anyone use backer rod??? we use a lot of it here in the NW.
What a great question flygirl!! A lot of people are missing out by not using it. It helps makes a 20 year caulk last 20 years! It has advantages for windows for both exterior and interior use. I think you were mainly referring to its use on the interior for insulation, but I'll touch on both interior and exterior.
Windows should be properly sealed for air and water infiltration during installation to perform correctly. To seal the window on the interior (we'll get to the exterior in a bit), one can seal the gap between the window frame and rough opening using backer rod and caulk. Loose fiberglass insulation stuffed into this gap does not stop air flow although it will slow it down, so it will insulate the gap but not seal it. A non-expanding insulating spray foam that will not bow the jambs or void window warranties will work also (that's actually my 1st choice, since it both seals and insulates, and is easy to use). However, since some companies still have warranties that discourage or prohibit the use of foams, the backer rod solution is a fine choice. See the following graphic for the use of backer rod for interior use:
Flashing, tapes, caulks, and backer rod are among the materials used to seal the exterior for air and water infiltration. Backer rod is recommended by many caulking experts and windows companies. It has lots of advantages.
1. Without backer rod a caulk joint will potentially fail quicker. The backer rod (a polyethylene or polyurethane rope foam material) controls sealant depth - it provides a backstop to prevent the caulk from sinking too far into the gap. The caulk should be tooled concave to form an hourglass shape against the backer rod for optimal longevity. Ideally a caulk joint should have the sealant depth 1/2 of whatever the sealant width is (but a ideally a minimum of 1/4" deep). Never completely fill a deep joint. Doing so wastes caulk and makes a good, permanent bond less likely. The depth of the caulk should never exceed the width. Companies vary on their minimum and maximum recommendations - some use backer rod in all situations, others (for example) in joints only wider than 1/4" or deeper than 1/2", but again the principle is the caulk joint depth should not exceed its width and preferably should be about half as deep as it is wide.
2. It provides a back-up seal in the event of failure of the primary caulk seal and insulates the underside of the sealant .
3. It provides a firm surface with backpressure to tool against forcing the sealant to the joint walls.
4. It prevents failure of the caulk due to "three sided adhesion." The caulk being used should adhere to the sides, but not the back, of the gap. If the caulk adheres to the back of a joint as well as the 2 sides, it limits the amount of movement that a joint can accept without inducing a tear. This is more crucial in large window composites or any joint that experiences a lot of expansion and contraction.
So backer rod, when used with sealants, provides a "belt and suspenders" seal system - two different materials working to perform the same function, and as a bonus each working to extend the longevity of the other. Here's two examples of the exterior use of backer rod: