because this insulation may be subject to moisture damage, fibreglass is not the best choice. Rigid foam insulation is a better option, due to its resistance to moisture damage and absorption. If the crawlspace is accessible, this insulation can be installed on the inside of the skirting and the dirt floor and inside of the insulation covered with a 6-mil polyethylene air-vapour barrier. This will prevent moisture intrusion into the newly heated space and condensation in the insulated skirting. The final step should be to add heat to the crawlspace, by extending a heating duct from the existing furnace ducting or installation of a thermostatically controlled electric baseboard heater. This will warm the crawlspace and provide a comfortable floor.
Far too often homeowners decide to change the usage of an old building or section of the building without adequate thought to the possible consequences. You have asked a very important question before making a decision about a change that could be very damaging to the older structure if not done correctly. If your “summer kitchen” has the shed style roof structure discussed, limited access to the shallow crawlspace, older single-pane windows or other problematic components, leaving it as an unheated 3-season room may be the correct option.