Insulation, Radiant and Vapor Barriers - Vapor Barrier Needed?

I see lots of posts about blowing in insulation in existing walls, but what about the need for a vapor barrier? In new construction, you put in a vapor barrier on the warm side. Why is it ok not to on an older house? Do the benefits outweigh the risk of condensation? Thinking about having my new house insulated this way when I replace the siding, but want to make sure I'm not overlooking anything important.

Hi Mark, It seems like a simple issue, but even here on the board you will find differences of opinion. Some of the confusion comes about because all of the other factors have to be considered. Warm or cold climate, or both. Humid climate or dry. Basement or attic. Current ventilation/air exchange. New construction or renovation. And there are more. The general rule as you stated is VB on the warm side. But there is also the rule, that no VB is better than one in the wrong place. Homes have survived without VBs for a long time, so, when the conditions are right, you can leave it out.

You say: “Thinking about having my new house insulated this way when I replace the siding” I assume you mean recently purchased as opposed to new new. If there is currently nothing in the walls for insulation, then blowing some in is good. To add a traditional VB would mean a major project. But adding insulation to an old 2x4” wall isn’t going to give you enough R-value if you are in cold country. A few months ago, (how quickly we forget), crude oil was headed to $150 a barrel and higher so people had concluded that 3 ½” even 6” walls could no longer hold enough insulation. That brings me back to your siding project, which can offer you the opportunity to add some additional rigid foam to the outside. AND that then brings in the issue of where the VB belongs, because many rigid foam boards will essentially become a VB. MY OPINION, is rigid added to the outside in cold country, needs to be a min of 2 inches. The extra thickness will in most cases keep the dew point within the rigid insulation allowing you to get away with a VB on the outside. The cost effective R-value of exterior walls was headed to R-50 at the peak of the prices we saw. Trimming and detailing is a challenge and I will have pictures of my 3 ½” rigid installation as it progresses. I’m including new windows and will hang the new siding in spring and I’m just trying to reach R-30.

Happy to expand on any issues,
Bud

Department of Energy - Vapor Retarders (Vapor Diffusion Retarders)
EERE Consumer's Guide: Vapor Barriers or Vapor Diffusion Retarders (http://apps1.eere.energy.gov/consumer/your_home/insulation_airsealing/index.cfm/mytopic=11810)

Department of Energy - Rigid Foam Insulation
EERE Consumer's Guide: Foam Board Insulation (http://apps1.eere.energy.gov/consumer/your_home/insulation_airsealing/index.cfm/mytopic=11620)

If researching rigid foam insulation, note that not all are created equal.
http://www.pima.org/SpotLightFiles/Press_RigidFoam.pdf

Thank you twelvepole, very good links. I don't often take the time to read all of the links available, but I did these.

Bud

I read your links, Bud! A recent link of yours about rigid foam sent me looking to see if they were all cracked up to be equal. They are not. I was curious because of the popular use of rigid foam in construction, especially in regard to insulative qualities, vapor emissions, and installation.

We had a poster a few years back who purchased a home only to learn there was extensive decay due to improper installation of rigid foam. That's pretty scarey. Thus, my interest has been peaked ever since. We've come a long way since the tar paper days.