Insulating Walls that dont have Weather Barriers

William Robinson  |  4/17/2009 1:06:36 AM

Hundreds, if not thousands, of older homes damaged in Hurricanes Katrina, Rita, and Ike are waiting to be rebuilt. These homes are gutted, stripped to bare studs and have weatherboards or other exterior finishes that do not need to be removed.

Rising energy costs, the focus on reducing carbon emissions, and the green movement are driving energy-efficient building practices. This has put insulation, air sealing and weatherization high on the safer, stronger, smarter rebuilding list of desirable features.

What usually comes to mind when we speak of insulation? Fiberglass, right? Fiberglass insulation has long been the material of choice for insulating homes for energy efficiency and comfort. It is available everywhere and the price is right. And to top it off, anyone can install it. Unfortunately, it is not that simple.

First, the fiberglass needs to be protected from moisture-water-rain. These old homes, for the most part, were built without the now-common water-resistant barrier, which we know as housewrap or building paper. This barrier, now required by code for all new construction, keeps bulk water out and allows water in the form of vapor to pass through to some degree. Without this barrier, any material placed in the wall cavity will get wet.

Framing is the other issue in many of these older houses. If the house is more than 60 years old, there is a good chance the framing is balloon style, with no bottom plate. You can tell if the framing is balloon style by looking down between the studs. If you can see light from under the house it is likely the home has balloon framing, which means the wall cavity is open to the area under the floor. There is no bottom plate to block circulation in the wall. In today’s style of platform framing, the wall cavity ends at the floor, making the wall cavity into a six-sided compartment.

This brings us to the other issue with using fiberglass insulation in balloon-framed homes -- fiberglass insulation needs to be against an air barrier and have circulation blocked on all six sides to be effective. The open wall cavity created by the balloon framing has no air barrier.

Finally, in considering energy-efficiency improvements to older homes, it is important to remember that, for many years, the weatherboards have gotten wet, leaked into the wall cavity, drained out through the opening at the bottom of cavity, and have done quite well. If we drastically change this dynamic by stopping the draining and drying, the least that will happen is the paint will begin to flake off and the worst is there will be significant damage, such as mold, mildew and decay, and health effects from retaining excess moisture.

So – what’s the solution? How do we improve comfort, save energy, and maintain the structural integrity of these beautiful old homes? We create a combination water and air barrier inside the wall cavity, leaving a drainage space between this barrier and the weatherboard. If we create the barrier with a material that has some R-value (for example, insulation) we can maintain the benefits of the balloon framing and have our insulation, too.

Creating a weather-barrier inside the wall

In new construction, the code-mandated standard is to cover the framing and sheathing on the exterior with a water-resistant barrier. Here we are not removing the weatherboards so whatever is done must be to the inside. Any water/vapor barrier applied inside the wall cavity must leave a gap for drainage. A reliable gap can be difficult to achieve using traditional housewrap or building paper. Something with rigidity, such as sheet foam, will work better.

The solution presented here provides a moisture, air and thermal (insulation) barrier by placing 1-inch rigid foam between the stud. It uses spacers to create the gap required for drainage between the sheet foam and the weatherboards blocks airflow by sealing the joints where sheet foam meets wood members.

When the detail is completed using 1-inch XPS, a minimum R-5 is achieved. Other rigid foam boards can be used, however the XPS has the best balance of R-value and cost. XPS and other rigid foam come in 4x8 sheets in thicknesses from ½ inch to 6 inches. Most common are ½ inch, 1 inch and 2 inches.

Furring strips can be thinner 3/8 inch fanfold foam, corrugated plastic sign board, or any other material that will create the minimum ¼ inch gap and withstand exposure to the elements, since the spacers are on the “wet” side of the weather-barrier and insulation. Other materials for this detail are foam-friendly sealant, spray polyurethane foam, and insect screen.

Once the initial insulation detail is complete there are several ways to bring the wall up to the R-13, which is the code requirement for new construction. The specific method will depend on budget, available remaining room in the wall cavity, and access to other insulation products.

The installation process

Begin with a strip of insect screen at the bottom stapled to the weatherboard and laying out onto the floor. Apply the 3/8-inch furring strips to the inside of the weatherboards using foam-friendly adhesive. (Some sealants will attack the foam and cause it to dissolve. Be sure to check labels to confirm the sealant and foam are compatible.) Fold the insect screen up and fastened in place to create a block for pest entry into the drainage cavity. This folded-up piece can be just at couple of inches. The 1-inch sheet foam comes next; try to cut the pieces so you get a reasonably snug fit. Once the foam is in place seale it on all sides with polyurethane foam. If there is no bottom plate, there will be a gap between the 1-inch foam sheet and the floor. Fill the opening with foam or wood blocking and seal to the bottom of the 1-inch foam sheet.

Now your walls are sealed for air, moisture and heat to a respectable level.

For additional insulation value, add spray foam, blown cellulose, loose fill fiberglass or insulating material of choice. The air and water barrier is in place, protecting the additional, interior insulation.

The keys to success here are the initial sealing at the 1-inch rigid foam and the ability of water hitting or condensing on the sheet foam to drain to the ground through the bottom of the wall cavity. The technique can be used on homes that have platform framing, provided care is taken to create a drain at the bottom of the wall. A piece of housewrap or other water-resistant material must be installed to usher water that runs down the sheet foam across the bottom plate and to the exterior.

This weatherizing-insulation detail also will work on brick veneer homes where the walls have been gutted from the inside. In this case, the sheathing or brickboard over the framing - between the studs and the brick - is not a complete moisture barrier. The 1-inch sealed foam detail will provide a first line for insulating your home. For the brick veneer application, there is no need to space the rigid foam away from the sheathing or brickboard. These materials form a drainage plane, where the gap is on the brick side. However, it is critical to establish a moisture barrier. This is easily done with the rigid foam, but closed cell spray foam will work equally well.

Video: Insulated wall in Old New Orleans Home with Balloon Framing

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