[wildseamus]

Looking for advice on insulating the roof of on unattached garage. Garage is 20'X24' with 4/12 asphalt roof; has peak vent and vents at the eaves. I know I need to maintain a minimum of an 1" air gap between the insulation and roof sheathing.

So one option I'm looking at is installing R15 batts which would give me 2" of air flow.

I would like to increase the R value if I could so I was also thinking about installing foam baffles the entire length of the rafter run (from eave vent to peak vent) and installing R19 batts. This would still maintain air flow but the batts would be slightly compressed to sit flush with the rafters. Has anyone ever done this; or see any issues I would run into going this route?

The R19 batts are available in my local area and the R15 batts are not. It's about a 4 hour drive one-way to a big box store that carries R15 batts 23" wide and the price is only $5 a bag less then the local cost of R19 batts 23".

I'm still undecided if I will sheet over the rafters once I install insulation might just run 1" X 2" across the rafters ever 3-4' to help get the insulation from sagging.

 

[Windows on Wash]

How accessible is the inside?

Full perimeter soffit vents?

What area of the country?

How handy are you?

 

[joecaption]

Why does it have to be a Cathedral ceiling?
If you use faced insulation it will need to be covered with sheetrock to meet code.
Going to be a whole lot harder to heat and cool, tons of lost storage space if you use wide enough rafters, that way you could even go with R-50 if you needed to.

 

[Yodaman]

location? R13 might be fine in Miami, but won't work in Boston

 

[joecaption]

https://www.energystar.gov/index.cfm?c=home_sealing.hm_improvement_insulation_table

 

[wildseamus]

location? R13 might be fine in Miami, but won't work in Boston

Location is in NE Oregon; below zero temps in winter and over 100 in summer.

 

[wildseamus]

How accessible is the inside?

Full perimeter soffit vents?

What area of the country?

How handy are you?

Live in NE Oregon, I'm pretty handy and the vents are at every eave (each birdblock is vented) and the peak.

 

[wildseamus]

Why does it have to be a Cathedral ceiling?
If you use faced insulation it will need to be covered with sheetrock to meet code.
Going to be a whole lot harder to heat and cool, tons of lost storage space if you use wide enough rafters, that way you could even go with R-50 if you needed to.[/QUOT

2" x 6" rafters on 24" centers and it is a cathedral ceiling

 

[wildseamus]

location? R13 might be fine in Miami, but won't work in Boston

NE Oregon, below zero temps winter and above 100 in summer

 

[jlhaslip]

Add to the depth of the afters by nailing 2 x 2's to the bottom of them.
This should allow for the ventilation air space and enough space to install deeper batts.

 

[wildseamus]

Looking for advice on insulating the roof of on unattached garage. Garage is 20'X24' with 4/12 asphalt roof; has peak vent and vents at the eaves. I know I need to maintain a minimum of an 1" air gap between the insulation and roof sheathing.

So one option I'm looking at is installing R15 batts which would give me 2" of air flow.

I would like to increase the R value if I could so I was also thinking about installing foam baffles the entire length of the rafter run (from eave vent to peak vent) and installing R19 batts. This would still maintain air flow but the batts would be slightly compressed to sit flush with the rafters. Has anyone ever done this; or see any issues I would run into going this route?

The R19 batts are available in my local area and the R15 batts are not. It's about a 4 hour drive one-way to a big box store that carries R15 batts 23" wide and the price is only $5 a bag less then the local cost of R19 batts 23".

I'm still undecided if I will sheet over the rafters once I install insulation might just run 1" X 2" across the rafters ever 3-4' to help get the insulation from sagging.

Location is NE Oregon; below zero temps in winter and above 100 in summer

 

[wildseamus]

Add to the depth of the afters by nailing 2 x 2's to the bottom of them.
This should allow for the ventilation air space and enough space to install deeper batts.

I did think of that but there is a loft storage area below the ceiling and adding 2" to the rafters is not an option.

 

[jlhaslip]

Also, I just re-read the opening post of the topic.
This is a garage, which nobody will be living in, so the codes for insulation will not apply; so don't be concerned abut meeting the required R-values for insulation. (At least here where I am, the code would not apply.)
Use whichever insulation value you have available; remembering that compressed fibreglas insulation will not be the same effective value. You are likely better off with a full R-12 rather than a compressed R-20 is what I am suggesting.

 

[Yodaman]

What will the space be used for? Are you going to heat/coo the space? Irregardless if you are concerned about meeting code, condensation from light insulation is a bigger concern. Especially so in cathedral ceilings.

 

[Windows on Wash]

Me personally...I would make my own full length vent chutes our of foil faced ISO board, seal up the vent chutes to the edge of the rafters to prevent air communication, fill the remaining cavity with fibrous insulation, cover the fibrous insulation with drywall.

 

[wildseamus]

What will the space be used for? Are you going to heat/coo the space? Irregardless if you are concerned about meeting code, condensation from light insulation is a bigger concern. Especially so in cathedral ceilings.

I'll be heating in winter with a wood stove and in summer I'm hoping by insulating the cathedral ceiling it will keep the loft area cooler that is below the cathedral ceiling.

 

[wildseamus]

Me personally...I would make my own full length vent chutes our of foil faced ISO board, seal up the vent chutes to the edge of the rafters to prevent air communication, fill the remaining cavity with fibrous insulation, cover the fibrous insulation with drywall.

Could you explain better how you would make vent chutes out of foil faced ISO boards.

 

[Bud9051]

Hi Wild and welcome to the forum,
Just some math. Your proposal to use r-19 instead of high density r-15 results in the same r-value. Fiberglass insulation is actually rated in r-value per inch so the r-15 is 4.25 per inch where the r-19 is only 3.45 per inch. Installing the 3.45 per inch into a 4.5 inch cavity gives you essentially r-15.
If you create your own vent chutes as WOW suggests with 1" foil faced rigid (r-6.5) spaced 1" below the roof deck and then add your 3.5" high density FG you get a total of r-21.5. And you can add another 1.5 (give or take) for the foil surface facing the 1" gap. That gets the numbers up to r-23.

Actually, I'm laughing while typing this as too many people get too caught up with the numbers, building inspectors included. But they do give us a way to compare.

If the garage will be exposed to excessive amounts of moisture we would want to rethink where the vapor barrier needs to go. Low moisture no problem.

Be sure to check to see if all of this requires following any codes and pull all required permits.

Bud

 

[Bud9051]

Just saw your question,
strips of 1" rigid attached to each side and then the foil faced across them.

Bud

 

[Windows on Wash]

Could you explain better how you would make vent chutes out of foil faced ISO boards.

Last page, upper left corner.

http://www.finehomebuilding.com/pdf/021221068.pdf