Garage insulation and polyiso boards?

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fastsvo

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Hey guys,

I'm about to insulate and drywall my garage and paying close attention to my east-facing wall because the sun is so against here in SoCal.

Currently the original tar paper is broken up or missing in several spots. My guy said the right way to do this is to remove the wood siding on the exterior side of the wall but this would be cost prohibitive. Is it worth stapling and patching the tar paper from inside the garage? I understand it's my only form of a vapor barrier while I don't see much rain here and my eaves are pretty deep, its still a concern.

That being said moving forward from the tar paper I'm interested in using some kind of polyiso board or perhaps a reflective atticfoil to work in unison with the R15 bats that I'm going to staple in. Would like to hear any suggestions or ideas on little tricks that I can do here before I close up this wall.

Thanks in advance!
 
CA is a world on it's own when it comes to code requirements.
However, I would thinkg that a foil barrier would reflect the heat back outwards and possibly heat up and damage the exterior siding.
Just my 2 cents.
 
The tar paper is more of a protection for the sheathing for water that gets past the siding. In your dry climate moisture from inside a unheated and uncooled garage is going to be minimal. If you were in a hot and humid climate and cooling your garage you could have condensation on the inside of the wall from humid outside air condensing on the cooler surface of the garage walls, much like water condensing on a cold drink glass. In your climate I bet you could just use unfaced fiberglass insulation.

Radiant barriers have been used, I'd Google it to see what if any damage the reflected heat could cause. The main cause of damage would be the direct sun and UV rays on the outside of the structure not the reflected heat from the radiant barrier.

Here is a decent article on the subject. If you poke around the Building Sciences pages you can find a wealth of information on designs that work for various climates. What works in hot and humid Florida isn't right for frozen Minnesota or mild and dry California.

https://buildingscience.com/documents/reports/rr-0410-vapor-barriers-and-wall-design/view
 
Sparky617 said:
The tar paper is more of a protection for the sheathing for water that gets past the siding. In your dry climate moisture from inside a unheated and uncooled garage is going to be minimal. If you were in a hot and humid climate and cooling your garage you could have condensation on the inside of the wall from humid outside air condensing on the cooler surface of the garage walls, much like water condensing on a cold drink glass. In your climate I bet you could just use unfaced fiberglass insulation.

Radiant barriers have been used, I'd Google it to see what if any damage the reflected heat could cause. The main cause of damage would be the direct sun and UV rays on the outside of the structure not the reflected heat from the radiant barrier.

Here is a decent article on the subject. If you poke around the Building Sciences pages you can find a wealth of information on designs that work for various climates. What works in hot and humid Florida isn't right for frozen Minnesota or mild and dry California.

https://buildingscience.com/documents/reports/rr-0410-vapor-barriers-and-wall-design/view
Click to expand...

So given the minimal moisture due to the dry climate, would just adding an R15 bat be "good enough"? Does it have to be kraft faced?
 
The #8 paper used as the underlayment for the siding and the paper back of the drywall is sufficient for this climate with either kraft or unfaced.
 
Snoonyb said:
The #8 paper used as the underlayment for the siding and the paper back of the drywall is sufficient for this climate with either kraft or unfaced.
Click to expand...
Got it. So just kraft alone wont cut it?
 
If the #8 is, as you have stated, damaged and in poor condition, it should be patched and/or repaired.

The kraft facing, if you choose to use it, should be placed so that it faces the inside of the garage, and the paper backing of the drywall is also a vapor barrier.
 
Snoonyb said:
If the #8 is, as you have stated, damaged and in poor condition, it should be patched and/or repaired.

The kraft facing, if you choose to use it, should be placed so that it faces the inside of the garage, and the paper backing of the drywall is also a vapor barrier.
Click to expand...

The attached pic shows the east wall and how the paper has been chewed up by mice over the years.
 

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Interesting.
My initial impression, what a nightmare, boy have you got your work cut out for you.

Rodent damage usually occurs near the base of the wall, and heat near the top caused by the dissipation of the asphalt impregnation no longer acting as a binding agent for the paper.

Beside the linked product you can also use asphalt impregnated kraft paper, which is the $8 referred to, or 15lb. roofing felt to repair that missing. It can either be cut, folded and stapled or glued with Scotch 777 spray adhesive.
 
Interesting project and I’m wondering if your plan after this is done is to heat or cool the inside of the garage? Or is this just to stop the heat transfer during the day from the sunlight coming in?


If it is the latter the best you can ask for is the inside of the garage to stay the same as the outside. Would you maybe do better with a power exhaust fan setup or better venting of the roof area?


I’m just not clear on what you want the end result to be.
 
How unusual. I really wouldn't bother repairing the tar paper. It doesn't add anything to the insulation.
I would use unfaced R15 batts and cut them to fit each box. The Kraft faced won't help in your situation and will make it more difficult to install the batts. The drywall will do the job of what a Kraft paper would do in your climate.

Dave Mason
 
I am trying to keep the heat transfer from coming in.

It makes sense that the Kraft paper won't do much in my climate but would it hurt? It only cost $0.02 more per square feet from the insulation supplier.

I'm guessing most people wouldn't consider fixing it though this is my only chance to do so.
 
Tar paper will do nothing for the heat transfer coming in.
The Kraft faced is a PITA to cut and you are doing a lot of cutting for those small boxes. Plus the Kraft requires stapling to the studs. Unfaced can just be inserted into the open box/cavity. You WILL like it much better.

Dave Mason
 
mabloodhound said:
Tar paper will do nothing for the heat transfer coming in.
The Kraft faced is a PITA to cut and you are doing a lot of cutting for those small boxes. Plus the Kraft requires stapling to the studs. Unfaced can just be inserted into the open box/cavity. You WILL like it much better.

Dave Mason
Click to expand...
I was just concerned about the moisture coming in in the areas where the tar paper is missing. In addition to what you recommended how about placing polyiso boards against the wall cavities before placing the insulation?
 
Asphalt impregnated builders paper is cheaper and easier to handle.
 
fastsvo said:
I was just concerned about the moisture coming in in the areas where the tar paper is missing. In addition to what you recommended how about placing polyiso boards against the wall cavities before placing the insulation?
Click to expand...
You can add the foam board against the wall but that's a lot of cutting. It will give added R value but you can only use 1/2" thick because anything thicker will cause the batt insulation to become compressed when you put the drywall up.
 
How deep are those studs / boxes?


They don’t look 3.5 deep in the pictures.


What is on the outside it looks like board siding?
 
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