Need Advice from a Home Builder

He needs to take another look at how the room is being heated. The system in your house might not be large enough to provide correct heating to the new room because it was chosen and installed based on the square footage of the original house without the garage. Once it's determined that the furnace can handle the additional room then he needs to install a return vent.

Hubby has built several houses including the one we're currently in and each heated room has a vent for heating and a return vent. The few times we've had issues with heating it's always been because the kids covered the return vent by accident.

Not a builder but I've watched several houses being built or renovated and plastic sheeting is not a normal part of that process, at least around here. Plastic sheeting would trap moisture and contribute to rotting and mold and isn't something you really want in your walls. Now..the construction that you describe sounds pretty typical *except* that most additions around here have a layer of Tyvek on the outside - so you would have your siding, then Tyvek, then the plywood, then the frame with insulation tucked into the frame, then wallboard. Tyvek is a breathable air and moisture barrier that limits air flow without trapping moisture in the walls in an unhealthy way. It's certainly not a requirement, but might help.

In any case, Tyvek would be applied to the outside of the house (you could take down the siding in the spring, add the home wrap and then reinstall the siding) so there's no need to hold off on the carpeting. Go ahead and install the carpet and see if that helps. Also, how do the heat vents in the converted garage compare with the heat vents in the rest of the house? Are the vents fewer and smaller? Do you have baseboards in other parts of the house? If the heating is different, you may need to consider installing more vents or larger vents. In the meantime, there are many safe electric heaters out there that you can use to make things tolerable for now.

Was a building permit, approved for all this home improvement and is it all up to code?

Insulation is not the same as a moisture, barrier.
Tyvek, was not used.
Tyvek is what most all construction companies, use per residential and commercial projects.
And you have no return vent.

More than likely it is the lack of a return. For whatever reason our builder thought the large return in the family room would be enough for the kitchen as well since it is open, nope! Add a return, you will see a world of difference.

I would never use a plastic moisture barrier in a garage, just asking for mold issues.

The only thing I can think of is that the insulation is not sufficient.
You could spend a lot of time/money tinkering with this or just hire a builder to get it done right once and for all.
An icebox with carpet in it is still an icebox - it's not going to fix the cold problem and will do nothing to fix it being too hot in the summer.
Consider getting a home energy audit - someone who will take infrared pictures of your home inside and out - and you'll see exactly where the hot/cold spots are.

http://www.oxbowinspections.com/energyaudit.php

http://www.dasolar.com/energy-audit/idaho/boise

Yes, the plastic should have been put back up as a water barrier. It however does not really help with the cold. It sounds like the insulation is good enough, but remember no matter what you seem to be surrounded by concrete. This hold the cold and heat no matter what you do.

My suggestion would be to get an electric heater or even one of the electric fireplaces. I have one in my living room and it helps keep that room a little warmer during the winter.

I am not sure about your area, but it has been a colder winter than it has been in the last 2-3 years. I was wondering why we have gone thru so much oil already and still happen to be cold. Both my husband and Dad had to remind me that we were having Jan.-Feb. type temps in November and December.

Garages are not built the same as living space so converting one will need a separate heating/air conditioning along with good insulation. Your heating/cooling system was put in to cover the square footage of the living space so since you have expanded you can install a larger heating/cooling system (expensive) or add separate heating/cooling to the added room. Adding carpet will not do much in warming the space.

Can you add a nice wood stove to add warmth? Something like that would also boost the home value. You can also install electric base board heaters which will also boost the homes value. In the summer you will need to add a window AC unit or invest in a small central unit.

I'm a GC in California. Obviously, our weather is mild here, but that's how we would build it (I am assuming you have a moisture barrier between the wood siding and the plywood - that is code and unless the exterior siding was removed, it is probably still there). The problem isn't the plastic sheeting/lack thereof in between your insulation and drywall, it's your HVAC system. Call an HVAC company and see what they'd recommend. It might be as simple as adding a damper system to your existing ductwork. At worst, you may need to upsize your HVAC unit and/or add a new unit. A lot of houses are built with a barely adequate-sized system, and with the addition of another 200-400 square feet, the whole system is on overload trying to keep up. They even make single-room ductless systems that can be installed inexpensively, and with a programmable t-stat, that might solve your problem.

If it turns out your HVAC is the right tonnage, then you can add insulation to the interior of the walls after the fact. I know you already have insulation there that meets code, but code is pretty minimal, really. They can shoot insulation into your walls and add more in the attic for a relatively small cost.