Why heat and freeze–thaw are the real clock

Heat is a multiplier, not a nuisance

The rule of thumb chemists use is blunt and close enough for a garage: every 18 °F of extra warmth roughly doubles the speed of the reactions that age a product. It is not that the shed is a bit hotter than the house. It is that the chemistry inside the container is running at twice the rate.

Put that in the terms of a real shelf. A bottle held at 95 °F is aging about twice as fast as the same bottle at 77 °F, and about four times as fast as one at 59 °F in a basement. A two-year window on a label assumes something near the middle of that range. Put the same bottle on a dark shelf in an uninsulated shed that reaches 120 °F on an August afternoon and the window it was sold with is not the window it gets.

It is a rule of thumb, not a law. Some reactions are more temperature-sensitive than that and some are less, and nothing ages on a tidy curve. The direction is never wrong, though, and the direction is the useful part: the same product, in two rooms, is two different ages.

Heat also banks. A can that spent July at 120 °F does not get that time back in October. The window on a label is spent against the storage conditions printed next to it, hour by hour, and nothing spends it back.

Freeze–thaw breaks what is stirred together, not what is dissolved

Cold is a different kind of damage. It is physical rather than chemical, it is usually permanent, and it happens on one bad night instead of over a season.

The products at risk are the ones that are not really solutions. Latex paint is water with plastic particles and pigment held in suspension; freeze it and the particles are pressed together, and when the can thaws they stay together — stringy, grainy, past saving. It is the canonical victim, and it is why a can left in an unheated garage through a northern winter is the most common single casualty in the room.

The same failure runs through anything water-based. Emulsions split into layers. Thickeners collapse and never rebuild. Dissolved salts fall out of solution and settle as grit that will not go back in. A product does not have to be water-based to have a cold limit either: liquid paraffin lamp oil goes solid near 15 °F and can come back cloudy and thickened.

The tell is the same across all of them, and it is something you look at rather than something you do: layers that did not used to be there, lumps, grit at the bottom, a skin, a stringy texture, or a color that is not the color that went in. Let a suspect container stand at room temperature, give it a gentle shake, and look again. Anything that will not come back together has not come back.

Where to keep it instead

Nearly every container in a garage carries some version of the same sentence: store in a cool, dry place, away from heat and direct sunlight, tightly closed. That is the manufacturer's own storage rule and it is doing more work than it looks like — the shelf-life window and the storage sentence are one statement, not two. Reading an SDS storage section is where the long form of that sentence lives.

Read as a ranking of places, best to worst for almost everything on this site:

• A conditioned interior space — a closet, a basement, an interior utility room. One temperature all year, and by far the cheapest fix available.
• An attached, insulated garage. It still swings, but slowly, and it rarely reaches either extreme.
• A detached, uninsulated garage or shed. This is where most of this site's corpus actually lives, and where most of it dies early.
• Anything sitting in sun — a metal deck box, a truck bed, a shelf under a south-facing window. Air temperature is not container temperature.

Two smaller rules come off the same label sentence. Keep containers off a bare concrete floor and away from exterior walls, which are the coldest surfaces in the room on a winter night. And keep them away from the water heater, the furnace and the dryer vent, which are the hottest surfaces in it every other day of the year.

Where this shows up

Products on this site whose windows are set by one of these two clocks:

• Latex paint — the canonical freeze–thaw casualty — one hard night can end a can
• Caulk — water-based cartridges a freeze can ruin before they are ever opened
• Liquid fertilizer — salts that drop out of solution when it gets cold
• Antifreeze — a water-based fluid with a storage sentence worth reading twice
• Lamp oil — the cold limit that leaves a bottle cloudy and thick
• Gasoline — the fastest heat clock in the room

The short version

Heat sets how fast; cold sets whether it survives at all. A number on a label is a promise about a room, not about a product, and the garage is the room that promise was not written for. If you move one shelf's worth of anything indoors this year, move the water-based things.

Everything on the site, ordered by how fast it goes · What "shelf life" means on a label