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How long does expanding foam expand for?

(20 Posts)
Katymac Mon 14-Mar-16 08:25:57

so if DH holds a board on one side of the hole & I fill the other side - how long does he have to hold his side for? & will it stick to the foam?

& how long do I have to shave off any extra on my side?

TheyreMadITellYouMaaaad Mon 14-Mar-16 08:36:40

It will stick to the board. Though there are some things that will peel off the foam (Silicon baking liner? Clingfilm? I don't remember.It's been a good few years since I rebuilt repaired my house with this stuff!)

The foam expands visibly, then stops. It will say on the can how long it takes to set. Once set you cut off the excess with a Stanley knife and sand it smooth.

Katymac Mon 14-Mar-16 09:48:36

Hmm I thought it might


PigletJohn Mon 14-Mar-16 10:00:35

It expands by reacting to moisture. If you use a garden sprayer to dampen the surfaces first, it will expand quickly and form a crust. If you spray it into a dry area with no circulation of damp air, it can take hours.

If you are using formers to shape it, wrap them in clingfilm. It will still stick to the clingfilm but you can take the formers away when fully set.

Cover all surrounding surfaces especially floor, shoes, clothes, hair, skin and eyeballs.

Change your disposable gloves as soon as they get foam on them.

Keep the garden sprayer handy to harden any drips or smears. They will form a crust and stop heing sticky.

Cut away excess with a serrated knife only when fully set.

Katymac Mon 14-Mar-16 11:14:59

Useful tips thanks

I may layer my gloves then smile

I have a water spray (for ironing rather than gardening but I guess it will work)

Do I do a little bit at a time or just go for it?

PigletJohn Mon 14-Mar-16 11:25:34

It expands more than you think so do it a bit at a time. For example do a bit on one side, then while it expands, do a bit on the other side, then go back to the first side. If you stop the nozzle will clog.

Katymac Mon 14-Mar-16 11:38:12

It's a 4 inch hole (from an extractor fan) through an external wall

SoupDragon Mon 14-Mar-16 11:46:49

Acetone removes un-set foam so check your nail polish remover and have it handy in case you get it where you shouldn't.

JeffreySadsacIsUnwell Mon 14-Mar-16 11:54:36

"It expands more than you think"


Wish someone had told DH that before he attempted to fill a crack using the stuff some years back! Took me weeks to get it off everything in the vicinity!

PigletJohn Mon 14-Mar-16 23:56:02

it is extremely sticky until fully set. You can actually (although opinions differ on whether this is good practice) prop a door frame into a hole in a wall, squirt foam into the gap around the edges, let it set, cut away the excess, and hang the door on the frame with no screws or nails holding it, just the foam.

Anything that drips on the floor will stick to your shoes and be trodden into the carpet or floor, where it will stick and harden, and be very difficult to remove. IMO it is easier when hardened, as it will scrape off and can be sandpapered away.

JeffreySadsacIsUnwell Tue 15-Mar-16 00:22:24

It was also quite difficult to remove from DH's hair. IIRC, I had to use scissors - and I was laughing too much to cut straight. Guess he was lucky I didn't think to use sandpaper - next time wink

engineersthumb Tue 15-Mar-16 05:42:21

If it's a cavity wall it's best to avoid bridging the cavity with the foam. If you did it would probably be ok as the moisture transmittance of the foam is low though it could produce a channel for moisture to run back at the wall. Also how are you finishing the surface? Most PU foams are not UV stable so need covering. Whilst they may be painted the finish isn't likely to be great. I've plastered over foam but never over a section that large and would worry that it would crack. If it is a plastered finish I would infill with mortar and pieces of block/brick/tile and then patch plaster, on the outside I would roughly cut brick and mortar in place, if your using soft brick such as a Dorset red then you can pulverise this and mix this into the mortar mix to make the mortar closer to the brick colour - though it surprising how it will all blend in anyway with a little care and some time in the elements. Foam quickly stops the breast around the knees though!

TheyreMadITellYouMaaaad Tue 15-Mar-16 07:43:59

"Foam quickly stops the breast around the knees though!"


lighteningirl Tue 15-Mar-16 07:47:49

Just place marking and thinking I might use foam to quickly stop my breasts grin

Katymac Tue 15-Mar-16 08:35:57

Behind a freezer on the inside

I was kind of planning to refix the vent cover on the outside blush

Bit worried about the gap now

engineersthumb Tue 15-Mar-16 19:00:15

Whilst its best not to bridge the gap foam does have a low moisture and thermal transmittance so you'll probably be ok. Keep an eye open and if you see signs of mould/ damp then carfully gouge it out.
Avoid contact with breast... or breazesmile

Katymac Thu 17-Mar-16 22:13:55


If it works I will update the thread (after I find a dry calm day) if it doesn't I will swear lots

engineersthumb Thu 17-Mar-16 22:32:21

I wouldn't hold up a board to retain the foam though. I'd gently apply in little blobs around the circumference gradually closing the hole. Let it expand and dry (usually about an hour but if you can leave it for 24 hrs as it will stop pushing), then cut it off. Its easy to trim, a carving knife or brad knife will do it well. Once cut you will see air pockets so not the nicest finish but if you can't see it behind the fridge you probably don't care. Remember it's not uv stable so you should at least cover with emulsion. Hope this helps.

CointreauVersial Fri 18-Mar-16 00:23:39

Ugh. Ghastly stuff.

I'm still chiselling it off our kitchen window frame a year after using it to fill a gap above. We thought it would somehow defy gravity and didn't really bother to cover the surfaces below. blush

Sheezus Fri 18-Mar-16 00:29:14

20 mins

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