Re: insulating crawl space
Frankly .... the mention there are open vents to the outside and there was a functioning heat duct in that space has me shaking my head in disbelief.
If the walls , floor and attic of this addition were well and properly insulated along with proper air sealing will be the first major step.
The way I see things ....... The vents to the outside should be completely sealed closed.
The 3 perimeter walls of the crawl space should be well and properly insulated and sealed. The insulation under the floor can remain .
Since the floor of this crawl space is covered with concrete you should be able to include this area as conditioned space by opening the heating duct and leaving the access window open to the existing basement. If leaving this open is an eyesore then have a louvered door or grate covering the opening allowing air circulation to this area.
I'm guessing the furnace is located in the basement with the heat and return ducts located at floor level in the addition ..... where they should be.
The questions that come to mind are ..... was the furnace size and the cubic feet per minute ( CFM ) delivery considered with the 16x28 addition ? Also ... the size , run and placement of the supply and return ducts?
This is one area I suspect that might be a contributor to the issues.
If there isn't enough air being delivered to this room because of undersized or very long run supply ducts or the furnace isn't producing enough this would certainly cause a cold room. In other words .... if the air coming out of the heat vents is cool and/or low volume would be an indication of this and wouldn't heat the room very well.
You might have a heating contractor evaluate your heating system .... they can measure the CFM and temperatures being delivered to this room.
Hopefully this makes sense and helps.
"" an ounce of perception -- a pound of obscure "