I have a 2-car garage that is not cooled or heated. Above the garage is an attic/loft space. The attic has beams (they are large beams, not 2-by-4s) that are spaced 16 inches apart. The beams have pink fiber insulation in-between them. The insulation has a vapor barrier on the underside. I did a lot of research online and think my questions will apply to a number of people. I couldn't find a single source that answered all of my questions.
1) Before I add a plywood subfloor do I need to add an additional vapor barrier (the insulation already has one)?
2) I watched a Georgia Pacific instructional video on YouTube and they said to leave 1/8 of an inch in-between the 4×8 plywood boards, I thought you were supposed to butt them up against one another?
3) Will 1/2 inch thick plywood work in my situation?
4) What kind of screws should I use? Will 1 inch decking screws work?
5) Should I lay the plywood perpendicular to the beams?
Best Answer
If the cords on the trusses are larger than 2x4 than it sounds like they used storage trusses designed to carry a little more load than normal.
1) No don't worry about another vapor barrier especially since it is a garage. The first one helps keep moisture out of the insulation which degrades its ability.
2) Good idea. A small gap allows the plywood to expand and contract with humidity and temperature without bowing.
3) If you are walking on the plywood then I like 5/8" since it doesn't spring as much under my weight (225 lbs), and even more so if you are storing items with significant weight to them.
4) Decking screws should be fine. I would go to 1 1/2" but you are just trying to keep the wood from shifting. 8d nails would work fine too.
5) Plywood and OSB have a higher strength to them lengthwise to span the rafters and carry the weight of the roof. So, yes lay the plywood or OSB perpendicular to the trusses. There is a good discussion of strength orientation here and they say plywood is generally stronger in its long dimension unless otherwise marked.
Good luck!