Thermal mass for cooling garage

We live in the Sierra foothills in California and summer temperatures regularly hover around 100F during the day and can stay in the 80s throughout the night. The house is on a slope and access to the garage from the house is downstairs. Atop the garage is a wide open living/dining area with ~18' ceilings. Quite lovely, and quite hot. Our deck overhangs the top of the garage and so we have like a 7' covered area adjacent to the garage. See attached photos cuz I'm sure my description is confusing

Our vision is to turn the garage into a summer hangout spot for when our upstairs is too hot.

The garage is a long rectangle, roughly 14' wide, 25' deep, with 8' ceilings. The back wall (photo included) is solid save for a door leading outside. That back wall catches sun for most of the day. All around the inside of the garage is a concrete ledge 9.5" tall with a 1" lip.

Things I've thought about to help keep it cool:

1) pouring more concrete on the floor to bring it mostly level with that concrete lip.
Pro - it would remove the need to install a step from the laundry room into the garage.
Questions...
-Would it increase the weight too much and necessitate external reinforcements since the house is on a slope?
-Home Depot says I'd need about 480 bags for that area which is ~$3k USD - could we add in gravel or something similar to reduce that cost a bit? Like hugelkultur but for concrete slabs? lol
- Could we pour the slab without adding the rebar in since it's technically on top of a floor that presumably contains structural support?

2) Padding the back wall with a couple layers of bricks or cinder blocks. Since the back wall is what catches the sun most of the day, my inexperienced brain thinks that making it thicker would help stabilize the temperature?
IDK if this would be as effective as increasing the mass of the concrete floor and will likely be more expensive.
Questions...
Would cinderblocks be a good or bad idea because of the air pockets?
Similar to above, would we need to do something further to reinforce the wall/foundation since we'd be adding so much weight?

3) I've tossed out the idea of using that covered area for water storage and putting a couple # gallon containers there. That would be great for passive irrigation and potentially help keep the garage cool if they're up against the wall? My partner doesn't like the idea of adding a bunch of weight to the slope. I'm inclined to agree because, as might be visible in the photos, I think that part will need to be reinforced sooner rather than later. For some reason the previous owners had the gutters spilling directly next to the covered area and you can see where there has been some erosion away from the concrete pylons.


I appreciate any and all feedback!

1. Are you going to pour 1" thick concrete to get to the lip level or 9.5"?
If this is 9.5" then:
-you need proper reinforcing, otherwise it will crack sooner or later and will be just weak. I consider concrete without reinforcing to be a joke and waste of time, money and materials.
-you would need to make sure that the terrain under the slab was originally cut or at least the garage was built on the fill dirt that was properly compacted, contractors in the mountains do not care about it and assume that random riding of heavy equipment will be sufficient - for the trailer probably yes, for serious masonry no
-if you are talking about 9.5" thick slab, it will be cheaper and much better to order ready mix concrete. It would be around 7.5 m3 which is roughly 10 cubic yards, so more than one truck. Concrete will be high quality, uniform and poured within short time - proper vibration is advised.

I'm not sure if making thicker slab would make any noticeable difference in the thermal comfort. If the slab was going down 9.5", below existing one then it would couple to the cooler earth down below.


2. I would start with stacking them dry to see if it helps at all. If it helps then I would recommend proper footing and then building the wall on the footing.

The existing floor slab is probably uninsulated and fairly well coupled to the earth, so I don't see much benefit to adding another mass on top of that.

You have more heat than you want all summer, day and night; adding thermal mass to the exposed wall will just ensure that the heat is held longer. What you want is more insulation on that exposed wall so heat doesn't get through as fast and the outside has a chance to cool to ambient temperature at night. An extra thickness of insulation will add a negligible amount to the weight of your structure.

I think you would be best served long term by taking measures that will reduce heat buildup in your upper floor. Things like a ventilated roof skin above the existing one, vents high and low and taking advantage of prevailing breezes, screening of walls and windows that are exposed to direct sunshine...

Cristobal Cristo wrote:
I'm not sure if making thicker slab would make any noticeable difference in the thermal comfort. If the slab was going down 9.5", below existing one then it would couple to the cooler earth down below.

This is enough to convince me, thank you! We definitely can't afford to dig up the current floor and then further than that so it would just be adding a layer on top. I see my understanding of this concept needs more development.

Glenn Herbert wrote:
What you want is more insulation on that exposed wall so heat doesn't get through as fast and the outside has a chance to cool to ambient temperature at night. An extra thickness of insulation will add a negligible amount to the weight of your structure.

Thank you for this explanation. Is it possible to add a layer of insulation on top of the existing drywall so we don't have to tear that down? Adding a 1.5" thick layer of poly iso rigid boards is way more cost effective and has an r-value of 9.6!

Our roof is pretty sloped-not quite A-frame but pretty close-so I'm not sure about adding a ventilated roof skin like you also mentioned. We can definitely look at adding insulation and better vents to the attic.

Thanks folks!!

Just going to add my $0.02 USD worth on top of what Glenn H. recommended, but from a different region and angle.  We have 3 nice steel-skinned, post-and-beam out buildings on our property all built by the same builder with essentially the same design and materials in each building.  However, the last building to be built was purposed for housing animals and my wife wanted it to be insulated.  So it got an insulated ceiliing, walls, doors, and windows.  What I find amazing is how much cooler that building is in mid-summer...even with all doors open!  The other two buildings with no insulation just seem to radiate heat throughout the entire building.  Almost needless to say, the opposite is true in winter where the insulated building needs the smallest amount of supplemental heat to bring it to a comfortable temperature...and it is 50% larger than the other two buildings!  I haven't yet decided what the priority should be on retrofitting one of the other buildings with insulation, but the effect is very clear.  So I would second that insulation, in some way, shape or form, to keep that garage space cooler would be paramount in your decision making.  Good luck!

As a Civil Engineer I agree with all the previous ideas.
I will add a few comments and questions;
- extra weight from a water tank will not cause a problem with the slope
- Installing a cool wall on the side that gets all the sun will reduce the heat load
- A cool wall is an additional skin with an air gap to allow warm air to rise away and an insulating sheet of sisalation . 2 layers of top hat are used to create that wall.
- consider a colour change on the dark walls.
- that upstairs area with the low uninsulated roof may be a really hot spot, changing the design may help.
- Consider an insulated patio roof upstairs, lift the height of that roof

I just want to clear things up, is the sun hitting the rear wall of the garage through the open garage door , it hitting the rear wall from the outside?

If it's coming through the open garage door, you need some shade outside, probably in the form of an awning.
If it's hitting the back wall from the other side, a trellis and grape vine could provide the shade , or a water tank or a bunch of other stuff.

If you want a thermal mass in the garage, a plunge pool could be a great place to stay cool.
Two courses of cement block and a liner, and you have a kind of impluvium, the pool in the center of a Roman villa.

Edit: keeping the freezer in the garage will increase the heat for any occupants.

Rather than trying to add an unknown that may or may not add a cooling feature why not add more shade?

Some sailcloth tied to trees?  A trellis with some vines?

Frozen water bottles with a fan blowing air across them?

Sprinkler or mist like some restaurants do on patios?

A layer of polyiso board on top of the existing wall is perfectly reasonable. It will need to be covered with drywall for fire resistance, and some minimal wood framing at drywall joints at least so that the joints will not buckle or shift. It is possible that gluing the polyiso to the wall and the new drywall to the polyiso can make the centers of drywall sheets rigid enough for good looks. If you will want to put any shelves on the walls, you will need regular (2x2) framing behind the new surface.

As you have an attic, putting more insulation and bigger vents in it would definitely be a good idea. The ideal would be vents that allow breezes to flow freely through the attic space in summer, so heat does not build up there.