Do Heat Packs Work in Extreme Cold? Testing & Results

Winter shipping often means facing freezing temperatures, snow, and even sub-zero conditions. One of the most common questions we receive is whether UniHeat packs still work effectively in extreme cold. The answer: yes—but with the right preparation, duration, and insulation. Let’s break down how heat packs perform in harsh weather and what our tests reveal.

Testing heat packs in sub-zero conditions

In controlled trials, UniHeat warmers were activated and placed inside insulated boxes exposed to outdoor conditions ranging from 10°F (-12°C) down to -10°F (-23°C). Data loggers recorded the internal temperature every hour.

Key findings:

• 40 Hour packs maintained safe warmth inside insulated boxes for short routes but dropped below ideal temps after about 30 hours in sub-zero weather.
• 72 Hour packs provided reliable heat for 2–3 days, even at -10°F, especially when paired with proper insulation.
• 96 Hour packs delivered the most consistent warmth, keeping internal temperatures above freezing for the full duration of long-haul shipments.

Why insulation matters

Heat packs don’t work in isolation—they need the right environment. Even the strongest warmer can struggle if placed in a thin box with no liner. By combining insulated containers, foam, or thermal liners with UniHeat packs, shippers can prevent cold air from overwhelming the warmth generated inside.

Tips for extreme cold shipping

• Choose the right duration — opt for 72 or 96 hour packs when shipping during winter storms or below-freezing temps.
• Use multiple warmers — in very large boxes or extremely cold routes, two packs placed apart can balance internal heat.
• Allow airflow — UniHeat packs are air-activated. Don’t seal them in airtight bags or bury them under dense insulation.
• Ship early in the week — avoid weekend delays when possible to maximize warmer efficiency.

Recommended UniHeat warmers

Based on real-world tests, here are the most effective options for extreme cold:

UniHeat 40 Hour Warmer

Good for overnight or mild cold. Limited use in sub-zero weather.

Shop 40 Hour

UniHeat 72 Hour Warmer

Dependable choice for 2–3 day shipping in freezing temps.

Shop 72 Hour

UniHeat 96 Hour Warmer

Best option for long-haul or sub-zero shipping.

Shop 96 Hour

Sampler Bundle (40/72/96)

Try all three to test what works best for your routes.

Shop Bundle

Frequently Asked Questions

1) Can heat packs freeze in transit?

No. Once activated, UniHeat packs maintain warmth and won’t freeze. However, if not properly insulated, cold air may overwhelm the heat in extreme conditions.

2) Do I need more than one pack in sub-zero shipping?

For very cold routes or larger boxes, two packs placed on opposite sides can ensure balanced heat coverage.

3) Will packs last the full duration in -10°F weather?

Yes, if you choose the right type. The 72 and 96 Hour packs performed reliably in tests, while the 40 Hour is better for short trips.

4) How should I pack shipments in extreme cold?

Always use insulated liners or foam. Leave air space for the warmer to function and avoid direct contact with live animals or plants.

5) Are UniHeat packs reusable?

No. They are single-use and designed to provide consistent warmth for the stated duration only.