One of the ways that Crusading knights/soldiers protected themselves in hotter climates was by wearing a cloth surcoat over (and under) their chain mail. Historians have speculated that the use of the surcoat was picked up by the Crusaders and brought back to Europe circa 12th century (see "Norman Knight AD 950-1204"). The use of a surcoat provided both ventilation and some protection from the sun. Covering up the chainmail prevented direct sunlight from heating up the rings on the chain mail, which helped the knight/solider keep cool. It seems that longer surcoats typical of the Crusaders provided more ventilation; after the Crusades ended medieval knights eventually adopted a shorter surcoat in later centuries. The use of the surcoat was likely adopted by the Crusaders from either the Turks or the Fatimids/Mamluks/Ayyubids, who are commonly depicted wearing full surcoats over their armor. I think it's a very viable that the Crusaders adopted this style from the latter or former, as the Crusaders adopted many customs from their encounters in the East (examples include scientific learning, bathing, and architectural forms and techniques). Interestingly, the Bayeux Tapestry (pre 12th century/Crusades) does not show use of the surcoat over chain mail, yet later depictions of armored soldiers/knights do show surcoats being worn (see the 13th century manuscript "Einhard vita-karoli 13th-century" from Der Stricker - Karl St. Gallen, Stiftsbibliothek, Ms. Vad. 302 II, fol. 35v, 13th century manuscript (ca. 1300)).

Another very obvious way Crusaders were able to abide and keep cool in the desert heat while wearing armor was to stay constantly hydrated. Without proper hydration, armored individuals would become exhausted and fatigued very quickly in the hot sun (much faster than an unarmored person would due to the exertion of wearing heavy armor). A fantastic example of the Crusaders failing to properly hydrate, leading to heat exhaustion, thirst, fatigue, and ultimately defeat occurred at the Battle of Hattin. The Crusading army left a source of water and on their way to attack Saladin's army, and was unable to secure another water source. The Crusading army was eventually defeated due in large part to the mix of thirst, heat, and exhaustion (among other factors, including desertion).

When you say heavily armored, are you referring to men plate? Because that would certainly have not been at all common throughout most of the crusades. Simple plate armour was 'readopted' (Some classical armies utilized a plate cuirass) by European armies in the late 13th century, but it was not until almost a hundred years after the end of the crusades that full suits of plate were in use. During the period of the crusades the most common form of armour utilized by the Europeans was a chain mail hauberk (if you could afford it) over a quilted gambeson (typically all your standard man at arms would have by way of protection).

When you are dealing with a hot, dry heat, the optimal garments are (you may think, paradoxically) capacious, all encompassing, and loose fitting (and possibly dark, though there isn't much beyond anecdotal evidence of this). All encompassing because the specific heat capacity of human skin is much greater than that of fabric. So while exposed skin would heat up more slowly, it also dissipates heat much more slowly. The result is that a large flowing garment shields the skin from direct sunlight, but also allows for warm air against the body to rise. That creates a chimney effect drawing in air, that by virtue of its movement is cooler against the skin.

As /u/hey_dale points out, the adoption of the surcoat was the manner in which that 'technology' was adopted by the armed forces of the region. By adding a looser layer of fabric over, and under their chain mail they limited the direct heat it would absorb both from their body, and from the sun. As /u/impfireball alludes to, this also creates spaces through which air can move relatively freely due to the structure of the mail. I cannot find any specific references to the use, or adaptions of the gambeson during the crusades, so i wont speculate on it.

So I have to ask, the scene from Kingdom of Heaven where Raynald of Chatillon pours water over his mail and it sizzles; is this nonsense?