The entire point of insulation is to minimize the thermal changes that occur between the outside and the inside. Currently this process is slowed by insulating in between stud wall cavities with either a combination of materials such as fiberglass and spray foam insulation, or a simple method using a recycled insulation material like cellulose. While all of these methods work well at slowing the thermal changes that occur between the outside and the inside, there is really only one solution to creating a better thermal barrier with minimal heat load and cooling load implications.
Exterior Insulation System
Applying sheets of foam board insulation has been used on many remodeling projects with great success and extreme failure. The hit and miss results have many insulation experts up in the air as far as promoting an exterior insulation system, but it does make a lot of sense from a thermal dynamic perspective. Take the following into consideration:
Walls
Two layers of 2 inch foam board equal and R-value of R-20. Running the sheets in opposite directions while taping the seams prevents air infiltration. The insulation provides code compliant insulation and requires no interior vapor barrier because the perm rating is met on the exterior surface. The sheeting runs up all the way past the soffit and to the bottom of the rafters/trusses. This creates a wind barrier to prevent wind washing. The insulation continues down the foundation walls as well.
Roof Deck
The entire roof surface is insulated with 4 layers of R-10 foam board insulation. An axillary sheeting is placed over the foam board to allow the roof shingles to be installed over the foam board. Closed cell spray foam is used to seal the gaps between the bottom of the roof deck and the wall insulation that butts up to the roof deck.
This type of system requires no interior insulation. Some minor air sealing may be necessary around plumbing and chimney penetrations, but other wise there are no areas for are to penetrate that cannot be controlled. This type of insulation system carries with it some other benefits:
No More Cold Windows
Because the windows would be set to the outside of the thermal envelope, the convective loop will stay in the extended portion of the window creating the illusion of warmer windows.
Air Quality
One of the worst things about new construction is the amount of dust that circulates after the project is done. This will not be as prevalent with the lack of interior insulation.
Faster Building Time
With an exterior insulation system, the subcontractors for drywall and the finishing of the interior will be able to start and finish sooner.
Like everything else that sounds great on paper, there are some definite disadvantages to this type of system:
Cost
This type of insulation can be expensive. At $20 or more per sheet (depending on the region), It could easily cost you $7,000 or more to insulate an average (1700 square foot) home. However, it will be much more energy efficient than conventionally insulated homes that cost between $2,500 and $3,000 to be insulated.
Applications
Because this is not a typical insulation method, things like fascia, window and door jambs, may be harder to install. This costs more because the contractor has to take time that would not have normally been used for these items.
Deterioration
Foam board has a life span of 60 to 90 days in the sun before it has to be removed and disposed of. Sunlight breaks down the polymers that bond the plastic particles of the foam board together. This puts a rush on the exterior which can cause shotty carpentry work.
Sound
Empty walls mean that the sound will carry throughout the house, so even though insulation is not needed in the wall cavities, it may be need to be added for sound deadening qualities.
Exterior insulation systems do not usually go to the extremes that I have illustrated here. They are usually a combination of interior and exterior insulation that is used today. However, the premise of exterior insulation systems is used on commercial buildings and is integrated into a small percentage of residential projects every year. This type of insulation method will grow as the concept of energy efficient building grows with it.
