I have a question with regards to polyurethane spray insulation for the basement. Will 2″ provide an adequate vapour barrier? I hear differing opinions about this. The plan is to build a frame wall flush to the 2″ of spray insulation and put batts in the wall. As to whether there needs to be a vapour barrier in front of the studded wall, I don’t know! The polyurethane seems a good idea in so far as it’s sealing capacity.
You have asked an excellent question that I may not be able to provide a definitive answer for. That is because use of polyurethane spray foam insulation for residential use in basements is a relatively new building practice. I am quite familiar with the product, having attended several seminars on its use and observed its installation on a few occasions. I will answer some of your questions and attempt to provide guidance for the vapour barrier installation part of your inquiry.
For those not familiar with Polyurethane foam insulation, it is a blown-in insulation that is stored in a liquid state and cures to a solid foam product after dispensing. There are many different chemical formulas used by different manufacturers, but two main types. The first type is the single component product that is dispensed from a can that often takes several hours or days to fully cure. The rate of expansion and curing is dependent on exposure to air, temperature and relative humidity. Many people may be familiar with the small cans of this material sold at home centres, used for filling small gaps and insulating around doors and windows. Several different types of this insulation, which have varying expansion rates, are excellent for use by both homeowners and professionals.
The second main type of foam in place insulation is a two-part material that is often referred to as Isocyanate, which is one of its main chemical components. This material differs from the single component type in that it is chemical cured rather than air cured. When the two different foams are mixed together upon application, they cure very rapidly. This allows the installers to regulate the thickness of application, which increases the versatility considerably. This is likely the material that will be used in your basement to provide the relatively consistent 2-inch thickness desired.
To answer the first part of your question, this heavy thickness of foam will provide an excellent air-vapour barrier as long as it is continuous in its application. Initially, this material was used specifically for air sealing around gaps and openings in buildings. Even in a relatively small thickness, it provides a very good air-vapour barrier due to its ability to flow into any shape of crevice in its liquid state. It is still used for this purpose today, in many commercial building, where accessibility to conventional air sealing techniques may be difficult to achieve.
As insulation, Polyurethane foam is quite good with a high R-value of between R-5 and R-6 per inch. The variance depends on conditions at application and differing manufacturers. What this means for your home, is that you may have an R-10 to R-12 insulation value installed over your foundation walls before any additional framing and insulation. Adding another few inches of fibreglass installed in the traditional manner with a 2 X 4 stud wall over top will give you an accumulated R-value of up to R –24, which will be equivalent to or better than most above grade exterior walls on new homes.
Now the dilemma with proper location of the air-vapour barrier must be addressed. When the liquid insulation is blown on to the inner surface of your foundation wall, it will provide an almost instant air-vapour barrier at the inside surface of the cured foam. Ideally, this should be left as the air-vapour barrier and will not require covering with polyethylene sheathing, but you desire additional insulation for better heat retention. The simple solution is to install an additional few inches of foam between the studs in the new basement wall, but that may be excessively costly. If you place approximately R-12 fibreglass over the foam, you may require poly over the studs.
The rule of thumb is that the vapour barrier does not have to be on the inner surface of the insulation, but should be installed no deeper than 1/3 of the way into the insulation. In your case the vapour barrier is already present 2/3 into the insulation and roughly at the ½ way point of R-value. This may allow warm air leaking into the wall cavity to remain trapped in this spot, causing moisture damage to the fibreglass. Installing a poly air-vapour barrier on the warm side of the studs seems like the simple solution, but now you have two air-vapour barriers, which may cause another problem.
As any professional who has torn apart an insulated wall can tell you, Polyethylene air-vapour barriers are never perfect. There are many holes and points where air and moisture can still penetrate into the wall cavity. This moisture may condense, due to the cooler temperature inside the wall cavity and must have an avenue to escape. The excellent air sealing properties of the Polyurethane foam insulation may prevent this from happening on inside of the new wall. Now you may be faced with a similar scenario to the one where the poly sheathing is left off, moisture damage in the wall cavity.
Unfortunately, I can’t steer you in one direction or the other because I have never seen the results of this scenario, in the long or short term. Speculation on the future effects of this insulation application is theoretical and only time will tell what will work. Also, you will likely get differing opinions on which way to go, no matter who you talk to.