Why Is My Mattress Only Expanding on One Side?

Why it happens

• Roll position inside the bag. If the mattress was rolled and compressed off-center, one side may sit closer to the bag’s end. That side gets less room to expand first.
• Foam memory characteristics. Memory foam and polyfoam have different rebound rates. A side with denser foam layers may expand slower than a side with softer foam.
• Cold temperature. Foam stiffens in cool rooms. If one side of the mattress was against a cold floor or wall, it will expand more slowly.
• Vacuum seal break. If the bag was punctured or the valve opened unevenly, air may rush into one side first, causing lopsided expansion.

How to check

1. Unzip the mattress cover (if removable) and look at the foam layers. See if any foam blocks are visibly shifted or bunched.
2. Feel the surface of the unexpanded side. If it feels stiff and cool, temperature is likely the cause.
3. Check the bag’s original position. If you unrolled the mattress on the bed frame and the bag was not centered, the unexpanded side may be the one that was pressed against the bag’s end.
4. Measure the height difference. A difference of 1 to 2 inches is normal for the first 24 hours. More than 3 inches suggests a mechanical issue.

What to do

1. Give it time. Most mattresses need 24 to 72 hours to fully expand. If it has been less than a day, wait.
2. Warm the room. Turn up the thermostat or use a space heater (kept a safe distance away). Foam softens and expands faster at 70°F or above.
3. Massage the slow side. Press firmly with your hands or walk gently on the unexpanded area (with shoes off, on a clean surface). This breaks up compressed foam cells.
4. Flip or rotate the mattress. If one side is against a wall, rotate it 180 degrees so the slow side faces open air.
5. Remove the bag completely. If you left the mattress in the plastic, take it off. Plastic traps cold and restricts airflow.

Common mistakes

• Cutting the bag too early and forcing the mattress onto the frame before it has any shape. Let it partially expand on the floor first.
• Placing the mattress on a solid, non-breathable surface like a plywood board. This traps moisture and slows expansion. Use a slatted or ventilated base.
• Assuming the mattress is defective after only 4 to 6 hours. Most mattress-in-a-box models take a full day to reach normal height.
• Jumping or kneeling hard on the unexpanded side. This can damage internal foam layers. Use gentle, even pressure.

When a new mattress makes sense

If you have waited 72 hours, warmed the room, and manually pressed the slow side, but it remains more than 3 inches lower than the other side, the foam may have been damaged during compression or the vacuum seal failed. In that case, contact the manufacturer. Most reputable brands will send a replacement under warranty. If the mattress is more than a few years old and this is a new problem, it may be time to look at a new model. Compare memory foam options that have higher density and better recovery ratings.

Frequently asked questions

Can I sleep on the mattress while it is still expanding on one side?

Yes, you can sleep on it, but it may feel uneven and uncomfortable. Body heat can actually help the slow side expand faster. Just be careful not to put heavy weight on the unexpanded area for long periods, as that can create a permanent dip. Read more about sleeping on a mattress before it fully expands.

Will the expansion issue go away on its own?

In most cases, yes. The unexpanded side will catch up within 2 to 3 days as air works into the foam. If it does not improve after 72 hours of trying the steps above, contact the retailer or manufacturer.

Does the type of mattress affect how it expands?

Yes. All-foam mattresses expand more slowly than hybrids with coils. Hybrids usually reach full height within a few hours because the coils spring back. Memory foam and latex can take longer, especially in cooler rooms. If you are concerned about expansion time, hybrid models are often a faster option.