This would be a great idea if you want to deaden sound or improve insulation EXCEPT:

These hollow doors are usually filled with a honeycomb arrangement of cardboard, so your foam wouldn't go very far into the door. I think you are wise to consider the less-agressive foam, I used some of the other and had it crush a central vac air outlet slightly, so there can be some big pressure!

I think you're better off looking for solid doors, if you're on a budget, maybe wait for a sale, or see if you can get some second-hand. Fire rated doors might be a good idea anyway.

I suppose your doors may be completely hollow, you could drill a test hole in the top edge of the door, and look with a flashlight.

If it's hollow then go for it, but make sure you put newspaper or cardboard underneath in case the stuff leaks, it's horrible (and difficult) to remove.

Thanks much. I'll drill and take a look inside.

Since they're about 30 yr old, maybe they won't have the honeycomb paper like the doors today.

There is some type of grid inside the door to prevent bowing of the face panels.
Modern doors often use corrugated cardbord, some older doors have thin strips of wood.
You could always strip off one entire face of the door, fill with foam sheet cut to size remove the bridging), then apply a new panel.

Sometimes it just doesn't pay to fix up what you have. You will use 4 or 5 cans of this stuff or more at $4 or more a can so you will be in for $25 or more. Also, your door will be full of holes.

Buy a new door on sale or see if you can get a slightly damaged showroom model at a discount.

Saw an article in Handy Man where a guy exploded his door into an oozing mess doing it. It was funny to look at but i am sure it was a total bummer when it happened.

Maddie

I just looked at a table of R values for doors: 2.1 for hollow core vs. 3.0 for solid core....so I decided to leave it alone and instead glue 1" of extruded foam board onto the back side of it (adding R 5) and be done with it.

Don't forget, if you glue foam to the back of the door you create a code violation.

Rigid foam cannot be left exposed in a storage attic and must be covered with a thermal barrier to keep it from igniting in a fire.

You need a new insulated door for this application.

Can spray foam on one pipe 1 1/2 by 4 inch Hollow inside and How because is for door frame Tankyou..

Just a general note for spray foamers.

It's best to drill at least two holes for an application, the size of the dispensing tube. Inject in one hole, the other vents. There is a degree of judgement on the spacing- more pairs of holes may be required, say for under a tub (large area). A small amount of oozing through the vent is desirable, but provide surface protection for overflow.

If the visible area around the holes are to be finished or meet cosmetic requirements, masking should be used. The foam residue is pervasive & entrenched-speaking of urethanes.

For the OP, you need to know what's behind the door skin.

I am a DIYer, but personally, I would not do what you are doing. Besides the fact that I hate the appearance of most of these hollow core doors, it would take a lot of time, make a mess, and cost you 1/2 as much as a new door. You can get a solid pine panel door at Menards for about $60 if it is on sale.

So I just had this "genius" idea, and knew I couldn't be the only person to want to save a buck trying this.

This was the first forum that came up, and even though the OP was 15 years ago, most of the discussion seems relavent....except for costs...

I haven't tried this yet, and if I do I'll let y'all know how it turns out, but I was almost completely convinced to just go with a solid door. Why not if on sale they are $60 like what one of the comments said. Well, lol, right now I'm getting anywhere between $300-$5K...for a door...at home Depot...

That's why doing this may...may...be worth it...

It seems even after resurrecting this 15 year old thread, it is still unclear to many how a DIYer should not try to fill a hollow core door with foam. Here is a picture to hopefully put that idea to rest:

So unless you want to guess about where to drill hundreds of holes and make a certifiable mess, buy a new solid core door slab and hang it on the existing door frame.

And, even during a pandemic, the price today of a solid wood stain-grade solid core door slab is around $75 . . . for a door . . . at Home Depot . . .