Normally you do not see much in the way of Long Exposure  photography in the Infrared world.  The reason is that it is necessary to use Neutral Density filters (ND) to cut down the amount of light reaching the sensor to enable very long shutter speeds.  The infrared spectrum will reduce the the effective value of any ND filter that you attach to your lens from between 20% to 30%.  Have you ever taken a photograph of a person wearing a pair of sunglasses in infrared and noticed that they are totally clear?  This is the effect that I am talking about.   The general rule is that the cheaper filters suffer much greater loss in infrared and the variable ND filters do not work well at all (yes, even the expensive Singh Ray). Should you desire to pursue this style of photography I strongly suggest that you purchase and use B+W ND filters which loose around 20%.  I am also told that Lee filters do not loose too much effect as well but I have not yet tested them even though I own them all…….

Source: markhilliardatelier.wordpress.com