How to minimize radiant warming of unfinished uninsulated garage in California summer sun?

Question

We have a 75-year-old attached but unfinished and uninsulated redwood stick-built garage, roughly 20x20 feet. In the unshaded California summer sun, the roof gets quite warm and radiates the heat into the garage, making it uncomfortably warm some days (e.g. 90F). How may I (DIY, less labor strongly preferred) insulate the roof to lessen the maximum indoor temps while not damaging the roof through overheating or adding fire hazard? Climate is no snow, some rain in winter, generally low humidity.

Garage construction: Wall framing seems to be redwood 2x4s, stick built. 90% of perimeter walls are exterior or garage doors; plywood? sheating, tar paper, wood shingles, then aluminum siding over those shingles. Roof is also stick built from redwood lumber, with 2x10(?) cross beams (no trusses) and 2x6s, 2x8s and 2x4s framing the hip roof with a very short ridge. Redwood 1x6s(?) form the roof sheathing, with tar paper on top and 40-50 year asphalt shingles applied about 25 years ago. No soffit vents or ridge vent on garage.

Garage is attached to L-shaped main house; house roof ridge runs into garage hip roof a few feet lower than garage roof ridge. House roof ridges DO have ridge vents and soffit vents and house attic space communicates with garage.

Aerial photo of garage and part of house shown below. Red areas are highest points of roof. Green shows vented ridge connecting main house with garage. Pale blue outlines actual square garage.

Sample photos of roof from underside; there is basically no ceiling - some junky ceiling tiles have been added in a few sections, but most is open from concrete slab to roof underside.

Proposal: Reduce summer sunshine-driven heating of garage interior by applying a layer of Class A fire-rated Thermax rigid insulation attached to rafter undersides, leaving perhaps 3" air gap to underside of 1x6 redwood sheathing. What sort of garage rooftop venting would I need to install? Garage ridge is only a few feet long, so would a single-hole vent of some sort in that area be sufficient? What size and number of soffit vents would I have to install? Thermax seems to be an easy-apply complete solution that doesn't require emptying the garage, hiring anyone, or applying additional layers to deal with fire issues.

Also, am I likely to be correct that the roof is the main source of the radiant heating of the inside? South is to bottom right in photo. Walls are painted "sand" color, so lighter than the roof color.

Edit: Would something like Reflectix do much the same as Thermax to cut radiative heating at lower cost/easier installation?

Edit2 - modified proposal (Based on reading about "short-circuiting" under-roof airflow by having multiple high vents): Install 20-30 4" round screened soffit intake vents around exterior garage roof edges (total open vent area 1-1.5 sq feet). Install reflective foil stapled to undersides of rafters to block radiant heat from roof, making sure to allow airflow passage around blocking. Leave gap in foil at top to allow airflow out into peak area of garage, from which it will hopefully flow to and out of slightly lower ridge vent along connecting ridge from house (circled in green on photo). Thus, no new top vent, only soffit vents and radiant barrier.

Answer 1

Ventilate the space under the roof.

You have about 36m2 roof surface, at noon this corresponds to about 40kW of heat to get rid of. Considering half of that will radiate and dissipate upwards in the air and the other half will radiate down, you need a ventilation that can get rid of 20kW.

Heat capacity of air is 330 kJ/m3 so you need about 100-200 cubic meters per hour (60-120 cfm) of air to get a temperature rise of a few degrees.

In other words, a standard whole house air extraction unit will do the job. Pick a cheap one, put it below the roof at the highest point where the hottest air will be, and vent to the outside. No need for intake pipe, just let it suck the air. Make sure to add cool air inlets on the sides. You can add a thermostat so it turns on automatically when it's hot.

Then you can put in a ceiling with insulation and a thermal radiative reflector on top, for example metallized mylar.

If you want a higher ceiling, you can just ventilate the space between the rafters. It's a bit more complicated to do:

Without ventilation, temperature above your insulation will reach ridiculously high values in the summer, more than 60°C. The amount of heat that gets through insulation is proportional to temperature difference across it.

So if you have 60°C above the insulation and you want 25°C in the room, temperature difference is 35°C, and the thermal power will be the same as for heating with 25°C inside and -10°C outside.

So without ventilation, insulation alone won't help much. With a cheap air extractor, you can drop the temperature above your insulation from more than 60°C to about the same as outside air, si you gain at least 20°C. Then your insulation will be a lot more effective. You can also add an air conditioner, and it will have a lot less work to do and use much less electricity.

In fact, if you're okay with the inside temperature being equal to the outside temperature, there is a much simpler solution. Get some heat reflector foil, like that:

and just hang it below the rafters, for example on a simple structure you build out of wood. Then evacuate the air above it with a, air extractor, and place the cool air inlet at ground level, on the coolest side of the garage, in the shade:

Then later you can do a proper ceiling, when you have time. Of course the latest proposal is not compatible with air conditioning.

Answer 2

Your core problem is heat absorption from the roof.

A typical asphalt shingle has an albedo of about 30%, meaning it reflects about 30% of solar energy and asborbs 70%. My supplier's best paint has an albedo of 91% (9% absorbed or 8 times better). One guess what color is on all the roofs I am able to paint.

And that works, boy howdy!

You mentioned you don't want to change your roof type or mess around with painting asphalt shingles. OK, but food for thought on next roof replacement (e.g. standing seam metal, takes paint like a dream).

Now, if you want to interrupt the radiant heat coming down off your roof, gosh - any insulation would do, either on the roof (slanted) rafters or on the flat ceiling joists if equipped. I would just go for plain old fiberglass batt. You could go full thickness if you wanted to, but R13 fiberglass (the stuff that's 3.5" thick intended for 2x4 walls) at about 60 cents a foot should be an impenetrable barrier for radiant heat.

Just brute-force it. Air condition.

At this point, you can get heat pumps in "mini-split" form-factor as high as 38 SEER. That is a tenfold increase over what you're used to in hokey, el-cheapo old window units or worse, 1-hose portables.

At 38 SEER, for an occasionally used workspace, it simply becomes more rational to throw cubic BTUs at the problem since you get so many for your buck.

And fun fact, that heat pump can also run in reverse and provide efficient heat. In fact, at typical metropolitan California temperatures, your heating COP will be very favorable, probably 4-6 on the most favorable units - meaning you get 6 "watts" of heat per watt of electricity. Look at this guy's research on heat pump efficiency in Chicago, for Pete's sake, and found 359 days a year, heat pumps beat gas-as-fuel.

So I think, push comes to shove, a nice mini-split is a better option than trying to retrofit insulation. Certainly less work.

And many of the mini-split systems can be DIYed. Electrical power only needs to be brought to the outdoor unit; the small power for the interior unit fans is carried with the line-set. How does that work, since you need EPA certification and recapture/recycle infrastructure to work with Freon? Two ways.

• A few units (e.g. MrCool) use very special UL-approved coupler fittings. The units and lines are shipped with the correct amount of refrigerant in them, and you follow the instructions. However you must use their line-sets, as they supply them.
• Other units (e.g. Pioneer) use ordinary line fittings, but the compressor has valves. The compressor is shipped with enough refrigerant for the whole system. You get to custom-build the line-sets conventionally, then using normal A/C manifold gauge set and vacuum pump, test the system with nitrogen gas as a "test gas" (the air you're breathing is 78% nitrogen) and evacuate with the vacuum pump. It's rather slick actually, since it lets you use old school A/C skills without handling refrigerant.

For insulation, I'd stick to fiberglass or rockwool

The reason is flammability.

The makers of petroleum-based insulating panels talk a big game about how non-flammable they are. That's nonsense. In a fire, they will greatly accelerate the fire - this sort of thing is why houses burn much faster today. The supposed "non-flammabilty" means they added some borate compounds to the plastic to help it self-extinguish if all flame sources go away. But that only works in the lab - in practical experience (Grinnell tower fire, Sunshine mine) this doesn't work.

And will emit toxic fumes, which is what makes modern homes so dangerous. The toxic fumes will incapacitate you so you can't escape. As such, Code typically requires this type of insulation be behind fire-stop material such as drywall.

So stick to rockwool and fiberglass. It works just fine. Apply vapor barriers according to local guidance.

But I would just use an efficient air conditioner myself, because this is an occasionally-used shop space.