Asked on Mar 27, 2012

Sealing around a vinyl siding mounting block

Clay B
by Clay B
I had to remove a wooden mounting block on my vinyl sided house (it was there when I moved in), of course the wood rotted. I purchased a jumbo mounting block to fill in where the old wood block was, and old J-Channel. I used a siding removal tool, installed the new mounting block like the one shown. I'm just not satisfied with the bottom corner in the red-circle, to keep water out. Now I installed this on an existing house, and have 2 light fixtures to do yet. I don't know how good the house-wrap is underneath, so I don't want any water seeping in over time and doing damage. I have searched the web, and found nothing. I've seen "Big Stretch" caulk, to seal the box to the siding, and then put the trim ring over it. Any other ideas?
  15 answers
  • Peace Painting Co., Inc. Peace Painting Co., Inc. on Mar 27, 2012
    The caulk idea sounds good. I would fill the gap deeply all the way around. CP
  • KMS Woodworks KMS Woodworks on Mar 28, 2012
    Some backer rod could help keep the caulk from failing
  • PFG PFG on Mar 28, 2012
    What about filling the void spaces with the spray foam that does not over expand for keeping unwanted pests out along with no R value breach? Then install a thick lining of this backer rod KMS Woodworks mentions above with extended gray wear caulking when setting it exactly as wanted with trial fit first of course.
  • KMS Woodworks KMS Woodworks on Mar 29, 2012
    PFG...If the foam is done if such a way as to support the caulk then the backer rod becomes unnecessary. You just needs to allow the foam to fully expand and cure...then trim an contour excess it needed.
  • Clay B Clay B on Mar 29, 2012
    Thanks to all so far, need more input.
  • You never want to spray foam into a wall cavity...the fumes can build up and actually be explosive. Use back rod and seal the back of it with a product called Through the's a clear elastromeric roof sealant by Sashco but it's not paintable. It will act as an adhesive and is good for 10 to 20 years in this application. You can get it at most Ace Hardware stores...just ask for it!
  • Clay B Clay B on Apr 02, 2012
    Hi all, I want ahead, and installed them, and used "Big Stretch" caulk, it was really nice stuff, and can finger smooth it out (unlike silicone types). Since siding moves due to temperature changes, felt this would be the best type. I caulked it around it, pushced the trim ring into the caulk, and caulked a little more, then cauked the top of th trim ring and down the sides of the first 'clapboard' to help shed water to the sides and away, as I didn't want ANY water getting behing it. My home has no house wrap, the walls are just vinyl siding, 3/4" foam, studs and fiberglass insulation, and drywall; there is no plywood on my exterior walls! Nice building codes in 1988 when it was built; I'm told there is plywood at the corners of the home. I plan on re-siding in a few years anyway with Hardiplank, so I'll plywood it then too.
  • KMS Woodworks KMS Woodworks on Apr 03, 2012
    No wall Sheathing? I've seen a handful around me that were sheathed with Celotex...over wimpy ply. But the Sheathing provides more than just a surface to apply prevent racking of the wall. You may have some in the corners doing this...older balloon framed walls would often have "let in" diagonal bracing to help withe the racking.
  • Peace Painting Co., Inc. Peace Painting Co., Inc. on Apr 03, 2012
    That's rough, Clay; nefarious. Our Hometalk lawyer friend Kevin, says it's a shame contractors cannot be tracked down.
  • Clay B Clay B on Apr 03, 2012
    Peace Painting...I'm pretty sure I know who built it (name on electrial panel), but from what I undstand, this was common practice back then (1988). I'm told there should be plywood/waferboard at the corners. I'm going to check further into it, if it was not legal when it was built, that buider may not be happy.
  • Clay B Clay B on May 15, 2012
    Did my own idea to fix water issue. As I mentioned, I was not satisfied with any water getting behind the siding, as I have no housewrap; just vinyl siding, insulated foam sheets, fiberglass wall insuation , then the interior drywall. So I go these Jumbo Outlet Mounting Blocks (with built in electrical box). I used Big Stretch caulk (by Sashco) in almond color to seal the edges, seams. No no water will get behind my siding here, and this super stretchy caulk, will move with the siding expanding/contracting. Got mounting blocks from Allied in Chesapeake, VA, blocks made by Mid America. There was rotten wood here before, looks much better, and I don't have to worry about water penetration like the first pic which allows water to get in behind the siding, this may be the 'way it's done', but I just did not like it that way. The Big Stretch caulk is very easy to work with, it's like silicone, works easy just like interior latex caulk, and can smooth with finger.
  • KMS Woodworks KMS Woodworks on May 15, 2012
    as with any caulking and painting project it is important to inspect them periodically this way if the caulk begins to fail you can make repairs before any damage in done.
  • Clay B Clay B on May 15, 2012
    I will keep checking it, this caulk it supposed to last a long time, and stretch really well. I did two light fixtures the same way, to replace the rotted wood mounting blocks.
  • Clay B Clay B on May 15, 2012
    Thanks to everyone for suggustions. Helped me formulate this solution.
  • Sheldan Picard Sheldan Picard on May 13, 2021

    What size backer rod would you recommend to use?