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Foam Rot Repair

Repairing rot in non-structural wood trim

I’ve repaired a lot of rotting trim in the past few years—mostly window sills, door framing trim, and garage door trim. I’ve used all of the commonly accepted practices—like cutting out and replacing the rotted piece and using structural repair epoxy—as well as not-so-accepted practices, like using Bondo. I’ve come to the conclusion that they all have their place in the hierarchy of repair options.

While simply cutting out and replacing the rotted material may be the preferred method, there are times when circumstances (or budget) just won’t allow for it. The same goes for the structural epoxy. It does a superb job, but at $5.00 a blob (that’s my unit of measurement for it; 1 blob = the size of a golf ball, @ 18 blobs per tube) you can quickly spend upwards of $100 for the epoxy alone. I’ve found that after explaining the cost-benefit analysis of epoxy vs. Bondo, most customers choose Bondo without a moment’s hesitation. And who can blame them? A 1-quart can of Bondo costs about $11, while the equivalent amount of epoxy is over $200.


A few months ago, I was doing a job as a sub for another service. The job consisted of putting up some PVC trim around a window to try to match all the other previously-finished windows. The job was a set price, so cost was definitely a consideration. There wasn’t money to do it right, but if it looked bad it would reflect badly on me.

The problem/opportunity was that, after replacing the old trim, there was a varying gap between the trim and the stucco wall of between 1/2 in. to more than 1 in. What to do?

While looking in the van for a solution, it hit me. I had a can of black expanding polyurethane foam from another job. I thought that, if controlled, it could be a very convincing stand-in for stucco. So I went back in and filled the gap. Here is where my accidental discovery came into play.

While impatiently waiting for the foam to cure, so I could begin cutting it away, I found that you could push it back in and compress it. You could actually rough-mold it to your desired shape. This was huge!

I shaped it to a reasonable facsimile of stucco and primed it. When it was all finished, you couldn’t see the repair.

I’ve since developed a new system of repairing rot in non-structural applications.

The Basic Components

(Click any image to enlarge. Hit your browser's "back" button to return to this article.)

The epoxy is an elastomeric wood repair epoxy. The brand I use is made by Advanced Repair Technology, called Flex-Tec HV. There are other brands, but this is stocked by my local Sherwin Williams.

The foam is basic expanding foam like Great Stuff or, in this case, Touch ‘n Seal.

The tools needed are a vacuum, drill with rotary rasps, 6-in-1 tool (not shown) and any other tools you may have to clean out the rotted wood fibers. I also keep a caddy handy with various consumables like paint sticks, epoxy resin, sandpaper, plastic flashing for shaping and mixing, rubber gloves, etc.

The procedure is quite similar to a dentist filling a cavity. Here are the basic steps:

Find the decay. The homeowner will show you what needs to be fixed.

Determine the extent of decay by poking or digging with an awl, screwdriver, or 6-in-1 tool.

Gouge out and vacuum all the debris until you hit sound wood.
Grind away to rough out the interior, then vacuum.
Seal the cavity with an epoxy resin.
Fill the cavity with spray foam, then compress until it’s just shy of the surface.

You have a small window of opportunity to compress the foam (between 10 and 20 minutes). If you press it gently and it doesn’t stick to your fingers, you still have time.

Don’t rush curing the foam after you’ve compressed it. If you top-coat it too soon with the epoxy, it will blow out the epoxy like a big blister and dry like that.

Smooth out exterior surface.
After 24 hours, sand the high spots and prime.

The beauty of this system is that the foam is water-resistant (see “Spray Foam Waterproof Experiment” below) and rot proof. Also, unlike Bondo, the elastomeric epoxy will move with the wood, so it won’t be forced out over time.

The only drawback is that the upfront cost of the epoxy gun is between $60-$100. However, they do sell one-shot tubes that fit in a standard caulk gun for around $20. Most of us already have a foam gun—if you don’t, you really need to bite the bullet and get one. A cheap gun is around $40 and a can of foam is $20, but once you have it you’ll be amazed at how many tasks it can handle. It can be used to glue in nailers for drywall in places where a nail or screw can’t reach (like a closet under the stairs), glue up drywall, caulk and seal wide cracks, used in place of “caulk saver” foam bead, and for rot repair, just to name a few.

Additional Tips

  • Keep a few feet of 3/16-in. ID plastic tubing—cut into 1-in. to 2-in. pieces—for use as disposable tips. Not only will it allow you to get into hard-to-reach places, but it keeps you from constantly having to clean off the tip. Just pull off the old one and put on a new one.
  • Keep a can or two of cleaner with you. If you have to change out a can you can quickly spray out the screw-on basket before you put on the new can.
  • Keep a spare can of foam handy. If you run out, take off the old can, spray out the basket with cleaner and screw on the new can. This will keep you from having to clean the gun.
  • Don’t leave the foam outside in the cold; bring it in at night with your batteries, caulk, and other weather-sensitive materials. It won’t freeze, but it will thicken to the point of uselessness.
  • If you DO accidentally leave it out in the cold, fill a sink or bucket with hot water and soak the gun for a while,; or prop it over the defrost vent of your vehicle and turn the fan/heat on high.

This is how I store my cans/guns in my van. Notice how the cans are stored upside down.

If you still have some foam left after a job, store the can upside down, attached to the gun (see photo, right). My supplier told me about this. There is a bladder inside the can, and, after use, air can get trapped in the bottom and render the can useless, no matter how much you shake it. I ruined two almost-new cans by leaving them in the upright position, but after taking this advice, I haven’t lost another one.

Foam spray gun cleaning


Before I ever bought my first spray foam gun, I read all the pros and cons I could find to be sure it would be worth the investment.

I decided that it would be a worthwhile investment, as long as I could stay vigilant about closing the valve—otherwise you could wind up with $45 (or more, depending on which model you bought) of useless junk. One error by you or one of your crew and you would have a gun with hardened foam in the tube.

Well as it turns out, it is almost impossible to always remember to shut down the mechanism. It’s like that truism about windows…there are two kinds of windows, those that leak and those that will leak.

The first time I accidentally left the feed tube open I had had the gun for about a year, so I felt like I’d already gotten my money’s worth. When I did it again a couple weeks later, I wasn’t feeling as unbothered.

I figured that the gun was already ruined, so taking it apart to attempt a salvage could be no worse than a waste of time. If, on the other hand, I succeeded, I would no longer have to buy a new gun every time someone (no need to point fingers here) carelessly left the feed tube open.

My motivation came from cleaning an M16 rifle. Anyone who has been in the military has spent countless hours disassembling, cleaning, and reassembling an M16. The first time you do it, it may seem daunting, but after the umpteenth time you can do it in your sleep. There’s really no trick to it: you take the gun apart in a logical sequence, laying all the parts in order, clean all the individual pieces, and re-assemble it. Eventually the parts don’t need to be in order because you know what each part does and where it fits into the over-all assembly. These foam guns are no different, and they’re easier because they have fewer parts. Also, with today’s digital cameras you can take pictures as you go, much like leaving a trail of bread crumbs.

To begin, we have a dirty, filthy foam-clogged spray gun.

Here’s a list of the tools I use to clean the guns:

  • Lacquer thinner
  • A sealable plastic that fits all the parts
  • A pair of adjustable pliers and two wrenches
  • An M16 cleaning kit.

You don’t need the M16 cleaning kit, but if you have something like it, it helps. The brush fits well enough to clean the inside of the tube, and you can even put a patch soaked with lacquer thinner on the end of the eyelet to clean the tube. Again, you don’t need this, you just need to be creative as far as finding things around the shop to scrub and scrape away the debris.

To begin, disassemble the gun down to its component pieces, being careful not to lose any of the smaller pieces (such as the “C” clip, or the shut-off valve bearing). If you’re not sure about the order, start taking photos for your re-assembly.

Scrape as much of the foam off as you can, and place the parts into the lacquer thinner to soak. Periodically remove the pieces and clean as necessary. This requires an investment of time, since different parts will take longer than others to get clean. So take your time and be thorough.

Be aware that in addition to the steel bearing (not shown)—on which the shut-off valve spring rides—there is a plastic bearing inside the brass screw-on can adapter. This plastic bearing does not come out for cleaning, but it needs to be clean enough that it can freely move in and out as you push on it with a tool. If it doesn’t move freely, it’s not clean enough.

Below are the individual (cleaned) components of this particular gun.

And the original gun after re-assembly…

Spray Foam Waterproof Experiment

After I stumbled onto this new procedure, I began wondering how waterproof the foam really was. After reading about the differences between open-celled foam and closed-cell foam, the major difference seemed to be density. If you are spraying the underside of a roof deck, you would use closed-cell foam because of the higher Perm Rating, which basically equates to “waterproof.” Open-celled foam is merely water-resistant. My theory was that by compressing the foam before it set, I was making it denser, thereby making open-celled foam more like close-celled foam.

Bottle experiment

I’m sure that in a laboratory it’s much more technical than that, but I’m working out in the field, not in a lab, so for my purposes this theory would suffice. All I needed was an experiment to confirm or dispel my hypothesis. So I filled the middle section of an empty drink bottle with foam. Since I couldn’t compress it the experiment would be done with open-celled foam.

In the picture (see photo, right), the foam appears glossy against the sides as if it is leaking down, but that is how it looked when I sprayed it in, before putting any water into it. Also, even though it has a cap on it, the cap is not tightened. I left the cap loose so the water wasn’t being held up in a vacuum—that way, it wouldn’t spill or evaporate quickly.

I started this experiment on Oct. 27, 2010. Today is Dec. 3, 2010; so far there has been no leakage.

Update: By Dec. 21, there were drops of water visible in the bottom of the bottle…

Theory Dispelled!

To take the experiment one step further I somehow needed to compress the foam in the water bottle. After some thought, I came up with a way to do it.

I took another bottle and sawed off the bottom. This allowed me to compress the foam from the top and the bottom. I hot-melt-glued the bottom back in place and filled the bottle with water.

I have to say I didn’t expect the results I got. Within an hour there were signs of water leaking through, and within a day, all the water was in the bottom of the bottle. I was pretty confident that it would easily outlast the original experiment, but I’ve come to the the conclusion that the expanding foam is NOT waterproof, merely water-resistant. This doesn’t mean that the foam rot repair technique is no good, it’s just something to keep in mind when you consider using it.


39 Responses to “Foam Rot Repair”

  1. Fong

    Great non-conventional application of expanding foam. I had never considered using it for external patches, only internal filler. I have used expanding foam to stiffen automobile frames for a more torsional rigidity on sports cars.

    Comment about converting open-cell to closed-cell: Each cell is roughly spherical like a soccer ball (the old polygonal ones, not the current ones they use in the world cup). The balls are broken in an open cell and not broken in a closed cell. It is true that closed cell is denser. This is because it’s more difficult to maintain a closed cell at lower densities (has to do with surface tension of the foam). When you smash the open cells together, you are collapsing some of the cells into each other. The cells will most want to collapse where the cell is broken. What you’ve essentially done is increase the density of already open cells but the cells aren’t ever truly closed. In fact, they’re probably more open but just mashed together. The density itself increases its water resistance somewhat and for intents and purposes, should perform better than the unaltered open cell. You are of course, decreasing it’s R-value but this isn’t what you’re using it for. Nice accidental discovery. Sometimes those are the best.

    Bondo is fantastic and I’ve used this most often in making custom enclosures for automobile subwoofer boxes. For internal making internal corners on with wood, there’s nothing like it. Anything more organic would require fiberglass but you can’t beat Bondo for both its price and workability.

    Bondo works great for filling up door jam holes when replacing a conventional door with a bifold. After sanding and painting, you’d think it was built that way.

    • Dan

      Thanks Fong

      And thank you for the explanation of the differences between the 2 foams. I didn’t know that but after your explanation it explains why my second experiment failed so miserably.

  2. gary

    The only time I have used foam like this was between drywall and a stone fireplace. I used it to act as an expansion joint and to fill the void. I mudded over it with a vinyl spackle. This was an interior application. I would have concerns with varying rates of expansion and contraction on an exterior repair like this. In addition, moisture will condense behind the harder surface causing more rot. I too have tried a variety of products but have found that the best method is to replace wood with wood and realize that no product lasts forever.

  3. Joseph

    Don’t forget when you do a rot repair you may get the big stuff but in most cases there is still life on the micro level i.e. fungi. We can see rot but the fungi that goes with it has in most cases continued to spread into the remaining wood. I think we call it dry rot. The fungi spreads unseen until before you know it more repair is needed. We might look into using a Borate solution (AKA Borax & Glycol) very low toxicity. Check it out. Google, Borate or Boracare or Fungi and Borate. I think you can treat the surrounding wood before you do the final paint. Also the glycol in the solution penatrates most finishes and carries the borate salts with it into the wood.

    • Joe Volkmann

      I discovered this years ago, repairing wood stringers in a boat. To kill remaining fungi in rotting wood, I use a wood preservative like Copper Sulphate. You won’t be able to kill it all, but you can get the vast majority. After meticulously cleaning away the rotted wood and vacuuming it, clean it with wood grain alcohol. Then stuff a rag or cotton batting (or similar) soaked in the preservative, into the cavity. Cover it with plastic (as best you can) to slow down the evaporation, and let it soak in for several days. Remove the rag, let it dry very completely and move on with the repairs.

  4. Jim Sear UK

    Dan, this sounds like fun.
    I’m a great believer in “if it works don’t knock it” so I’m going go out and find me some rotten wood and do some experimenting.

  5. Lee Place

    Who is the maker of your foam gun? I have two guns that I can not take apart. I like the rack you store them in and the idea to use the short length tubing on the end. I have used longer lengths of tubing to foam or caulking in areas where the guns will not fit.

    • Dan

      Just went out and checked. I didn’t see a company name, it just had “Made in Italy” on the handle. I can tell you that where I bought it, it wasn’t the cheapest model but the next model up. The cheaper one was a molded unit that couldn’t be dis-assembled. I bought it from Warehouse Bay Corp at I found them on the internet then when i checked there address I realized the were within 15 min. of me, so I started buying my foam and guns from them. They are the people who told me about storing the can as you would use it.

    • Wayn Goodman

      Lee, Dan’s gun looks very much like the ones we buy from (Energy Federation, Inc.)called PUR Shooter… they’re imported here by Todol Products (the suppliers of my favorite foam gun, the Pageris) and are a better idea than the Pageris for crew – since crew doesn’t ever take as much care of a foam gun as bosses do…!

      And thanks to Fong for the good input regarding open vs. closed cell!

      And, finally, thanks to Dan for the article that started the convo – I’m already forwarding the link to my crew leaders for them to see the storage device AND so the crews can see what a CLEAN GUN looks like!!!

  6. Tim Strayer

    This is a great solution! We have been presented with some tasks where replacement of the entire damaged piece is cost prohibited. Filling is obviously the only answer then. This saves cost and doesn’t compromise the overall effectiveness of the repair. Now, if we can only come up with a way to do this on ‘structural’ repairs. :) Nice article and information!

  7. tim Kearns

    I’ve been using foam for backing when caulking large cracks and seams in wood, especially around brick mould. I started out using Great Stuff, but wasn’t happy with quality. I have been using Hilti’s product and have been very satisfied. For every case of foam they give you a gun, which is not rebuild-able but lasts at least for two years. I do a lot of insulation retro. and use the low expansion flame retardant foam.

  8. Steve DeMetrick

    Interesting approach, I will definitely try it out. My only concern would be about what the repair looks like after a few years years. We know from experience that Bondo, because it moves at a different rate than wood, fails too often when used for wood rot repairs. I’ve used the Abatron products with great success, but they can be expensive, and you have to plan ahead because they aren’t available at the local hardware/paint store. My local lumberyard started carrying a product called PC Woody. It is a 2 part epoxy paste filler that works just like Bondo, but is engineered for wood repairs (unlike Bondo, it even sands like wood). It’s only a few dollars more than the similar sized container of Bondo, very easy to work with, and should hold up very well. The oldest repair I am monitoring is almost 5 years old.

    Thanks for sharing your method.

  9. Emanuel

    Hey Dan, Nice article. Very well written. I also own a couple of spray foam guns. I still have the first spray foam gun I bought 7 years ago. Just used it last week. I really enjoyed all the info on the upkeep of the foam gun. Thanks for sharing all the info.

  10. Mike Vega

    Great article, I just love this site and all the articles. I too have used the open cell foam for cavity filling and by accident over filled and compressed it into the cavity to allow for epoxy application. I have had good success with using Borate rods around the repair areas and would like to find a less expensive way to treat rotted repair jobs. I have tried various Borate mixtures to treat repairs and would like to find some tried and documented solutions to continue to make the best repair possible.
    Thanks for sharing your knowledge.

  11. Doug Mink

    Very good article and its good to see “accidental discoveries”
    My question deals with a waterproofing issue.
    My shower stall has a fiberglass pan on the base and it is cracked and leaking. Instead of replacing the unit (expensive), could I try some waterproof expanding foam? I’d drill a hole in each corner of the 60″x30″ base for filling and maybe two small release holes near the drain. Is the closed cell the best to try?
    How would you suggest I cover the holes I drilled?
    Any further suggestions or ideas /

    • Gary Katz

      I suspect the foam would seal the crack in your fiberglass pan and stop it from leaking, but I’m not sure how you’d finish the surface, or finish the surface where you drill the holes. Maybe someone else will chime in here with a solution.

    • Dan

      Hi Doug,
      Are you expecting the foam to totally fill the void between the pan and the floor?

      IMO this won’t work because 1. the foam isn’t really waterproof (merely water resistant) and 2. even if it was there would be no way to tell if the foam spread evenly in the void. Just because it came out the weep holes you provide wouldn’t guarantee that it completely filled the void. And besides back to number 1, in my experiment it proved to be not waterproof.

      We had a crack in our fiberglass tub that I repaired well over 10 years ago with an epoxy crack repair filler. While the color wasn’t a perfect match the epoxy itself fixed the hole and we never had another problem with leaking. I found this online,

      I don’t remember the brand I used back then but this is basically the same stuff. Read the reviews to see if this will work for you and as they all point out repeatedly you need to work fast b/c this stuff sets really fast…

      hope this helps.

  12. Emanuel

    Hey Doug, I would suggest some kind of pvc filler, the ones used on exterior pvc products. Any brands like Kleer, Azek, Koma, etc supply their own filler, this would probably work. It works on the exterior why not interior. Hope this helps. Please keep me posted.


  13. Susie

    Hi! Thanks for this! I’m running a painting business and this helps me keep subbing costs down, since I can fix some of it myself! Kudos!

  14. bigbob

    I had this issue where the cost of replacement of the windows would have been extraordinary. I elected to use a 2-part epoxy with a 2 part sealant from the Rot Dr. It was slow to set up, but when put in place, it stayed put. It finished just like wood and was much better to work with than bondo. I tried to upload before and after, but it wouldn’t work for some reason.

    • Dan

      Coincidentally I just repaired a window today with the Flex Tec. The stuff you describe sounds similar. What’s the cost of it? Do you have to buy the 2 part gun for the epoxy or is it in the single tubes like you would use in a standard caulk gun?

  15. bigbob

    [img] picture.jpg[/img]
    [img] window.jpg[/img]
    [img] sill before.jpg[/img]
    [img] sill 2 before.jpg[/img]
    [img] sill interm 2.jpg[/img]

  16. bigbob

    [img] sill intermed.jpg[/img]
    [img] sill.jpg[/img]
    [img] sill2.jpg[/img]

    Took three steps:
    cut out and seal;
    fill with caulk gun and shape as close as possible;
    finish with sanding and carving

  17. Junk8punk

    Wow, thank you for putting this up, very helpful for those starting mini projects and not well knowledged in the construction area like myself. I’ve done a few projects myself and had success with my final results.

  18. christopher

    Thanks alot would have never thought of doing that with the epoxy and foam! genius!! just in time too because i didnt know how i was going to tackle a rot job on the door casing and inside framing on a tight budget homeowner. thanks!!

  19. Mike

    Thanks for the info which (was/is helpful) and for actually taking the time to educate.You should be a teacher.


  20. John

    Dan, great info. Just to think outside the box, we are all so focused to do the right or best job. I guess thats why we have good references from previous customers. But lets think of how we can convince that “tight budget customer”(tbc). Case in point. Several years ago, I had a job for (tbc) and bid on the job based on good practice and mindful of their take. But to my surprise, they gasped at the “low” bid. I should of walked away, but the slow down through the holidays said to me, discount and take the job… got to eat and pay bills, right! Well, the last day of the repairs, cleaning up and all, had to move out of the way for a delivery of a 60″ plasma TV. Back then not sure of $. So here I was stupid and mystified. I thought they have the money to by the TV but cheapskated me for good work. Lesson learned of course, but how can we convince (tbc) what it takes to do the job right. They call some trade folks and see (tbc) write a check freely, lambs to the slaughter, and not look back. Here you are trying to do a handyman’s, carpenters, and wood engineers job. Skill, experience and good info helps always. So any thoughts about re-training those “tight budget customer”? Now we need to learn psychology, LOL.
    John (JK)

  21. Annie

    Hi. Will this spray work on my16 door jams are now too short after removing all of the layers of floors at my 1954 ranch. They kept urging baseboards as they added floors.

    Thank you

  22. Rachel

    Hi. We have rot in the exterior widow and door frames around our older home which we need to get repaired. I want to get the damage fixed and then get the frames aluminum wrapped. I have two bids for the work but they are approaching it with different techniques. The higher bid wants to replace the frames with new wood frames and wrap those with aluminum. I am thinking that for what they are doing the bid is not unreasonable. The more affordable bid wants to cut out the rot fill it with spray foam and then cover that with the aluminum. I don’t want to make a stupid decision just to spare my tight pocketbook. I would appreciate any thoughts on whether I should just get the frames replaced with new wood or if there are questions I can ask the other company that if I get the right answers make the more affordable spray foam option a reasonable choice. Thank you so much.

  23. Rachel

    A correction, I am actually talking about rotten window casements or the wood trim around the window on the outside of the house.

    • Gary Katz

      ‘Casements’ refer to the window sash itself–the window frame and glass that opens.
      If you’re talking about filling the wood trim with CLOSED CELL FOAM and then covering it with aluminum, I see no problem with that, as long as the trim and foam are pained VERY WELL, 2-3 coats, before they cover it–that would be necessary whether you fill with foam or not.

  24. Allan Apel

    Dan, thanks for your awesome article which I stumbled across whilst googling for evidence of anyone using foam as a large capacity filler for wood rot voids. What started as a small hole in 1 inch thick hardwood shutters soon grew to a fist sized gap as I gouged out more soft rotten wood fibres and quickly abandoned any idea of using wood filler, passing through the need to cut out and replace with hardwood blocks, then finally thinking that a spray foam solution had to be a more efficient way to fill non-load bearing decorative panels. After all, isn’t that what surf boards are filled with ? Thanks again and best wishes from the Netherlands

    • Stuart Thomas

      I followed the conversation here started by Dan way back in 2011!
      To you, Allan, I too got here for info on a situation similar to the one you described; difference being that the situation I need to address for a (TBC) [see post #21 preceding] is rotted exterior wall shoe/sill and bottoms of studs. Wall is not carrying any load beyond its own dead load weight. So I am considering injecting a good quality expanding foam product in the cavities to lock up all surfaces with a contiguous cohesion.
      Would welcome any commentary on this.
      P.S. This site is going in my favorites list, great to get reliable and experienced minds together!

  25. Larry

    I am a bit confused. After everything has been prepared and the foam has been compressed and cured. Do you apply a two part epoxy dough? Or epoxy resin? And then do final shaping?
    Thank You,
    Larry K

  26. Mark

    I would like to know what is the best adhesive for attaching PVC to a latex-painted wall? The PVC is being used as a tile edging. It is 1/4″ x 1/4″. Also do think that it needs to be attached with some type of nail as well?

  27. Shopforn Otes

    But will it work if you just have a dumb spray foam gun instead of a smart spray foam gun with a brain like yours?


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