I have never compared DHT11 or DHT22 to a calibrated reference but am surprised you are getting ±30% discrepancies. The datasheets claim ±2% on temperature and ±5% for relative humidity and I don't see a lot of posts from people using them saying they are 'worse than useless'. When I used them they gave similar readings to all the other non-calibrated devices I had.
There is an independent assessment of the DHT22 by someone who appears to know what they are doing here ...
http://www.kandrsmith.org/RJS/Misc/Hygr ... dht22.html
"My devices initially performed impressively. Whilst I never achieved ±2% absolute accuracy over the full range 0–100% RH, they did when first tested deliver a higher accuracy than I had expected for such a cheap device. (My expectations were pretty low!) The maximum errors I measured in my initial calibration was +6% RH and that only on one of my six devices. Typical errors were 2–3% RH at a constant room temperature (20°C)".
Could it be that something else is wrong other than the sensors themselves ?
What code and libraries are you using to read the sensors ?