Mylar and temperatures.

Ive literally just got a few metres of mylar in the post... but before I put it up (well, Im gonna go put it up anyway) will it increase the temperature in my closet, and by how much? The temp is about 23-24C in there during the day, and about 18 at night. Its 5 feet high, 2.5 feet wide and 3 feet deep. Ish.
Is heat gonna be a big issue for me, or will it only go up a few degrees? Also, my lamp (600w HPS) is about 3.5 foot above the ground, and its 'dangling' (I know, I know, its not really the right word but its early morning and my brain isnt working yet) by a chain, so heat can easily dissipate upwards, plus Im not gonna mylar anything above the lamp for obvious reasons.

swizzy, the heat went up a tad bit in my closet when I put it up but I put to much , I lined the WHOLE closet.. depending on your set up you only have to put it where the plants can use it , I would think if your worried about the heat that much , just put it level with your growing medium and to the level of the light , you shouldnt see a difference in the temp ...I wouldnt think any way