roof insulation w/ open trusses

[brywisco]

I am scratching my head on the best way to add insulation to the roof of my detached garage. Here are the specifics:

The garage is 32X24

The roof is constructed of standard 2x4 trusses spaced 24" apart

I want to leave the trusses open and insulate between the rafters

There are soffit vents but no ridge vents

I would like to do this myself

The location is central Wisconsin

I just want decent insulation to allow me to heat the garage/shop to 50-60 degrees when I am working in it.

I am adding f-glass batts + plastic + OSB board to the 2x4 walls

My options as I see them so far:

Spray foam (urethane) directly to the OSB sheathing - expensive but fairly easy and quick (??)

Fiberglass batts stapled between rafters - difficult for me to fight up in the trusses etc. but cheaper. Might need to add ridge vents and maintain air space between insulation and sheathing (thus might have to add 2x2 spacers to underside of rafters before stapling in the batts)

Blown cellulose - add netting material to underside of rafters then blow in cellulose. I have never fooled around with this method. Would I need ridge venting and space between the insulation and sheathing? Help!!

Other: Any other ideas I should consider?

Thanks!!!

 

[blkhonda1991]

to my knowlege if you insulate between the trusses you will need to add a ridgevent, why dont you just drywall the bottom of the trusses and blow cellulose on top? is there a reason you want to maintain the open truss setup?

 

[autoist]

&, if you insulate between the rafters, you don't need to staple...they make these little wire things that pop between rafters to hold the insulation in place.

 

[brywisco]

Thanks for the inputs.

I must keep the trusses open for storage. I store alot of summer toys up inside the trusses during the winter (kayaks, water skis, etc...). I know it would be better to drywall the ceiling but it would not work well for me at all.

Has anyone ever attempted the netting + blown cellulose thing? A local insulation company suggested it but their price was a bit too much for my tatse.

 

[brywisco]

What if I just staple some of this prodex stuff to the underside of the rafters maintaining a 3.5" air gap between the prodex and sheathing?

http://www.prodexsa.com/

 

[Notch1988]

How tall is your garage? Sounds like a decent size, don't know why you couldn't add a bunch of J-hooks to the walls to mount kayaks, water skis etc.

Do you have the room on your property to build a $6-700 shed to store all that stuff?

 

[rsanter]

I vote for spray foam

you could also get the rigid foam and cut them to be a friction fit between the rafters

 

[brywisco]

I have much more than just kayaks etc. in the trusses so I am being pretty stubborn about keeping the trusses open. I know that limits me somewhat and make the insulation more problematic.

I do kind of like the rigid foam board idea. I was thinking of using 2" thick boards fit between and flush with the bottom of the rafters. That would leave a nice airgap above them. Some foil on the bottom of the foam board would help with radiant heat loss also. I was looking at the P2000 stuff which is basically EPS with mylar facing. The cost is still a mystery though.

 

[twostory]

I was looking at the P2000 stuff which is basically EPS with mylar facing. The cost is still a mystery though.

I was researching the P2000 also, the best guess on price I found was $45 for a 1/2 inch 4x8 ft sheet.

I just bought some 1/2 4x8 blue foam at lowes for $10.

 

[om-nc]

While I am certainly not an expert, you will still need an air gap between the roof decking and the insulation to keep from "cooking" your shingles. They make a rigid vent, thin styrofoam, that I picked up at HD and stapled directly to the decking and then I stapled FG insulation to the trusses. If you can close off an area near the ridge, and add gable vents that open to the enclosed area you can avoid having to retrofit a ridge vent.

Just another $.02 worth.....

 

[mleichtle]

I don't like the blown insulation idea, its just going to settle over time and leave the peak uninsulated. And every time you figet with your kayak, the vibration is going to speed up the process. Whats the roof pitch?

here in wisconsin, 3-1/2 inches doesn't make a noticable difference with heat, it'll go right through. And with 1" air gap for the roof vent makes its less yet. You need at least 6". Imo, the tiny savings in heat cost does't justify the time, effort, and cost.

What about installing some kind of nailers, 1X4s, to side of the trusses, 6 to 12" down and parallel to the roof pitch? Then ya could shoot up some rolled attic insulation or at least the r19.

Oh, and the problem with blue foam, its flamable if exposed. I'm not familiar with the spray foam directly on the underside of a roof. But Mike Holmes does alot up in Canada if i'm not mistaken.

 

[brywisco]

My roof's pitch is 4/12.
I also am wary of the blown insulation up there. I think you are right about the best bet being to install spacers on the edge of the 2x4 rafters to allow me to use insulation made for 2x6 walls. I am considering doing this and using the foil faced insulation or maybe adding that reflective bubble foil on the inside over kraft-faced fiberglass. I think the reflective thing would really help keep the heat in the garage and greatly help the lighting situation.
It would be more labor to add the spacers, but would really be worth it in the long run.

 

[smooth72]

Just had an estimate done on mine. I have a 10 pitch 30x40, with 2x6 studs. Spray in foam it will give me an R38 with no air movement, it totally seals the area. Came out to @ $2500. Your's would be who lot cheaper with a 4.5 pitch. If you plan on staying there it would be worth it. Measure the area that you want to insulate and call the insulating place, they can give you price per sq footage. What is nice about the spray in is that it totally seal everything, even if you get the same r factor with some other method it want insulate as well. Have you ever taken a switch plate of in the house and feel air movement with a batting house. I have the rest of my house done with the spray in stuff and it is tight and quiet. Also the spray foam is allot lighter in your rafters. Sorry for the long post just got finish with estimate about an hour ago and the info is still fresh.

 

[brywisco]

Was the $2500 to do both walls and roof?
Are they suggesting spraying the urethane foam directly onto the underside of the roof sheathing?

 

[smooth72]

Mine is in my attic of my garage that I am turning into a room. The price is the ceiling and 4 foot walls. They spray it directly to the sheathing of the roof. The roof expands to 4 inch to give you @ R38 and the walls expand to 3 inch to give @ r30. If you have 2x4 for framing they cut off anything pass the 2x4s so that you can be sheet rock over it. Also found out you can still chase down a wire through the insulation if you need to do it later for additional wiring.