If you’ve owned a hot tub for more than a few years, you already know the drill: the foam core in your cover eventually soaks up water, the whole thing gets heavy enough to strain your back (and your cover lifter), and the insulation value you paid for quietly disappears. Sadly, it’s been the industry standard for about 40 years and it’s also the industry’s most consistent customer complaint.
Thrml, a Washington-based company founded by a longtime spa industry veteran, is trying to solve that with the AIRCAP 1 — a hot tub cover that skips foam entirely in favor of a sealed, inflatable air core. We took a closer look at how it works, what the manufacturer claims, and where buyers should read the fine print before switching.
How AIRCAP 1 Actually Works
Instead of a styrene foam-and-vinyl sandwich, the AIRCAP 1 uses drop stitch construction which is the same basic technique used in high-pressure inflatable paddleboards. Thousands of internal threads connect a top and bottom skin, which is then coated in waterproof polyurethane. When the cover is inflated, those threads hold the two skins apart under pressure, creating a rigid, sealed panel of trapped air that does the insulating.
The pitch is straightforward: air doesn’t absorb water the way foam does, so the cover shouldn’t get waterlogged, shouldn’t gain weight over time, and shouldn’t lose R-value as it ages.
Foam vs. Drop Stitch: What Changes
- Water absorption: foam soaks up water over time, adding 50 to 75+ lbs. AIRCAP 1 uses a sealed air core; the manufacturer says it doesn’t absorb water.
- Weight over time: foam gets progressively heavier. AIRCAP 1 stays a consistent 35 to 40 lbs, per Thrml.
- Typical lifespan: foam covers run 3 to 5 years. AIRCAP 1 is rated 10 to 15+ years by the manufacturer.
- Styrene foam as it ages: foam degrades as it gets wet. AIRCAP 1 is claimed to hold its R-value while properly inflated.
- Warranty: foam covers commonly carry a 1-year warranty. AIRCAP 1 carries a 3-year warranty from Thrml directly (some retailers advertise 5 years — worth confirming with your dealer).
- End of life: foam covers aren’t recyclable, are a common landfill item, and terrible for the environment. AIRCAP 1’s PVC drop-stitch material is mechanically recyclable, per Thrml.
The Energy Savings Claim
Thrml’s marketing also leans on an energy-cost argument: as foam covers absorb water, moisture displaces the trapped air that provides insulation, so heat escapes faster and your heater runs more. The company claims this can mean up to 50% more heat loss through an aging, waterlogged foam cover compared to the AIRCAP 1, translating to roughly $120 to $180 a year in extra energy costs (materials we reviewed put annual heating costs at $400 to $600 for an aging foam cover versus $280 to $420 for the AIRCAP 1).
That’s a plausible mechanism as wet insulation performs worse than dry insulation, full stop, but the specific dollar figures are estimates and actual savings will depend heavily on your climate, tub usage, and how far gone your current cover already is.
Durability Claims
Thrml points to the drop-stitch material’s roots in inflatable paddleboards and stand-up paddleboard gear, which does see real-world abuse from UV, temperature swings, and impact. The company states the AIRCAP 1 has held up under testing for hail impact, temperature extremes, load-bearing weight (citing a scenario involving a 4,000-lb vehicle), and water exposure. It also cites compliance with ASTM F1346-23, the current version of the voluntary safety standard for spa and hot tub covers.
We’d treat these as manufacturer-reported results rather than independently verified ones — but ASTM compliance itself is a meaningful, checkable claim, and it’s worth asking any retailer for documentation if that matters to you.
Fit, Installation, and Price
The AIRCAP 1 is built for square spas in the roughly 77 to 94 inch range, across six sizes, and Thrml says it fits about 90% of hot tubs on the market. Installation is marketed as a 15-minute job thanks to a zip-on skirt system (branded ZipTech) that also lets the fabric skirt be replaced separately from the air core if it gets damaged which is a nice touch, since replacing an entire cover over a torn skirt is a common (and annoying) expense with foam covers.
Pricing directly from Thrml is a flat $1,195 across all sizes, which includes the pump and tie-down straps. That’s competitive with or cheaper than some other foam-free options on the market, though it’s still a step up from a basic foam replacement cover, which can often be had for around $800 thru your local dealer.
Who This Makes Sense For
If you’re tired of replacing a heavy, waterlogged cover every few years, or you’ve had a foam cover fail early because of local weather (hail, intense sun, heavy snow load), a sealed air-core cover addresses the actual failure mode rather than just being a marginally better foam cover. The incremental cost for the benefit seems to be well worth it. The Aircap is also considerably cheaper than other premium hard covers like the Smartop, Airframe, or Advanced Spa Cover as these cover start off at $2,000 and can climb as much as $4,000 for a Smartop.
If your current foam cover is only a year or two old and still reasonably light, there’s less urgency to switch. But if you’re already shopping for a replacement cover, it’s a legitimate alternative worth putting on your list alongside traditional foam options.
Hot Tub Insider receives regular promo codes and discounts from Thrml Covers. Please complete our contact form to inquire about current promo codes or specials.
For more information on how to shop and compare hot tubs before purchasing please visit our hot tub buyer’s guide pages. You can also click on the pink boxes at the top right of every page to get a free local quotes from dealers in your area. Or, click our “Spa Buyer’s Consult” link above to get one-on-one expert advice on how to shop around for the best hot tubs available in your area.
Have you tried a foam-free hot tub cover, or made the switch from foam? Let us know in the comments below.
