Inside a Newer Sleep Number Bed: Glued Foam Layers, Rails & How to Measure
If you own a newer Sleep Number bed, the foam inside isn't a stack of loose pieces anymore. On many newer models, the comfort layers are glued to each other and to the foam rails, so the whole assembly comes out of the bed as one piece.
That changes how you shop for replacement parts. To show you what's actually in there, we took apart the foam from a 2020 Sleep Number i8 with a split head (FlexFit) top and photographed it from every angle, inside and out.
This guide covers how these beds are built, what to measure, and what to expect when you order new components.
Quick answers:
- Newer Sleep Number beds often have the comfort layers and foam rails glued together into one piece.
- Rail height varies (we've seen 6" and 7" on similar beds), so always measure your own.
- Rails taller than 6" usually come with a hidden spacer foam under the air chambers.
- Most owners replace the full foam set; if you separate parts, the rails come off easiest.
How newer beds are built: the foam "tub"
The comfort layers and foam rails are bonded together into one unit that works like an upside-down tub. The comfort layers form the top, the rails are glued around the perimeter underneath, and the air chambers sit inside the cavity they create.
From the outside of the bed, you only see the rails and the top of the comfort layers. Flip the unit over and the construction becomes obvious: one continuous piece with an open center where the air chambers go.
- Comfort layers: two layers of foam glued to each other, forming the sleep surface
- Foam rails: glued to the underside of the comfort layers around the full perimeter, in sections that butt together at the corners
- Air chambers: sit inside the tub, surrounded by the rails and under the comfort layers
What we found in a 2020 i8 Split Head (FlexFit)
This bed measured about 12" from the bottom of the rails to the top of the comfort layers. These are measurements from one bed, not a spec for every i8. Worn foam compresses, and construction can vary by year and model.
| Part | Approx. thickness | Notes |
|---|---|---|
| Top comfort layer | ~3" | Softest foam, the sleep surface |
| Second comfort layer | ~2" | Firmer transition foam, glued to the top layer |
| Foam rails | 6.5"–7" | Glued to the underside of the comfort layers; height varied along the bed from wear |
The split head: On a split head (FlexFit) bed, the comfort layers and the head-end rail are cut down the center from the head of the bed partway toward the foot. That lets each side of the head raise independently on an adjustable base. If your bed is a split head, note it when you order and measure how far the split runs.
Don't miss the spacer foam
If your rails are taller than 6", there's very likely a spacer foam layer inside the tub, and you can't see it from outside the bed. In our experience, it almost always lies under the air chambers, though sometimes it sits on top of them.
The spacer raises the air chambers so they sit flush with the top of the taller rails. Without it, the chambers sit too low, leaving a dip in the middle of the bed.
Why this matters when you order:
- If you're replacing rails, make sure the spacer is still in good shape, or replace it too
- If you're switching to a different rail height, the spacer thickness may need to change to match
- When you lift the foam to check, look under the air chambers as well as on top
How to measure for replacement parts
Rail height is not one size fits all, even on the same model. We've seen beds that measured 6" and beds that measured 7". Measure your own bed before you order.
- Unzip the cover and fold it back so you can see the foam and rails.
- Measure rail height in several spots. Hold the tape straight up, not at an angle, from the bottom of the rail to the top. Check the sides, the foot, and the corners. Worn rails compress unevenly, so readings may differ by half an inch or more.
- Note the reading you get most often, plus the highest and lowest. If your readings fall between two sizes, send us photos and we'll help you pick.
- Measure rail width (the thickness of the rail wall from inside to outside), from the top.
- Measure each comfort layer's thickness if you're replacing those too.
- Measure the overall length and width of the foam.
- Check for a split head. If your bed has one, measure how far the split runs from the head of the bed.
- Look for spacer foam under (or on top of) the air chambers, and measure its thickness.
- Take photos with the tape measure in the shot. They make it much easier for us to confirm the right parts.
Replace the whole set, or just the parts?
Most owners of newer beds are better off replacing the full foam set. Because the layers and rails are glued together, pulling out one worn piece usually means fighting the glue.
If you do want to take it apart, the rails are your best bet. They run only around the perimeter, so the glue line is narrow and they're the easiest part to separate. The comfort layers are a different story. On the bed we tore down, those layers were bonded so well they couldn't be separated without destroying the foam.
Glue strength can vary from bed to bed, so yours may come apart more easily or less easily than this one. Go slowly, and if the foam starts tearing, stop and plan to replace that layer.
| Option | Best for | What to expect |
|---|---|---|
| Full foam set | Most newer glued beds; worn comfort layers | Simplest swap, everything matches in height and fit |
| Rails only | Comfort layers still in good shape, rails worn or collapsed | Peel the old rails from the perimeter; glue line is narrow but may still tear foam |
| Comfort layers only | Rails still firm, top layers worn | Hardest; bonded layers often can't be separated cleanly |
Signs your foam is worn out
Foam breaks down slowly, so most people notice the symptoms before they ever look inside the bed. Here's what to check for:
- Sagging or rolling off the edge: rails that have softened or collapsed no longer support the perimeter
- Uneven rail height: readings that vary along the bed, as in our example, mean the rails are compressing
- Cracks or tears in the comfort layers: often along the edges where the foam flexes most
- Yellowing or crumbling foam: a sign the foam is aging and losing support
- A dip in the middle: can mean missing or worn spacer foam under the air chambers
Frequently asked questions
Are all newer Sleep Number beds glued?
Many newer models use glued construction, but it varies by year and model. Lift the cover and check yours before ordering.
What rail height do I need for an i8?
It depends on your bed. We've seen 6" and 7" rails on similar setups, and worn rails measure inconsistently. Measure in several spots and send us photos if you're unsure.
Do I need the spacer foam?
If your rails are taller than 6", likely yes. It keeps the air chambers level with the top of the rails.
Can I reuse my old comfort layers with new rails?
Sometimes. If the layers are still in good shape and the rails separate cleanly, you can glue new rails in place. If the foam tears, a full set is the better option.
Need help choosing parts?
Send us your measurements and a few photos with a tape measure in the shot, and we'll help match the right components to your bed.
- Split Head FlexFit complete foam rail set
- Complete set of foam rails (standard tops)
- Split Head FlexFit replacement medium regular foam
- Split Head FlexFit replacement gel memory foam
- Replacement medium regular foam (standard tops)
- Contact us with your measurements
Magic Sleeper sells compatible replacement parts for Sleep Number beds. We are not affiliated with or endorsed by Sleep Number Corporation. Sleep Number, i8, and FlexFit are trademarks of their respective owner.
