Nearly everyone has heard of expanding foam, or “expandy foam” as I call it. It’s the stuff that comes in a spray can at your local home improvement store. It has a long tube that screws on to the top, and when you squirt it out, it usually looks like a thick, solid yellowish foam that expands over time as it dries and hardens.
Generally speaking, most people use it for insulating gaps, sealing around newly-installed windows, sealing cracks or gaps in old roof structures, etc.
What you might not know is that it has another particularly handy use.
First, what exactly is expandy foam? Expandy foam is usually a “single-component polyurethane foam”. According to Wikipedia:
A polyurethane, commonly abbreviated PU, is any polymer consisting of a chain of organic units joined by urethane (carbamate) links. Polyurethane polymers are formed by reacting a monomer containing at least two isocyanate functional groups with another monomer containing at least two alcohol groups in the presence of a catalyst.
Well, isn’t that… interesting.
The benefits of expandy foam are that it provides a superior air barrier, waterproofing, sound insulation, and it also provides an added degree of structural support. Since it expands as it dries, it’s perfect for spraying inside wall cavities. In short, it’s handy stuff.
I just happen to have a can of the stuff sitting here in front of me. It says:
Uses: Insulation, all types of sealing, filling of cavities, gluing of panels, tiles, corrugated iron, assembly, or weatherstripping of pre-hung door or window units.
It’s the “gluing of panels” part that is most interesting. In fact, if you’ve ever used the stuff, you will have quickly realized that expandy foam is really, really sticky. It’s darn near impossible to get off your fingers – unless you have acetone.
Acetone dissolves expandy foam on contact. If you don’t use the entire can of foam, immediately get your bottle of acetone, unscrew the spray tube from the can, and rinse out the spray tube and the top of the can with acetone. This will dissolve the foam so that you can use the rest of it later. Otherwise, the foam in the tip of the can and inside the spray tube will harden, and your half-full can of expandy foam will be utterly useless. So, before you do anything with expandy foam, make sure you have a big bottle of acetone on hand.
So, as I was saying, expandy foam is ungodly sticky. The can also says it’s good for gluing tiles and such. Well, one day I just happened to be installing a window in a stone house. I needed some way to secure a wood sub-frame to the stone, and then the window itself would attach to the sub-frame. I had some serious anchors and long, fat screws to secure the pieces of wood that would form the sub-frame, but it wasn’t a square frame. It was just two pieces of wood, one on the right side of the window, and one on the left. I needed some extra rigidity somehow, and a cross beam would have been impractical.
It was at this point that a little lightbulb in my head went off. I used my trusty 1100W hammer drill, Goliath, to drill 3 holes in the stone for each board. I then drilled pilot holes in the board for the 3 long screws. Before attaching the board to the stone, I sprayed a generous line of expandy foam onto it. Then I positioned the board, screwed the 3 fat screws through the board and into the anchors in the stone wall, and let it dry.
Before the expandy foam had dried, the board moved a fair amount. It was firmly attached, but not rock solid. So, I waited.
When I returned, the expandy foam had gooshed out the tiny gap between the board and the stone wall. It had tried to expand, but couldn’t because the board was screwed tightly to the wall. After I cut away the excess hardened foam, I tried to budge the board. I couldn’t. I grabbed the thing and pulled and pushed and yanked as hard as I could, but it seemed that the board had literally become superglued to the stone wall by the “compressed” expandy foam!
I then repeated the procedure for the other side of the window opening, installed the window, and all was well. Oh, and of course, when the gap between the top of the window and the window opening presented me with an empty cavity that needed to be filled, I sprayed some more expandy foam inside and boarded it up!
The moral of the story is that expandy foam is excellent as a glue – especially for securing wood to brick or stone. I also used the stuff to glue 2 short pieces of wood to a concrete fence post in order to hang a gate. I couldn’t drill holes into the post because of the iron rebar inside, and I needed some quick and easy way to secure the gate hinges to the fence post. After spraying the expandy foam on the pieces of wood and sticking them on the post, I put 2 metal bands around the wood and the post just to hold them tightly until the foam had dried. Well, it dried alright, and despite beating the hell out of the gate, it is still firmly attached to the concrete post!
So, the next time you need a powerful strong bond, I reckon you should try expandy foam!
Top tip, off to try it on some skirting boards where the plaster doesn’t reach the floor.
ive been thinking of welding closed a back sliding door of my van. you think i could use this to seal the door?
I think that would work pretty well! It would definitely be weatherproof. 😉
I had a similar “think outside the square moment” when my very old western red cedar patio table collapsed after all the joints and fittings became rotten. I nearly threw it out but it was a useful work stand for all sorts of gardening chores so I wanted to salvage it if possible. The lightbulb went off, and I grabbed a can of foam and sprayed it around all the joints and across the underside of the panels for good measure. Thick runs of foam bulged up and around the joints. After it set, the damn thing was rock solid! The foam stuck to the old wood no problem. A marvellous result.
Okay, that’s pretty awesome! 🙂
Is it safe to use on aluminum?
I have before and never noticed any negative effects.
great, I work at a aluminum plating shop and looking for a product like this to mask parts. I would like to do some more research to find out how it would hold up in our anodizing tanks. Thank you for getting back to me.
The tile flashing on my Velux window flaps badly in very windy conditions. Could I use foam in the gap between the flashing and the tiles below to bond the two? I think the flashing is thin metal and the tile looks like slate.
Yes! That should work quite well.
I have a caned rocking chair where the canes in the seat are breaking at the inner wood frame. The caning wraps completely around the wood frame, making an inner cavity. I’m thinking about injecting foam in between the top and bottom level of caning to make a solid seat and stop the breakdown of the caning. What do you think?
I have a bouncy spot in my subfloor that we plan to put vinyl plank flooring on…could I use expandy foam to stiffen up my subfloor. The joist have 16 inches apart.
If the foam was spread over a large enough surface area, it would be strong enough to support the weight of a person or a piece of furniture once it’s expanded and dried (assuming that the foam isn’t simply transferring the weight onto drywall underneath or something like that). But instead I’d probably pull up part of the subfloor and either replace bits of it or tack in a crosspiece to reinforce the bouncy spot.
I have a deer mount that can not be changed what do you think about using this it would hold screws that go in wall
If mounting a non-solid wall like drywall, I’d get fancy anchors (the kind that expand when you turn the screw in), or one of the epoxy-based systems where there’s a screw, anchor, and epoxy that gooshes into the wall and hardens. Expandy foam isn’t very strong or rigid and probably wouldn’t support much weight. The screw would just pull right out.
Now, if you wanted to use the foam to GLUE the mount to the wall, that might actually work pretty well. The weight would be distributed and you’d just have to find a way to keep the mount pressed to the wall while the foam dries. Of course, the foam will also goosh out around the edges and make a big mess… But as a glue, expandy foam is VERY strong.
I’m thinking of using foam to fill a gap on the overhang or eaves of the roof. Some mice have found there way in. The label mentions “flammable gas”. That concerns me. Is it safe to spray outside? I also dont want it to look bulky. how can i smooth it out if at all? never used this stuff before. 11/16/19 Ginny
Just avoid flames when applying the foam, and you’ll be fine. As for your application, it’s perfect for that!
Just be conservative with your foam squirting, since it tends to expand A LOT sometimes.
If you over-foam and it starts gooshing out everywhere, do not panic. Just let it expand and goosh and drop all over everything. Then, let it dry completely. Once the foam is dry, it’s relatively easy to clean off surfaces.
If you try to clean it off before it’s dry, you end up with a giant sticky mess that eventually dries and hardens, and is even more difficult to clean up.
Absolutely DO NOT try to use acetone to clean wet foam off any surface. Acetone dissolves many finishes, painted surfaces, certain types of plastics, etc. But it won’t hurt your skin (altho you should wear gloves and/or wash it off with soap + water soon). Acetone tends to dry out skin rather effectively.
Once the foam is dry, you can cut it easily with a long knife. You can also then apply another product over it, like caulk, mastic, paint, whatever. It usually tends to remain flexible and somewhat squishable once dry, so you can also trim with a knife, push the foam back into a hole let’s say, and then cover it with a piece of wood or siding or whatever.
Lemme know how it goes!
Thank you for the information. Good day.
Some nice info thanks, I’m thinking of using pu foam to build a pygmy wall for a shed from breeze blocks ….
Should I try?????🙏
If there’s enough contact surface area between blocks and the foam is squished flat between the surfaces, it should dry and make quite a strong bond. In fact, I may give that a try myself one of these days… If it falls over, well, I learned something!
Will it work as support and insulation under a fiberglass bathtub, do you think? Any pre-cautions other than possibly bracing the skirt?
I would only use it as filler and make sure the tub is supported by something more solid.
Is it heat safe? I’m thinking of trying to use it to adhere my perimeter heating to the wall. The children keep on standing on it and it comes loose then they put small toys behind it that are very difficult to get out again. I figure this could not only attach it to the wall but also get rid of the gap where the toys are being dropped.
It should work quite well for that kind of thing.
Do you know of any brands/hacks to have a very rigid/hard curing spray foam in a can?
My golf cart’s front seat is wood and is rotted where the side handle is screwed in. I am going to try the foam to fill the wood seat on the underside and fill the wood gaps. Will this work?
Expandy foam is good for filling empty spaces and gluing things together, but it’s not rigid at all. So, if you need to screw a handle into the filler material, epoxy is better.
All good but no need to reinvent the wheel. Expanding foam has been used as a fixing foam for plaster boards, skirting and everything else ever since it’s creation.
They do make specific fixing foam that is much more expensive than the ordinary expanding foam. It does not expand that much and indeed it is a better glue. Yet, even ordinary expanding foam can be used for most projects.
Has anyone ever used it to spray inside a car seat that is sagging? I have a small hole that I could squirt it in. I was thinking I could tape it closed after spraying. My one worry is that it might “melt” the existing foam in the seat?
I’ve never tried it, but I don’t think it would dissolve the existing foam. Aside from being really sticky, I’ve never found a material that expandy foam doesn’t like.
My concern is PRESSURE. Once I removed a length of fascia board and sprayed expanding foam into the area between rafter ends — as one of several means of trying to stop a squirrel from entering my attic. (Finally found a solution, but that is not the issue here.)
The foam in expanding put pressure on the fascia board and pushed it out by inches. (I don’t think it moved the rafter ends any farther apart.)
Now to my question: I have an area of wall with no insulation in between exterior brick and interior sheet rock. And I need to put insulation in, between the studs.
I do not want to use expandy foam if it is going to crack or burst the sheetrock on the interior.
What kind of expandyfoam may I buy, if any, that will insulate without putting excessive pressure on its surrounds?
Wow, that’s some serious foam! The standard expandy foam I get at the local home improvement store (including the ‘high expansion’ type) would never expand enough to push the sheetrock out. For the particular application you’re talking about, I’d probably just use some rolls of good insulation. If you do go the foam route, I’d check with a builder or somebody who really knows, tho. Not all foams are created equal, and getting a good R-value is kind of important.
Thank you for responding to my question.
As stated, my entire house was built without insulation between the studs and not even any sheet insulation outside the studs. Yet — thanks to moderate weather in this area — our heating and cooling costs have not been high, and there has not been any mold issue.
In the room our sons slept in, growing up, the north wall sweated inside in winter, and the room felt “clammy,” so I cured that issue years ago by cutting holes about 3″ X 3″, high up, manually pushing blow-in-type insulation in to fill the empty space, and then filling the holes, re-texturing the repairs and repainting the room. It took many hours of do-it-myself time, but that wall sweated no more, and the room no longer felt clammy compared to other rooms of the house.
To put in sheet insulation now, in the closet where moisture through the brick has caused discoloration, would require making a much larger hole between each stud, so the known solution in the boys’ room might be a better alternative than that — UNLESS I can be certain that spraying expanding foam through holes woud not result in damage to sheet rock.
I aim to apply a water sealer to that area of brick wall, outside, regardless. But am not content simply to apply several coats of Kiltz to the inside of the sheet rock in the closet involved, and paint over it.
I recall hearing someone say, years ago, that there is a kind of expanding spray that is soft after it expands — like the sleeping pads used for camping.
Ah, I see… Well, one problem with using expandy foam is that it’s very light. You might have a hard time getting it to spread properly inside the wall. If it hardens before gooshing downwards, for example, you could end up with huge air bubbles instead of a properly-foamed wall. So, your manual blow-in method might just be the way to go!
Thank you for the effort.
I’d like to attach a hinged tabletop to some open front bookcases (create a desk when raised and looks like cabinet door when lowered). Got the brackets and attached to 3” wide bookcase side support. Grrr it was hollow. Will foam placed into the hollow support the bracket anchor screws?
Nope. It’s not solid enough.