wrote:

I have a garage that is 20' depth x 28' wide and the ceiling is 8' high.

That's 560 ft^2 of ceiling and 768 ft^2 of walls, including windows and doors.

The walls are block and the outside I had 3/4" insulation board with
furring strips and siding added on for aesthetics.

Maybe US R6.

I put 4 new 36" x 36" double pane windows with argon in the garage as well.

That's 36ft^2/R4(?) = 9 Btu/h-F of window thermal conductance.

There are 2 on the east wall and 2 on the west wall.

They might collect 0.5x36x415 = 7470 Btu of sun on a 30 F Jan day in Phila.
What's the south wall like? You could glaze south doors like this:

http://builditsolar.com/Projects/Spa...tor/garcol.htm

A 28'x8' tall version might collect 0.9x1000x28x8 = 201.6K Btu and lose
6h(T-30)28x8/R1 when the doors are open + 18h(T-30)28x8/R6 when they are
closed. If the rest of the garage loses 24h(T-30)116 through the ceiling
and non-south walls, T = 74 F, over an average January day.

... there were only cables holding up the roof rafters.

Cables holding up roof rafters?

After the ceiling rafters & I-beam were installed I placed R12 or R19
insulation with a covering of plastic over the insulation and ceiling
rafters. I then added 3/4" insulation board over that as one side is
shiny silver and helps with lighting...

.... 560ft^2/R25 = 22.4 Btu/h-F of ceiling conductance.

After all of this I noticed I got a humidity problem and the humidity
levels were near 80%. So I bought a whirlpool 70 pint dehumidifier.
I can get the humidity levels down to 48% but no lower.

No vapor barrier under the floor? Paint it?

I spend alot of time in the garage and I need heat. At least to 65-69
degrees while Im in there. 55 when Im not. I bought two charmglow
propane wall hanging heaters... only to find out that I cannot get
them hooked up if I house a vehicle in there. Which I do...

You might run it for heat and pipe the exhaust to the outdoors and run
tools from an inverter, with a CO detector...

If the south wall has no doors, you might cover it with 28'x8' of U0.58
Thermaglas Plus twinwall polycarbonate ($85/4'x12' sheet from Palram at
800 994-5626) with 80% solar transmission and collect 0.8x1000x28x8
= 179.2K Btu on an average January day. If the air behind the twinwall
is T (F), it would lose about 6h(T-30)28x8xU0.58 = (T-30)780 Btu during
the day and 18h(65-30)28x8/R7.7 = 18.3K Btu at night. The windows and
non-south walls might lose 24h(65-30)116 = 97.4K Btu/day. The ceiling
might lose 24h(T-30)22.4 = (T-30)528 Btu/day. Putting this all together,
you could make an average ceiling mass temp T = 84.3 F.

If sun-warmed air keeps the garage 70 F for 6 hours on an average day and
you store 18h(65-30)154 = 97K Btu of overnight heat and the thermal mass
in 508ft^2x5 = 2540 Btu/F of hollow blocks (which would work better with
holes at the top and bottom to allow room air to move through the blocks)
stores (70-60)2540 = 25.4K Btu, you need to store 97K - 25.4K = 71.6K in
ceiling mass. If C Btu/F cools to 60 F by dawn and warms to 60+2(84.3-60)
= 109 by dusk and (109-60)C = 71.6K, C = 1473, eg 13 4"x20' thinwall PVC
water pipes under the ceiling, with a slow ceiling fan and a room temp
thermostat and an occupancy sensor to keep the room 70 F as needed.

On cloudy days, you could circulate hot water up through the pipes from
a large well-insulated heat storage tank to heat the garage.

Nick