I have a large garage - about 700 square feet/65sqm. It has a concrete floor, timber frame and metal siding and roof. I'd like to add a plasterboard ceiling (and walls eventually) so that I can insulate and regulate the extreme temperatures inside. I'm a novice when it comes to DIY at this scale... The roof is a series of 6 trusses, each around 1.8 metres (6ft) apart! So, i need something to hang the ceiling from. I'm thinking of installing some 2x4s in between each joist to bridge the 1.8m gap. Either at 1.2m spacing or 600mm (24"). Would I also need to run battens perpendicular to the 2x4s? I've added an image to help explain. [ATTACH]76296[/ATTACH] I just want the cheapest and easiest solution so that I can attach plasterboard and pop some insulation on top. I realise this is going to be a big job but I'd like to keep it as simple as possible. Any advice would be greatly appreciated

i originally had some type of thick plastic sheets on bathroom wall. before i ripped them out i put a square in each corner and they appeared to be very square. i'm putting up a tile wall so the underlayment is going to be 1/2 cement board. i'd like to assume that when i put up the boards that the wall will be square? Is there a best method for figuring this stuff out before putting up the wall? it seems that every 2x4 in the wall isn't perfectly flat against the next, so i'm starting to get concerned. thanks for any suggestions.