Styrofoam insulation under vinyl siding (new construction) and interior insulation

[1233user]

I am getting ready to side my new garage with vinyl siding, and I want to put a layer of 1/2" styrofoam insulation on the wall before the siding to give a little extra insulation, and also for noise reduction. From everything I have read the styrofoam will act as a vapor barrier, so I am assuming when I insulate the inside of the garage I should use unfaced insulation so that any water vapor can escape to the inside of the garage? Also, on the 4x8 sheets for the outside, would the foil backed be any better then the standard blue or pink styrofoam? The R-value of that is slightly higher. Any tips would be greatly appreciated!

 

[Nowater]

Usually the vapor barrier goes toward the conditioned space. I have heard reports of Styrofoam installed in the manner you describe as causing the wall to accumulate moisture and fail. I suggest you search the internet for some alternatives first. Hope this helps.

 

[pattenp]

Foam board is placed under vinyl siding all the time. It's recommended not to use foil faced in hot climates because the reflected heat can over heat the siding causing it to expand more than normal making it warp.

 

[KINGY2]

I agree with Nowater, it is still a common practice i see it going up all the time.
However i have seen it rot out walls very fast. Moisture will get trapped in the walls and they will fail.
My opinion dont do it .....let the walls breathe!

 

[1233user]

Thanks for the replies! I knew there could possibly be a moisture problem, thats why I was planning to use unfaced insulation between the studs and figured that it could breath to the inside of the garage instead of the outside. Is this a bad plan?

 

[Nowater]

This is a bad plan. The foam traps the moisture. Try to block the moisture inside from getting into the insulation. Let the outside face of the insulation breathe. I tried to say it nicely, but did not succeed. Consult a book on insulation practice or codes on insulation.

 

[Lugnut30]

The approach to this type of construction really depends on what part of the country you are in. Cold climate vs. warm climate. I am in cold climate zone (MN) and the vapor barrier is required (with good reason) to be on the warm side of the insulation. Think of your tall frosty out on the deck in summer. Water vapor in the warm air collects (condensation) on the cool surface of the glass. In your wall, you want the vapor barrier prior to the insulation so that the vapor is blocked from entering the insulation where it will condense as it cools when it moves from interior to exterior of the wall. Based on temperature difference between ext/int surface temps, your dew point actually moves within the stud cavity. You don't want the moisture to condense in the stud cavity where it has hard time to dry out, etc. potentially causing bad things to your wood framing.

 

[Lugnut30]

If you are in cold climate, check the "perm" rating of the foam you are going to use. This is a measure of how permeable the foam is. You want the foam to have a higher rating than the interior vapor barrier so that the vapor that does get by the barrier, (rips, outlet boxes, etc) has a chance to escape.
I have built many homes with Dow brand foam sheathing (the blue stuff) with success. Adds good R value, etc. Be aware that using foam may effect how you adhere any siding materials over it. Some sidings will not warranty and some attention should be paid to the nails used since they may have a longer shank due to thicker foam or more wiggle to them due to the lack of a "solid" plywood sheathing being used...