If your main interest is heating, I'd add a through wall, exterior vented gas heater.

If heating and cooling is desired, I'd suggest a heat pump PTAC (like in motel rooms) or a heat pump in the configuration of a through-wall air conditioner. Both will work, both may require some wiring be done because the better ones are 220V.

It's not unusual for an added room to be inadequately serviced by the existing HVAC system, especially when done in a half-fast way without supplemental provision for heating and cooling.

I have a room like that, but mine is cold in the winter due to being directly over an unheated basement garage.

Since it's my library and the place I spend more time than anywhere else other than bed, I have to have it warm for me. I have inexpensively solved that problem by buying a space heater. Mine is similar to this one but is several years old.

https://www.amazon.com/Honeywell-HZ-789-EnergySmart-Electric-Radiator/dp/B009YKYH92/ref=sr_1_fkmrnull_3?keywords=honeywell+space+heater+oil+filled&qid=1551900110&s=gateway&sr=8-3-fkmrnull

I like it as it's a "safe" space heater. I can leave it on when I'm not in the room. I keep it near the chair where I sit - sort of the the side and slightly behind it. I'm amazed how it heats the entire room. Yes, I'm sure it does affect my electric bill, but even that's a lot less expensive than any other alternative.

In the summer when I don't need it, I just roll it directly behind the chair so it can't be seen.

It's been a great solution for me!

I suggest you have someone check the capacity of the existing HVAC vis a vis the new capacity required with the additional room. If it's undersized for the new requirements, then no amount of air balancing is going to remedy your problem. Your utility company might offer this service or could suggest someone who can do it for you.

Assuming the unit is adequately sized for your home including the new addition, then it's a case of getting the unit to perform properly. A HVAC technician should be able to give your system a routine check up and recommend what you need to restore it to normal operation.

If the unit has the required capacity and is working normally, then you can investigate duct sealing to minimize the amount of conditioned air that flows into unconditioned spaces. The older your system is, the more likely you are to have considerable air leakage.

Once the leakage is addressed, you can balance the system to make sure you've got the proper air flows to each space.

Another inexpensive option would be to add an in-wall electric heater. They can be set to a certain temp level to keep the room comfortable, and the extra bonus is they can be turned off to save on utilities when the room is not in use. We had a very good experience with them in one of our houses.

Find a new HVAC guy! You should first make sure that the joists above the crawlspace are insulated. Then make sure that you have no leaks in that room. Finally, you need more warm air to that room. You can accomplish this by rebalancing the ducts to all the rooms and, if necessary, adding a return from that room. This should cost a lot less that $3000!

Follow recommendations first mentioned by CR above.

If that's not enough, seal and insulate the crawl. Or, at the least, use closed cell spray polyurethane foam (ccspf) on the underside of the bedroom floor.

Need more info on the current insulation situation first

You have listed a room with every heating sin possible (and still pass inspection)/

Is the crawl on the addition separate from the rest of the house? Is it vented? Is it insulated in anyway?

Skylights lose a lot of heat. Heat likes to rise.

Large windows obviously lose a lot of heat.

Are there curtains on the windows? Cellular blinds? Easy first step.

Can you cover the skylight? A operable cellular blind would be helpful - tight to the frame.

Seal the crawl (if it is unsealed). Insulate the walls of the crawl. A well insulated and sealed crawl should be close to 50 degrees or higher (the goal is really to condition the crawl - so 70).

It is hard to imagine a scenario where buying a full on heating appliance for a single room makes financial sense. Then again what is the sqft of the room?

The benefits of trying to get the existing HVAC system to do the job are twofold: first, that you won't need to purchase and install additional equipment you may not need, and second, that whatever you spend on getting it to work better improves its function in every other room it serves.

Good luck!

a 3 season sunroom...repurposed as a bedroom

I think (s)he's got it!

I once rented a home that had exactly that arrangement. It made for a wonderful office in the summer; winter was another matter entirely.

I'm not sure if the room used to be a 3 season sunroom.

The room is 14x14.

Crawl space seems to be separate from the house. There are 2 crawl spaces in the house, and the one below that room is exactly under that room only. Crawl space wall is insulated, but I don't know if it is sealed or if the joists above it are sealed.

I was leaning towards the wall heating/cooling unit, because there can be a chance the room will be still cold and need additional system, even after insulation/air sealing. I didn't want to spend X2 money.

I will try to find a guy specializing home energy efficiency, someone who does both insulation/sealing and HVAC. The HVAC guy I brought in was a salesperson for mini split ACs.

Adding a mini split would create a separate zone for this room, and allow you to set the temperature you want in summer and winter. However at $3K-$4K it is an expensive solution. But if you are going to be using this room everyday, then it is worth it.

I highly doubt you will find a company does insulation installation and HVAC. These are two very different skills. You should improve the insulation first, then install the correctly sized mini split.

Over sizing of HVAC systems is more common than under sizing. Depending on how the heating and cooling load of the single room compares with the total load for the remainder of the level (or the entire home if a single zone system,) you may find that it can handle the additional load. Assuming routine maintenance is up to snuff and ductwork is properly sized, then you're probably looking at air sealing ductwork to minimize leakage of conditioned air into unconditioned spaces or better insulating ductwork.

If the existing system can't handle the additional load, then you can either reduce the load through a combination of insulation and air sealing of the home or install additional equipment where it makes most sense (e.g., a mini-split for the single room addition) or, if the current system is nearing the end of its useful life anyway, replace the heating/cooling unit with one sized for the total load including the new room.

Before I dropped $3 - $4K (sound stupidly high) on a heating unit that will just heat and cool one room. Especially being this close to Spring and Summer.

I would check to see what type of duct work runs to the rooms registers. If it is flex duct. I would then look at two things.

1. Checking the speed of the fan motor (some can be adjusted to a higher speed thus increasing the air flow).

2. Check into installing an Inline Duct Booster fan. They can be bought for as little as $20+. It would increase the air flow to that room by 150+ CFM.

DO NOT waste your money of a Duct fan that replaces the registers grill. I have saw them from $40 to $300 and not a single of of them work very well.. So do not believe the reviews from no source.

An inline duct booster fan will not solve this type of problem in my opinion. You can try it but I think it will be a waste of money.

Mike Your right it will not be a permanent fix. But I would rather spend $20 giving it a try before I shut down my heating system or go tearing into a major insulation job especially this time of year.