The building codes are changing, adapting to both the rising cost of heating and the information on thermal bridging and air leakage that building science has uncovered over the last 35 years. The newest energy codes have requirements to address thermal bridging and blower door CFM50 standards.
Building techniques like SIPs, ICFs and advanced framing techniques are gaining new relevance to address building shell issues of thermal bridging and air tightness. Double stud walls are another approach gaining in popularity for efficient building.
What are Double Stud Walls?
The double stud wall system is a high R-value, lower air leakage method of wall construction. The builder constructs an interior stud wall and an exterior wall with an intervening gap, which eliminates thermal bridging. This building method produces a 10-12 inch deep wall cavity (depending on the size of studs used) and much more space to insulate with.
The most common variation (and for obvious reasons, there are many) is with a 2 x 4 structural (meaning load bearing) exterior wall and a 2 x 3 non-structural interior wall. A vapor barrier is installed usually 6 mil poly but sometimes foil faced polyisocyanurate foam board. Between the interior and exterior walls is left an approximately 2 inch gap.
The wall is insulated with dense pack cellulose and the gap between the two stud walls is filled, again with cellulose. The exterior is finished traditionally with either OSB or plywood sheathing, water resistant barriers and siding.
R-Value – The obvious winner here is R-value. Without resorting to any high price spray foam or adding difficulty to the project with exterior foam board, a double stud wall achieves spectacular R-value compared to standard construction. Effective R-values land in the R-30 to 35 range typically.
Thermal Bridging – Double stud walls go a long, long way toward eliminating thermal bridging. The framing around windows, doors and the top and bottom plate still bridge the width of the building enclosure, but overall the double stud wall eliminates most thermal bridging and greatly increases whole wall R-value.
Ease of Construction – I wouldn’t call building double stud walls easy. More like relatively easy. Compared to SIPs, ICFs or advanced framing, double stud walls do not require additional training beyond what contractors do every day. They’re just doing it twice.
Insulating a double stud wall can be tricky as well. If attempting dense pack cellulose in a 12 inch wall cavity, experience is essential.
Accommodations need to be made for windows and doors through a one foot wall assembly, but otherwise the whole project is within a standard contractor skill-set.
Air Leakage – Knowing that dense pack cellulose will slow air flow a great deal better than fiberglass, it makes sense that a whole lot of dense pack cellulose would retaird air flow even more. And it’s true. 8-10 inches of dense pack cellulose does a bang-up job of tightening the wall assembly, getting closer to the air leakage standards.
Important caveat is that in no way does it count as an air barrier.
Cost – An obvious disadvantage will be cost. No matter how you twist things around, doubling up the walls will cost more money. Even mimicking advanced framing (24 inch on center, eliminating redundant framing) still leaves a double stud wall with about 50% greater material cost than a standard wall. And unlike SIPS or ICFs, where economy of scale could drive down material prices, there’s no chance of that with double stud walls.
Green Material Use – This point is highly debatable and bound to get a lively discussion going on green and energy audit sites. Just how ‘green’ is double stud construction? It uses much, much more wood framing than a standard wall and unless it is a LEED Platinum or similar ‘super green’ standard, they are not recycled material.
The opposing case is that cellulose has a much lower embodied energy than fiberglass, the amount of energy and heating fuel saved far outweighs the material costs and that double stud walls achieve high insulation levels without resorting to petrochemical based polystyrene foams or spray foams. Like I said, bring this up on the right forum and watch the fireworks! As an auditor, I lean toward the energy issues rather than green material ones, but that’s me.
Double stud walls are a great way to significantly boost R-value without mucking around with exterior foam board or SIPs. Just about any contractor could manage this project. The downside is that adding that much additional wood and insulation really bumps the project cost up. Double stud walls are a technique that’ll become more common as thermal bridging and air tightness requirements work through the system.