Insulation Selection for 2x4 Trusses, 2ft OC?

[moparx12]

Hey guys, I am new here, figured I'd start with a question since most of my google searches end up here. I can't seem to decide on a solution from what I have read so far though. I'm in the southeastern US, and am looking to install a mini split in my 24x24 garage before the summer is over.

The garage is attached to the home, with insulated 2x4 walls. Although the walls and ceiling are drywalled, only the walls are insulated. I think the attic must be vented based on what I have read, but I don't remember seeing any light shining in when I was up there. I will go up later and look more closely.

Anyway, my question is, how should I insulate the attic? i want to use the space for storage, so I can't use blown-in. these are those factory-made trusses on 24" centers.

Thanks for the input!

 

[njhoudini]

Just lost my post. R30 should cost you under $400 for batt insulation, R38 should come in under $800. I would recommend calling some installers to find out if they recommend using a vapor barrier since you wouldn't want to trap moisture if you aren't supposed to.

The venting is usually occurring at the soffits which is usually perforated vinyl in newer homes. If doing it yourself, make sure you leave space around light fixtures and junction boxes. If you're in a ranch, is there a wall in the attic separating it from the rest of the house?

 

[moparx12]

Thanks houdini. On second inspection I see that the attic is in fact vented at the ridge and the soffits. Not a ranch, but it connects to the house via a small entrance way, which is insulated at the ceiling but open to the attic.

About how much for blown in insulation would you say? (I will definitely be doing the work myself by the way.) I am thinking I can retain some storage space by building additional structure on top of the 2x4 material at the bottom of each truss. I would also have to build something around the access ladder to keep the insulation from falling into the garage!

 

[TRWham]

Tread lightly. Trusses usually don't have much capacity for load on the bottom chord, and adding anything to strengthen them might just make it worse by adding deadload without reinforcing anything effectively. The connectors are frequently the weak point.

 

[dmcintosh]

Not really a direct answer to your guestion, but you may want to verify that the trusses are capable of supporting a load before planning to use that area for storage. Depending on what you find out, you can make a better decision on how best to insulate.

 

[jbwilkins]

"Typically" unless designed for 'storage', trusses shouldn't have any additional loads on the bottom cord. You could be fine, you could cause problems......

Also, keep in mind, you can blown/batt in whatever R value you want, but once you compress the insulation it will lose R-Value. EG a 10" thick R-30 batt compressed to 3.5 inches is only r-13 or so....It's the air space in the insulation that actually provides the R-Value and compressing it just reduces that air space. So there's no reason to install R-30 everywhere if you're going to put decking in and compress a large part for storage....

 

[big_bake]

Keep in mind that attic venting has nothing to do with the insulation of your conditioned space. It's to keep the roof deck at the same ambient temp as outside during the winter to prevent ice dams. Only thing you need to worry about is making sure you insulation does not block the soffit vents. They make baffles for blown in insulation, with batts you just place them right.

You can also DIY the blown in stuff, there is a minimum of like 10 bags at Lowe's but they will rent the machine to your for free for a day. If you use less than 10 bags, just bring back the remaining for a refund. Also in a humid climate I would recommend fiberglass over cellulose for the attic. You can do your own research though. I didn't want my attic to turn into a moldy newspaper.

 

[alexb2000]

Down here almost every new house has the rafters completely foamed and the attic is conditioned space. At first the concept seems crazy, but when you think about all the insulation compression, penetrations, thermal bridges, etc. in a typical attic, it starts to make sense. Not only that, but the air handler/ducting is now in conditioned space.

I would find out from an insulation contractor if sealing it all up with foam is an option in your area.

 

[moparx12]

Wow thanks everyone. What a helpful bunch y'all are! I will certainly look into some other storage solution, as I also had the same concern about the bottom chord strength. I will be going the blown in route for sure, and will make sure not to clog the soffits. It looks like i will need something like 13 bags according to a lowes calculator i saw. Does that seem right? Also, this was with R38. I think I am right at zone 4 on the R value map. Raleigh looks like it might be in the zone below though/right on the edge so not sure.