Standard, even "premium," foam covers use polyethylene plastic film up to 4 mils thick as the vapor barrier . It looks waterproof but it actually allows moisture to pass through it at a slow rate, and over time that adds up to significant water gain.
A 4 mil film has a water vapor transmission rate of about 0.34 g/100in2 per 24 hrs. That doesn't sound like much, but adds up to several gallons per year of water. Expanded polystyrene itself is a closed-cell foam, but it's made by expanding small beads of the material (sometimes it's called beadboard for this reason.) Water on the surface of the board is drawn into the spaces between the beads, eventually getting deep into the material.
This moisture does several things, none of them good for a spa cover:
Directly adds water weight to make the cover heavy, making it hard to open.
Reduces the insulating value (R-value) of the cover, allowing heat to escape.
Weakens the bonds between the beads, reducing the strength of the cover.
Creates an environment for mold and mildew to grow, generating unpleasant odors.
Causes the material to swell and expand, causing the cover to warp.
DuraCore cover uses an entirely different kind of insulation, a commercial grade polyisocyanurate (polyiso) insulation. This insulation is classified as a Class I vapor barrier used in commercial building construction. Inside, the insulation is truly closed cell, and on the outside each side of the board is wrapped in two layers of 1 mil aluminum foil. This foil vapor barrier has has a water vapor transmission rate of approximately 0.0003 g/100in2 per 24 hrs, which is 1000 times less than that of the 4 mil plastic film. Over five years, this would amount to less than two ounces of water weight gain over the ten year life of the cover.
With no water absorption, the cover stays light, maintains its insulating value, and develops no unpleasant odors.