How to improve insulation in a single room

Question

I would like to stabilize the temperature in our nursery. Currently we need to overheat/cool the rest of the house (using central forced-air heating and cooling) to maintain our desired temperature in a single room. The room is on a southwest-facing corner on the top floor and has two large windows on the exterior walls. We are located in a very temporate region, where the highs in the summer are rarely above 85f and the overnight lows in the winter almost never get below 40, and we do not have much humidity in the summer (Zone 3, near the coast).

Currently the windows are aluminum double pane, but the seals on the panes have "blown out" and air can move between them. Otherwise they are in good shape and there are no obvious cracks or drafts. There is no insulation in the walls, and there is 4" of paper-backed fiberglass batt with some gaps in the attic above. The building is modern construction (1994) but has very little insulation due to our locally moderate climate.

My first step will be to rearrange the attic insulation to eliminate the gaps and caulk/foam any gaps around the ceiling fixture box.

My question is, after that, where will I get the most bang for my buck? Should I install new windows, blow cellulose insulation into the exterior walls, or add another layer of batting in the attic? Will any one of these help without doing the others? Am I wasting time trying to improve one room without looking at the rest of the house?

I also have access to a thermal imaging camera if that will help make this decision.

I found a few related questions, but none of them directly address my situation (single room, willing to do moderate improvement projects.)

Most efficient way to improve my house's heating

Most bang for buck approach to insulating home

Insulating my Room for the Winter?

Answer 1

Your #1 enemy is solar gain

The southwest location of that room is the major problem on keeping the room conditioned to spec. It catches the brunt of the sun, which affects south more than north (for obvious reasons), and affects west more than east (because the east side of the house is still cold from the night).

This means the house's balance completely changes hour by hour.

I hope you're pausing to consider what that means...

"Wait. So if I have a one-zone heating system with one blower, that distributes heating to different rooms via ducting... and that ducting balance is tuned either by design or by adjustable flaps... and that is the normal setup for most homes... that is imbalanced half the time?

Yup, that's what I'm saying. Not half the time exactly, it might be balanced for sunless times, in which case the sunny rooms are "too warm" but the rest of the house is right.

It gets a lot messier if the thermostat is located in a sunny room. Now you have the tail wagging the dog: the thermostat is treating its amount of solar gain as if it's heating the whole house. So solar gain makes the rest of the house get colder!

Solar gain is around 340 "BTU" per square foot that is straight-on to the sun. So if the sun is at a 45 degree angle to the wall/surface, that means 340/sqrt(2) or 240 "BTU" per square foot. Regardless, we're talking a lot of heat gain.

When I say "BTU" I mean the HVAC industry unit, which is actually BTU/hr.

Insulation will only help somewhat.

Insulation will reduce overall heat gain/loss... somewhat... but it cannot eliminate the fact that south and west will catch a lot of heating from the sun. The balance of a 1-zone house will still be thrown off.

The only way to really balance it is to have a multi-zone setup, or have an auxiliary heat pump just in the room which adds heat or cooling as needed.

The cheapest way to keep the room consistent is to place the thermostat there. Of course that'll be the tail wagging the dog, and the rest of the house will take a wild ride.

Answer 2

By all means make use of the thermal imaging camera but I believe you'll find that the uninsulated walls and the "blown out" windows are your largest culprits.

You can probably put 6' of fiberglass in the attic and not make a dent in the heat loss.

If you really want to do this properly, replace the windows and while you're at it, add insulation to the walls. The "blown in" through holes drilled in the walls may be your best bet.

Answer 3

Take window replacement for a test drive: install a shrink film kit on the interior side of them. These kits include a roll of double-stick tape for affixing the clear heat-shrinkable film to the window frame. It's virtually the same shrink wrap film retail products have been packed in for decades.

This film will effectively stop the air infiltration/exfiltration problem, simulating replacement of the failed seals. It'll also add a little bit of insulating value, somewhat comparable to three-pane or coated glass units.

The result of the test may excite you (new windows first!) or it may disappoint (maybe insulating the walls first might yield better results).