user8144: I've had a bit of experience with DHT11/DHT22 sensors in a greenhouse environment, and believe that when they get wet they stop working altogether.
Are there any low cost humidity sensors that I can hook up to an Arduino that don't suffer from this problem, or is there a way to protect these sensors from damage when near water? (I'm not talking about immersing them in water - but do expect condensation to get sprayed with water and/or get condensation on them from time-to-time).
Am I correct that the SHTxx sensors are able to withstand this kind of abuse, and that these sensors are about 5-10 times the cost of DHT sensors?  (I did not want to spend that much, but if it's the only possibility I might consider it).

user36750: 2022 UPDATE: After ~5 years since I wrote this answer, I'd like to mention that I don't like BME280 anymore, because of its (1) inaccuracy and (2) requirement to make some non-trivial computations to get real values from sensor's data. If you already got a BME280 and your project requires only approximate values of temperature/humidity, it will work, go ahead. Otherwise, consider using SHT3x!
See also this, this, and especially this one.
Original answer:

I'd consider using Bosch BME280 digital sensor. Although it's a bit expensive, it has very good sensitivity, precision and amazingly fast response for humidity changes (for ex. in my project it detects dry human hand lying in 2" away within a 1-2 seconds).
So its features make possible to put it into an enclosure to avoid direct water contact with water spays and it'll still be very responsive for greenhouse needs, I suppose.
Datasheet warns:

Liquids shall not come into direct contact with the device.

Also, I'd use electronic-friendly protective acrylic lacquer (Plasticote 70 for instance) and put two layers to sensor's PCB, avoiding small black tiny vent hole on the chip itself:

UPD: Here's DHT11 with cover removed:

It might be the easiest and cheapest solution to cover all internal contacts and IC (avoiding humidity sensor's stripes) of DHT11 with Plasticote 70.

8144: <p>I've had a bit of experience with DHT11/DHT22 sensors in a greenhouse environment, and believe that when they get wet they stop working altogether.</p>

<p>Are there any low cost humidity sensors that I can hook up to an Arduino that don't suffer from this problem, or is there a way to protect these sensors from damage when near water? (I'm not talking about immersing them in water - but do expect condensation to get sprayed with water and/or get condensation on them from time-to-time).</p>

<p>Am I correct that the SHTxx sensors are able to withstand this kind of abuse, and that these sensors are about 5-10 times the cost of DHT sensors?  (I did not want to spend that much, but if it's the only possibility I might consider it).</p>
 36750: <p><strong>2022 UPDATE</strong>: After ~5 years since I wrote this answer, I'd like to mention that I don't like BME280 anymore, because of its (1) inaccuracy and (2) requirement to make some non-trivial computations to get real values from sensor's data. If you already got a BME280 and your project requires only approximate values of temperature/humidity, it will work, go ahead. Otherwise, consider using <strong>SHT3x</strong>!</p>
<p>See also <a href="https://forum.arduino.cc/t/compare-different-i2c-temperature-and-humidity-sensors-sht2x-sht3x-sht85/599609" rel="nofollow noreferrer">this</a>, <a href="https://www.kandrsmith.org/RJS/Misc/hygrometers.html" rel="nofollow noreferrer">this</a>, and especially <a href="https://wiki.liutyi.info/display/ARDUINO/Test+i2c+humidity+sensors" rel="nofollow noreferrer">this one</a>.</p>
<p>Original answer:</p>
<hr />
<p>I'd consider using <strong>Bosch BME280</strong> digital sensor. Although it's a bit expensive, it has very good sensitivity, precision and <em>amazingly</em> fast response for humidity changes (for ex. in my project it detects dry human hand lying in 2&quot; away within a 1-2 seconds).</p>
<p>So its features make possible to put it into an enclosure to avoid direct water contact with water spays and it'll still be very responsive for greenhouse needs, I suppose.
Datasheet warns:</p>
<blockquote>
<p>Liquids shall not come into direct contact with the device.</p>
</blockquote>
<p>Also, I'd use electronic-friendly protective acrylic lacquer (Plasticote 70 for instance) and put two layers to sensor's PCB, avoiding small black tiny vent hole on the chip itself:
<a href="https://i.stack.imgur.com/zvnot.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/zvnot.jpg" alt="BME280" /></a></p>
<p><strong>UPD</strong>: Here's DHT11 with cover removed:
<a href="https://i.stack.imgur.com/D3Zro.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/D3Zro.jpg" alt="DHT11 inside" /></a></p>
<p>It <em>might be</em> the easiest and cheapest solution to cover all internal contacts and IC (avoiding humidity sensor's stripes) of DHT11 with Plasticote 70.</p>
