NM® HD+™ Core 6" Extra-Firm High Density Foam Mattress - Double/Full

Comfort IQ™ · Scored on what actually matters

NM HD 6″ Extra-Firm High Density Foam Mattress — Performance & Feel

Comfort IQ™ scores eight dimensions 0–10 from National Mattress's in-house testing + construction — the same scale on every model, so two beds compare on what actually matters. Results vary by body type, base, and room temperature.

The NM HD 6″ Extra-Firm High Density Foam Mattress holds the spine in a flat, neutral line with no soft pockets for the hips to sag into — a dense, supportive platform built for back and stomach sleepers. Four inches of high-density poly over an extra-firm HD core deliver steady, on-top support at a firm 9/10, along with a quick recovery and a genuinely solid edge you can sit and sleep right up to. Sitting at the value end of the lineup, it is the pick for back and stomach sleepers who want firm, dependable support and a clean, no-frills feel without paying a premium.

Performance Scores

In National Mattress's in-house Comfort IQ analysis, the NM HD 6″ Extra-Firm scores: Overall 7.7/10, Cooling 7.3/10, Pressure Relief 6.2/10, Motion Isolation 7.9/10, Edge Support 8.6/10, Responsiveness 9.2/10, Spinal Support 7.8/10, Durability 7.9/10.

Best For

Considerations

How It Performs — The Measured Detail

Feel & Firmness · 9/10 Extra Firm

The NM HD 6″ is built extra-firm at 9 out of 10 — a flat, dense, support-first surface with almost no contour. You ride on top of the high-density poly rather than sinking in, and because that poly is responsive it springs back the instant you move. It is the firm, on-top feel some sleepers need, not a cushioned cradle.

Cooling 7.3

For an all-foam build, the NM HD 6″ Extra-Firm runs surprisingly cool. Peak surface temperature stays in the 30.1–32.9°C range and the heat rise over baseline holds to 8.1–10.9°C, both at the cooler edge of what shows up across mattresses. With no coil layer to move air, the work falls to the breathable cotton-blend cover and the airy poly comfort layers — and the thin 6″ profile helps, since a body rides high on dense foam instead of sinking into a heat-trapping pocket. By morning the surface settles into the 25.0–29.2°C range, shedding the night's warmth.

Pressure Relief 6.2

Honest framing first: this is an extra-firm bed, so pressure relief is the softest part of its story by design. On the NM HD 6″, sinkage stays in the 1.17–2.19 inch range — shallow, more a floating-on-top feel than a deep cradle. The firm poly comfort layers spread weight across the surface instead of letting the shoulders and hips drop in. Back and stomach sleepers, and anyone who dislikes a sinking sensation, will read that as exactly right; dedicated side sleepers who need sharp give will likely want something plusher.

Motion Isolation 7.9

All-foam construction is the best architecture for keeping one sleeper's movement off the other side, and the NM HD 6″ delivers it. Peak transfer lands in the 1.48–4.78 m/s² range and motion settles out in the 0.54–0.76 second range — the dense poly layers swallow a roll-over or a climb-in before it can ripple across the surface, with no springs to carry vibration sideways. For a value mattress that is a genuine benefit: a restless partner, or a kid sneaking in at 2am, won't jostle you the way a cheap innerspring would.

Edge Support 8.6

This is an all-foam mattress with no perimeter coil, yet the edge holds up better than the build alone would predict. Edge sinkage measures in the 2.55–4.62 inch range, at the firmer end, and lying support grades Good. The extra-firm high-density foam simply does not collapse much when you sit on the side or sleep near the rim, so you get usable surface right out to the edge — uncommon at this price. It won't feel like the braced wall of a coil bed, but for a 6″ foam mattress the perimeter is genuinely solid.

Responsiveness 9.2

Despite the firm build, the NM HD 6″ moves quickly. Total bounce travel measures in the 6.46–11.19 inch range and the surface recovers fast — it returns completely in under 2.93 seconds and is mostly settled in under 1.43 seconds. Because the comfort layers are responsive poly rather than slow memory foam, the surface springs back the instant you lift off, so there is no fighting a sticky sink when you change positions. Combination sleepers who roll between back and side will feel this firm foam keep right up with them.

Off-Gassing 8.5

Break-in odor is minimal. The initial smell registers Low and the off-gassing clears in 12–24 hours, well ahead of what most beds need. Two things drive that: the foams are CertiPUR-US certified to strict low-VOC limits, and the mattress is made locally in Ontario, then unboxed within days of build rather than sitting compressed for months like an imported bed-in-a-box. Less time crushed means less trapped scent to release — air it out for a day and any faint smell is gone.

Sleep Position & Body Weight Fit

The extra-firm 9/10 build is firmly a back-and-stomach-sleeper's surface — the flat, dense platform keeps the hips lifted and the spine level, with no soft pocket to sag into. Average and heavier bodies benefit most from that uniform, full-contact support. Dedicated side sleepers will likely find it too firm for the shoulder and hip; lighter sleepers will feel it as very firm but get steady, supportive contact.

At a Glance

Full construction, per-size dimensions and layer details are in the product description above. View 365-night trial details.

Frequently Asked