Will sealing make a pool deck hotter underfoot?

Start with what is missing. No document read for this answer measures the surface temperature of a sealed paver against the same paver left unsealed, in the same sun.

What can be said is built from two different kinds of source on either side of that gap: a paver document about color, and pavement research about heat. They are kept apart below because neither was written about the other's subject.

ICPI's line about paver sealers is a color line, not a heat line: most sealers make the paver surface darker and intensify its color. It sorts the sealer families by finish, from flat to a glossy wet look, rather than by how much they darken, and that family-by-family comparison is laid out under whether sealing makes pavers darker.

Lawrence Berkeley National Laboratory's Heat Island Group writes about pavements in general, not sealed pavers: dark pavements get hot in the sun because they absorb 80 to 95 percent of sunlight, and its infrared image of a road with light and dark segments shows the light segment about 30 F cooler than the dark one.

EPA defines cool pavements as paving materials that reflect more solar energy, enhance water evaporation, or have been otherwise modified to remain cooler than conventional pavements. A pilot study in Arizona that it cites found that conventional paving materials such as asphalt can reach surface temperatures up to 152 F at midday, while cool pavements stayed 10 to 16 F cooler. Both sets of figures belong to the pavements and roads those studies measured, not to a driveway or a pool deck, and neither is a paver number.

A color fact from ICPI and a pavement-heat fact from LBNL and EPA sit on either side of the gap without closing it. Most sealers darken a paver; darker pavements in general run hotter in the sun; how much, if at all, a particular sealer changes the temperature underfoot on a particular deck is not settled by any document read, and no number is offered in its place.

Slip is a different question, one ICPI does address for sealed pavers, and it is answered under slip on a sealed paver surface.

What the makers do publish about finish is about appearance. Techniseal lists its (iN) Natural Look Protector as matte, penetrating and water-based, and says in its own words that it “Preserves the natural look of pavers.”

That is an appearance claim from the product's maker, not a temperature claim, and no document read attaches a heat figure to it or to any other finish. On a pool deck, then, the sourced choice between a natural look and a darker, glossier one is a choice about how the surface looks and the curb appeal it adds, with heat underfoot left unmeasured. The named matte and natural-look products are compared under whether a matte or natural-look sealer exists.