Have you ever pealed strips of silicon or silicon-based caulking after a few years? You know, when it easily pulls off like a long elastic band? Not a good seal any more, apparently.

I have read that silicon is very mildly corrosive, you can see this on a failed old seal, there can be a small layer of dusty looking material on the contact area. My experience has seen this worst on metals (particularly bare aluminum), and concrete (including stucco). Pretty common materials that you encounter when making seals on the outside of a house!

I ran across Polyurethane Caulk in major retail building stores about 10 years ago. About 1/2 again the price of other premium caulks.

My late 1970s home was a sieve after 15 years (thin coat stucco simply butted up against jumbo concrete brick chimney for example). There were 1/4″ gaps right to the outside in the crawlspace! Inspection showed plywood sheeting underneath up to 1 1/2″ gaps from the chimney. That kind of stuff everywhere. Believe it or not, the foundation has no footings, the 8″ walls just sit on the dirt. Municipal inspection branch looked this up for me, apparently yet another “cost-efficient” building by-law experiment that was allowed due to high strength of clay soil in the area… Well the entire house has settled, cracks in foundation, in one room upstairs the floor has a slope of 1 1/2″ per 8′ (good for drainage, I guess). So, cracks in exterior in many places.

I also found the mortar separating from the concrete bricks all over the fireplace chimney (yes, I immediately stopped wood-burning and purchased a gas fireplace insert with a zero clearance chimney run up inside the old chimney). But meanwhile, moisture readily able to enter the house exterior through the cracks.

Everything was sealed up using the polyurethane caulk in standard squeeze tubes. I just checked today again, every seal is amazingly strong after 10 years. If you try to separate with a tool, concrete or stucco comes with the caulk. As flexible as day one. I am impressed.

Plus:

• Bonds to exterior house material very, very well for a long time.
• Retains day-one flexibility for expansion & contraction.
• Takes paints like a dream, paint adhesion is excellent
• A homeowner can handle this application themselves, but watch out for items below.
• Actually, really seals foundation wall cracks, but I found concrete needs to be dry so dig out to air and do this in summer.

Minus:

Biggest problem is with application.

• Very stiff and hard to squeeze out of the tube, near impossible at even 5C. I had to purchase a gun with ratchet lever. Good work-around was outside barbecue, one burner set very low, keep a couple of tubes on warming rack while you work & keep swapping out. Doesn’t have to get hot to start flowing again, seems around 30C is good temp.
• REALLY sticky, don’t expect to be able to smooth beads after it comes out of the gun. Apply as carefully as possible & let it be.
• Horrible clean-up, I found acetone was only solvent & that stuff makes me nervous as soon as I open the lid!
• Apparently the caulk is very toxic as well. Need protective gear per label instructions. Rubber gloves for sure!
• The tube product seems to have disappeared from standard retailer’s shelves, I haven’t seen it for a few years now. Maybe above problems are to risky, particularly when other less effective caulks will be needed again in 5 years.

My house has had a tight exterior seal since this work was done, at a tiny fraction of cost to other remedial measures. I swear by this stuff, but only outside the house where aesthetics are not so much an issue.