Temperature Control Problems in Bedroom

Your question almost exactly sums up the situation I had at my home last August.

What I found then was that despite a properly sized air conditioning unit and what I believe to be good insulation (the house holds heat well during the winter), the unit still couldn't keep up.

I don't have full proof, but I think a few factors caused my situation and might be worth checking for you:

1. The condenser (outdoor unit) sits in an "inside corner" with the house on two sides of it and a chimney and small storage shed that partially blocks a third side of it. This prevents good airflow across the unit. [I can't do anything about this without relocating the unit, which is not a DIY job for me.]

2. We had very little wind at all during the heat wave, which also added to the lack of airflow across the condenser.

3. The condenser was visibly dirty, which makes heat transfer to the outdoor air less efficient. Lots of dirt, dust, pollen, grass clippings, etc. build up on the fins over time. Normally rain will clean it off somewhat, but we hadn't had rain in ages.

Despite some advice I've seen not to do it, I used a garden hose to wash off the fins of the condenser (it gets wet from rain, right?) very gently, taking care not to make it full of mud or to bend the fins.

I could see water evaporating immediately on contact, and I have an infrared thermometer I used that showed a 30F or so drop in temperature in short order. The AC performed much better after that, but we also started to get a gentle breeze that helped things along.

I grew up with vermiculite (now rare due to most of it being contaminated (in the ground) with asbestos), moved on to fiberglass, and am now a blown-cellulose convert.

They all work, for various values of "work."

Cellulose is inexpensive, gets MORE effective when cold, and blocks airflow well enough that a vapor barrier is (possibly) optional according to some researchers. It's dusty during install, but otherwise innocuous. I'd suggest getting a bale from each supplier (or each different brand from suppliers) and inspecting it - or from the supplier you'd go with on a cost basis (first) and if that fails your inspection, from others. I found the sample I got from a major home improvement chain was contaminated with a lot of scrap plastic; as it turns out, not only were the bales from my local home improvement non-chain cheaper per pound, they were also good clean cellulose with no plastic scraps.

Fiberglass compares well at 70F (where R values are measured) but gets quite terrible at -20F, just when you want it working hardest. It's far more prone to air movement (whether in batts, where seams are are a problem or as loose-fill.) Plus there's that whole scratchy thing going on with glass fibers.

According to "belt and suspenders" thinking and "it's cheap enough" my cellulose is sitting on top of a vapor barrier. One more way to stop air movement. Since your climate is primarily heating, your vapor barrier goes on the inside (generally, the "warm" side - complicated in climates where heating and cooling are similar.)

If you are not using the space beyond the wall, insulate the wall (and perhaps add some furring strips to get more insulation on the wall where there is room.) If you insulate the roof, you need to provide cold air channels against the roof surface to vent the roof appropriately (though with that steep of an edge, it might be difficult to get a serious ice dam, which is what roof venting/cold roof design is trying to prevent.)

Since you are gutting it, you may also want to furr out the endwall to make it thicker and give you more space for insulation, since you won't get a lot of R-value with any insulation in the space available. Alternately, and at higher cost, you could sandwich a layer of sheet-foam type insulation over the studs and under the drywall on that wall.

Generally you will also want at least a few cans of polyurethane spray foam. While you can do all your insulating with spray-foam, it's very expensive, relative to other kinds of insulation. But it's great for sealing irregular cracks and crevices.