Tempered Hardboard for Walls

Ok, So I am setting up a grow box. I was going to use plywood, but have heard from others how heavy that makes the box, so I went to Lowe's last night and found this:

http://www.lowes.com/pd_7060-46498-SSP2202450_0_?productId=3043230&Ntt=tempered%20hardboard&Ntk=i_products_cascade&pl=1&currentURL=/pl__0__s?newSearch=true$Ntt=tempered%20hardboard$y=13$x=30

This is tempered hardboard that is 3/16 " thick and looks like it would be perfect for my setup seen below in this layout:



I will be lining the hardboard walls with mylar sheeting that I picked up from my local Hydro shop. I will be using either 2x2 or 2x4 or some combo of the two for the frame. I was thinking mainly 2x2s and then use a set of 2x4s in the flowering room as frame for the wall to use to support a 2x4 I would be putting across the top to hang the 400W HPS from. What do you guys think, particularly the hardboard?

Someone told me this hardboard swells up bad when a little moisture hits it. What about using this instead since it says it is moisture resistant?

http://www.homedepot.com/h_d1/N-5yc1vZ1xh9/R-100541850/h_d2/ProductDisplay?langId=-1&storeId=10051&catalogId=10053

I was wanting to stay away from pre-painted building materials as I want to be sure no toxic gases will come off it when heated up with lights.