If you're the tenant, your landlord is obligated to fix this. You deserve healthy, secure, comfortable, pest-free housing for your monthly rent.

If he or she won't, and you don't want to try to force the issue legally, then I wouldn't spend any more on it than absolutely necessary. I'd install an extra wide aluminum door sweep (I've seen them a little over 2 inches wide). If that wasn't wide enough to close the gap, I'd add a length of 1x2 (3/4" wide by 1.5" high) or something similar to the bottom, fastening it with wood glue and screws, maybe with a length of thin plywood on the outside tor reinforcement.

I think I can see daylight on the side of the door too. It looks like it lacks a real stop. So I'd also add a weatherstripped stop all around the door. This could be simple molding, or even more 1x2 furring lumber.

No way would I replace an entire door for a landlord. I'm a landlord myself, and not only would I not ask or expect a tenant to do that, I'd specifically tell him NOT to.

If you're the landlord and you're doing this for the tenant's comfort and security, that's a different situation. In that case you can get a door slab at a lumberyard, and trim it to size.

Others here may have better suggestions. Good luck.

I am just a renter. But I have an agreement where I remodeled this place and pitched in money, in exchange for beneficial lease terms. It's a good thing!!

If you feel confidently that a sweeper and some weather stripping will air seal this door as well as a new slab door would, that's what I'll do! No need to reinvent the wheel.

Would you add a small strip if wood to bottom first, or just a wide sweeper?

I don't want a giant draft barreling up the stairs into our living space all winter and outlandish heating bills. (yes, I realize it's an old barn and semi inevitable, but I'm mitigating what I can!)

What you are calling a "Z" Brace is to prevent the door from sagging and thus racking.

The sheet of plywood you are planning to install is made of many, many, laminates that form the plys in the plywood and serve to prevent racking.... however, the door will be at risk of warping. You'll need to properly seal and paint all six sides of the door!

Salti..... Do you see a way to use the current door and make it airtight?

What do you suggest?

Are their slab doors for sale anywhere that are exactly 3/4" thick I can cut down to size?

What would you do?

I see your door is 28" Wide... 28" is a "Standard" width for a prehung door. They come in various lengths also - such as 78" and 80".

Home Depot, Lowes, etc have them.

You'd need a door sill also to seal the bottom of the door.

Interior prehung doors are like $120-$150 - sills I'd guess $40.

As suggested above, don't know if you want to spend hundreds on a replacement door... you would need a lock set also - you know, to keep the horses and sheep out.

If I were a tenant, I wouldn't spend any more than necessary on this project. For sure I wouldn't spend hundreds having a pre-hung door installed, although that's what your landlord should probably do. Of course if her or she did, you'd lose the visual charm (I guess that's what it is) of the old barn door.

I wouldn't mess around with making a new plywood door, either. This one looks plenty solid. Caulk any cracks between the boards, if necessary.

It wouldn't be pretty, but if you need more weatherproofing, gluing a layer of foam board insulation to the outside of the door would help. It would be kind of ugly, and wouldn't exactly be to code, but hey, it's a barn, right?

I would definitely add the stop as mentioned above, with a weatherstripping gasket.

Rethinking the door bottom problem, if you really have a 2" gap there, even an extra wide sweep probably won't do the job.

You could indeed add a length of 1x2 on the bottom of the door, but something better occurs to me. In addition to the 1x2 filler, or instead of it, I might try screwing a length of 1x4 or even 1x6 to the bottom of the door on the OUTSIDE, so that it extends an inch or so below the top of that step. I'd put a gasket on the inside of the 1x4, to seal against the outside of that bottom step.

I like the idea of adding an aluminum threshold, but I don't know how practical that would be, because from your first photo it appears that the door closes flush on the outside with the bottom step. Maybe others here have some suggestions for that.

BTW, when your kids grow up and some infuriating authority figure asks one of them "Were you raised in a barn???" he or she can say "As a matter of fact, I was." :)

The sill would need to go on that stair tread shown, which sounds like a tripping hazard...

How air tight can I get this thing with weather stripping and a sweeper?!

Doesn't look like there is enough room to fit a sill on that tread. And it appears that the stairs are in the way of installing a prehung door.

Something like this:

Draft stopper

Perhaps the easiest way to seal this up is, if you have space outside the door, would be to frame up an anteroom & insulate it. Frame up to fit whatever exterior door you can find. There are loads of used exterior doors around for cheap or even free. I would look for an intact prehung door to keep life simple.

Use pressure treated material to frame the 'foundation' of the anteroom and insulate it before installing the floor. I would use sheet stryrofoam and spray foam the gaps. (I just demolished an old insulated dog house and learned that carpenter ants will eat, tunnel through both sheet styro & fiberglass bat insulation so, if that may be a concern...)

" Doesn't look like there is enough room to fit a sill on that tread. And it appears that the stairs are in the way of installing a prehung door."

One of us is missing something. The door does open out - away from the stairs. Right?

A pre-hung door would slip right in. (unless, gasp, I'm mistaken! lol)

Way too involved with the construction project.... Lol!

Salti.... The door opens out, away from the stairs yes. I realize it won't be air tight without a sill. But how close can I get it? This is technically an INTERIOR door, as the entrance is inside of a room, inside the barn.... No weather will ever effect this door....

Forget the sill, then. Buy a pre-hung door to fit the hole. Slap it in.

If you wanted a door sill, build the lowest step out with maybe a 4x4 or two 4X4s. Need to be careful to not create a trip/fall hazard.

Basically get the bottom gap as small as possible with and install the smallest sweeper after that to seal it up?

Where in the world do I find a 3/4" thick door that isn't hollow and can be cut down to size? I looked at home depot, didn't see any, and that's why I was going down he path of a piece of nice finish plywood...

Why can it not be hollow? Why 3/4" thick? Get Pre-hung hollow core door.

Dozens of 28" Solid Core Doors.

You are not helping yourself. What size door do you need? H x W x T? Sill? (Y/N?)

Couple of folk above tried to help - they seem smarter than I - but you won't seemingly allow them.

It's more than 28 wide....its 28.75 roughly.... And the frame is not square, so I'm going to need to cut/plane/mess with, this door down to fit. Hollow door won't work, unless I am missing something and these can be cut down accordingly?

The height is also odd at something like 78" if I remember correctly.

A sill on top of a step seems dangerous don't you agree?

I want a solution to make this as air tight as possible, without some dig hundreds of dollars, and to be safe.... That's all

One of us is missing something.

Salti, LOL, wouldn't be the first time. And you weren't expecting me to know what I was thinking yesterday, now, would ya?

As stated above, I don't know WTH I was thinking yesterday but I think Saltidawg is right & that a prehung hollow core door can work.

Treegeek...per using a prehung door, it is the size of the rough opening that matters. The opening after you remove the door, door trim and the door jambs.

1. A prehung interior door doesn't have a sill so that isn't a worry.

2. A prehung is fitted into an opening and shimmed to the rough opening, keeping the door jambs plumb & square. Rough openings are just that, rough. Seldom if ever are they square, so no worry about that.

3. Trim is nailed to the jambs & wall to cover the shims and the exposed rough opening. Before applying trim, you would want to insulate the voids. Commonly used is low expansion spray foam, the one that is specified for windows & doors. The other foam will expand too much and cause the jambs to bow out.

4. It is possible to cut a hollow core door. There is some allowance for trimming but I don't know how much & it probably varies from mfg to mfg. Anyway, you can cut the a door in half, if you want to. You just fit a piece of solid lumber of the appropriate thickness into the void and glue the filler piece to the door skins & add some clamps til the glue dries. Always trim the bottom edge, not the top edge.

I turned two 32" hollow core doors into bi-fold closet doors doing just that^^. I needed short bifolds. I cut a couple of feet off the length of the doors and then ripped them in half lengthwise. Added some clear 2 x 2 solid pine ripped to 'just fit' for the fillers, bottom & lengthwise.

Was extra work but the doors were new & bought cheap from a Restore several years ago. Needed them out of the way, so went to the extra effort to put them to use. So, almost free vs shelling out for custom doors & me w/ more time than $$. I ripped some 1/4" thick hardboard strips and applied to the front of the doors to give them a panel look.

The entire structure is illegally converted. Be far more concerned about that lack of safety than the cold air under a door of a added on tack room. Just getting in and out of that death trap is a life safety hazard.

Your landlord should be reported. That isn’t a safe dwelling to occupy.

BTW 78" is one of the "Standard" Door Heights.

The OP has not done any basic research and all I see is hands being thrown up by the OP and obstacles placed in the way of meaningful dialogue!

I quit and hope others have more patience than I.

I want a door to fit in this opening. Not taking te molding off, shimming it, etc. Something that will bolt right to these ore existing hinges, that's it

I see now my terminology was very wrong and sent you all in the wrong directions! My apologies!