Foil Faced Foam Board on Rafters?

[bad_idea]

I have a 30x40x12 2x6 studded detached garage/workshop. I have vented soffits and a ridge vent. The roof is a 6/12 pitch with scissor trusses. Vinyl siding and architectural shingles. 5/8 OSB roof decking and 7/16 OSB wall sheathing. No interior surface finishes. I am building the garage out as time and money permit. I am thinking about installing foil faced foam board to the bottom side of the top chord of the trusses and using expanding foam to seal the edges. I figure this will accomplish a few things: create an air path for the roof venting, seal it off from the interior of the building, provide a radiant barrier for the summer months, and provide a minuscule amount of insulation.

Long term I plan to install a metal ceiling with blown fiberglass. My thought is the foam board will help now and improve the performance of the fiberglass later. Thoughts? Any negative consequences?

 

[firebirdparts]

I use it a lot for exactly that. If I want to seal up a space to heat and cool I almost always use it. Ease of use. It stops air and that is job 1 if you want to heat and cool. If I had trusses I would be less enthusiastic due to the difficulty.

 

[bad_idea]

I agree it will be no fun installing this to the bottom of the top chord of the trusses. The peak is 18' up. There is no deck on the top of the bottom chords. I am thinking a few 2x12 planks across the top of the bottom chords will make a decent scaffold to work from. Will still be a lot of ladder work though.

 

[wrenchguy]

u got a decent plan.... is that hassle worth it? 2me i'd save $ & effort towards the end goal. ur in decent climate with summer being most uncomfortable, buy a fan. good luck with ur project.

 

[like2wheel]

Do you have room to do that? How far apart are your trusses?

I have 6/12 scissors on a 28' width (24" centers) & I can't figure out how to fit in there to blow in insulation after the ceiling is up. Let alone weave in some sheet goods.

 

[volleyball]

I'd put up a layer on the bottom of the trusses, then a layer offset of zip foam board under that. With trusses you won't have work or storage space anyways. The offset will seal a lot of leaks and the using zip tape will get the rest. To be safe 5/8 drywall in case of fire.

 

[Bert_]

Do you have room to do that? How far apart are your trusses?

I have 6/12 scissors on a 28' width (24" centers) & I can't figure out how to fit in there to blow in insulation after the ceiling is up. Let alone weave in some sheet goods.

Have somebody net the ceiling then blow insulation. Can blow from several different spots then fill the access as they gets closed up.

 

[3onthetree]

If you plan on a ceiling with blow-in later, the rigid board won't do much at that point. It is very difficult to cut the boards around all the web chords and glue the long edges together to completely seal it off (tape don't last). So you'll realistically have a lot of leakage, which means the R2 or so rigid will be above the actual attic space. If you want the benefit of a radiant barrier and an easier install, just go with a radiant film, and leave the gaps created by the web chords. With the blow-in, make sure to install the air chutes for the soffit vents now unless your scissor trusses have an 18" or larger heel.

 

[TheEquineFencer]

If you cut the F/F/F a little long, it'll cover the edges and you will not need the foam. Go buy an electric stapler! That's what I used, it's money well spent. Use the foil tape too to cover the seams. You're about two hours from me, if you're in the neighborhood sometime, stop by and I'll show you the horse barn the way we did it.

 

[Nicks522]

I’m going to give the foil faced foam a try on the ceiling also. I currently have kraft faced insulation against the roof decking. Which is giving me moisture and now black mold, not my doing, but I will undo it that’s for sure! Also adding gable vents and a ceiling vent that vents directly through the roof. Does anyone have advice on damp floors if the gable vents don’t cut the moisture enough?

 

[NUTTSGT]

It's going to be a royal PITA getting those sheets up through the trusses and I doubt they fit full size. I'd bet you'll be ripping them down to 2' widths.

I think I would be inclined to nail the sheets up to the bottom chord of the trusses. That's if the spacing is 24" O.C. and not 48".

1/2" foil faced pol iso will sag under it's own weight 24" O.C. so I would use 3/4" minimum.

 

[danfromsyr]

It's going to be a royal PITA getting those sheets up through the trusses and I doubt they fit full size. I'd bet you'll be ripping them down to 2' widths.

I think I would be inclined to nail the sheets up to the bottom chord of the trusses. That's if the spacing is 24" O.C. and not 48".

1/2" foil faced pol iso will sag under it's own weight 24" O.C. so I would use 3/4" minimum.

necro-thread bump

one could always apply firring to the underneath of the FFF and I've been using the expanding foam that comes in the BIG cans and has a application gun. just have to be very very dilligent to use the cleaning spray on the tip each time you stop for more than 1minute..

yes it's a tad tedious but a quick spritz and wipe with a ****** rag and the gun will last you years..
forget to and you'll have a PITA to clean it and lose the rest of a can of foam.
I leave the partial filled foam cans on the gun with the valve closed tightly for months at a time..
it's easily saved me $$ vs the cans with a tube on my house project..
plus the foam gun is very manageable aside from it's odd shape&size.

 

[jjblbi]

I'd nail furring strips onto the cords parallel to the roof deck (rafter chord of truss) and then attach 8' x 22 1/2" sections of your chosen rigid insulation. Should go pretty quick, little waste and latex caulk the seams.