Good morning Mr. Marantz. I would like to start by saying that my wife and I very much enjoy your weekly advice column, having recently purchased our first home. I found your previous article on knee-wall ventilation especially interesting because our home is exactly like the home described in the column. Any way, enough rambling, I’ll get to the point! We were wondering how difficult it would be to convert the wasted space behind the knee-wall into hanging closet space? Are there engineering/load considerations that must be taken into account? We are hoping that this could help us in our desire to create more closet space in the home without a major renovation, as it seems there is a lot of wasted space behind the knee-walls. Any advice you could provide would be greatly appreciated.
My first advice to you is to approach any changes to attic spaces with extreme caution. Changes that restrict air movement and proper ventilation in attics can cause severe condensation problems, moisture damage to roof sheathing and framing, ice damming and damage to roofing. The spaces in the knee-wall sections of your upper floor may seem like wasted space, but they are integral attic spaces that are designed to be just that!
If extra closet space is to be installed in these attic spaces, some load considerations may be necessary if larger openings are cut into these insulated dividers, often called knee-walls or pony walls. Often these walls act to partially support the roof rafters and prevent sagging in the roof structure. If large openings are cut for door access to the spaces behind these walls, proper headers should be installed to carry the load of the studs removed. The new doorframe studs, which carry this header, will have increased load and this must be taken into consideration, also. If these knee-walls are built over load bearing walls in the main floor below, there may not be much concern. If these short walls are supported only by the ceiling joists from the main floor, extra support may be needed to prevent cracking and sagging of the ceiling below. This may be difficult to add without tearing out a substantial section of the flooring above or ceiling below.
Adding flooring in the new closet to support the traffic for the homeowners should not create much of a problem. The floor structure in this area should be the same as the entire upper level of the home, so adding a subfloor and flooring will simply create an extension of the existing floor. Insulation and air and vapour barriers between the joists may have to be removed and new ones installed before completion of the closet. New knee- walls may also have to be constructed as dividers for the sides of the closet, or to support clothes rods.
These considerations must be addressed, but the major concern is proper insulation and venting of the new closet area. The space between the rafters in the existing attic space should not have insulation installed, and new insulation will be required in this area in the conversion to a closet. Often the rafters are 2 X 4’s, which will not allow much room for insulation and ventilation space. A bare minimum of 2 inches of ventilated air space below the roof sheathing is needed for proper construction of the new vaulted ceiling. To accommodate this with traditional friction fit insulation, the roof rafters will have to be increased in depth, with additional framing on the underside being installed. Blocking above the new fibreglass will be required to maintain the air space above. The requirements for this may make the ceiling in the new closet uncomfortably low, or impractical.
Another alternative is to install rigid foam insulation on the underside of the rafters, which will allow better coverage and allow the entire space between the rafters for ventilation. This will also reduce the height of the ceiling in the closet, but may not be as dramatic as the other method. The final choice is to install insulation stops between the rafters and have foam-in-place insulation installed, which will provide the maximum insulation in a limited area, but will be the most costly alternative.
Whichever insulation method is chosen, the bottom and top of the vaulted ceiling must be partially open to allow outside air to cool the small air space below the roof sheathing. If the current soffits are continuously vented, then this may be accomplished simply by leaving openings at the top of the vaulted area to allow air to pass into the existing main attic. If the soffits are not vented, enough new soffit vents will have to be installed to allow air to enter all the cavities between the rafters. If the house is built without soffits, or the design makes this impractical, installation of gable or roof vents may be possible by creating a smaller insulated knee-wall at the back of the closet.
If this all sounds rather complex and difficult, that is because it is. Too often, in older one and a half and two and a half story homes, the areas behind the knee-walls are not provided with properly installed insulation and minimal venting. I spend a great deal of time during pre-purchase inspections explaining to homebuyers the proper requirements for these troublesome areas. It is often quite easy and inexpensive to convert a badly insulated and vented knee-wall space into a healthy attic. It may be quite the opposite when trying to turn this attic into living or closet space.