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saxman2u 09-15-2012 05:35 PM

Insulating Existing Garage with Polyiso Foam Board
I am new here and need some advice. I live in Austin, Texas and purchased a home with an existing garage that is detached from house. Hardie plank is on the exterior. The bare studs can be seen from the inside of the garage. No type of vapor barrier/paper was used and the backside of the hardieplank has a thin piece foam of insulation on it. There is a small amount of sheathing in each corner, say about 4 feet in each direction. So, most of the garage all you see are the studs and the blue foam which is attached to the hardie plank. It has a gable roof, which I can see all the way to the top. Pre fabbed trusses were used.

I would like to renovate the garage interior and add R-19 batt insulation into the walls. I would then like to add at least 1 in of polyiso foam board to increase my R value. I might over lap the 1 inch board to give me 2 inches to even get a better R value. I am aware about furring out existing doors, outlets, etc...The goal is to insulate well and then install a small Mitsubishi unit to cool the garage in the summer.

Here is my question: I am not sure where the vapor barrier needs to go and/or if the foam board with a foil lining will act as a vapor barrier? Can someone help me with this. Should I add additional sheathing to the interior for extra rigidity?

I do plan on installing drywall over the foam board and also install a flat drywall ceiling and then blowing insulation in the attic part. I am also going to install a radiant barrier on the rafters of the roof.

Thanks for your help.

GBrackins 09-15-2012 05:51 PM

your climate zone is 2A, (see page 2 of this link)

this link will provide you information on construction in Hot-Humid regions from the Building Science Corporation

Dr. Staube and Dr. Lstiburek, principals of BSC are two of the leading researchers into how buildings work with their environment. Dr. Lstiburek worked on the Department of Energy's "Building America Program" (see this link for information on Hot-Humid Climates

hope this helps!

post back with any questions

saxman2u 09-15-2012 06:15 PM

ok, thanks for the link. Being that I am in zone 2A, it states that do not need a vapor barrier. We want the wall to breathe. Therefore, would an open-cell type of foam board be better than a closed type of foam?

Windows on Wash 09-17-2012 07:25 AM

If you put a foil faced ISO board in that application, be aware that is a Class I vapor retarder (i.e. Vapor Barrier).

Austin should be relatively dry so the point should be somewhat moot but one of the biggest issues that people do in warm and muggy climates is put a vapor barrier on the inside/cold side of the home (i.e. like vinyl wallpaper) and create a vapor trap.

Be sure that you use and unfaced foam and depending on what you are using the garage for, it may be overkill.

GBrackins 09-17-2012 08:41 AM


Sorry for the delay in response, somehow I missed your post.

Building science has separated the classification of vapor barriers into three categories: Class I, Class II, and Class III.

Open-cell foam is considered a class III vapor retarder at normal thicknesses (sometimes as low as a class II retarder at greater thickness)
Closed-cell foam is considered a class II vapor retarder

Class I - Very low permeability vapor retarders - rated at 0.1 perms or less. Sheet polyethylene (visqueen) or unperforated aluminum foil (FSK) are Class I vapor retarders.
Class II - Low permeability vapor retarders - rated greater than 0.1 perms and less than or equal to 1.0 perms. The kraft facing on batts qualify as a Class II vapor retarder.
Class III - Medium permeability vapor retarders - rated greater than 1.0 perms and less than or equal to 10 perms. Latex or enamel paint qualify as Class III vapor retarders.

The recommended best practice for the South is either to build a breathable wall, one without any vapor barrier at all or to install a vapor retarder at the exterior of the studs. This makes sense if you think about it. Vapor barriers should go on the warm side of the wall. That is the outside of the wall in the South.

Polyiso is a closed cell foam product, therefore it should NOT be be installed on the interior side of the wall, as this would be a Class I Vapor Retarder. You could install an open cell product as this would be a Class III Vapor Retarder, basically the same as latex paint. Any foam board should be covered with gypsum board to protect the foam from heat/flames.

You do not state whether your studs are 2x4 or 2x6. R-19 fiberglass insulation was originally designed for 2x8 floor systems. When used in a 2x6 wall assembly it is actually compressed and will provide about R16.7. The 2009 International Energy Conservation Code ( requires an R-13 wall cavity insulation for Climate Zone 2. In a 2x4 stud wall you can use high density fiberglass batt insulation with a R-15, in a 2x6 wall cavity you can use high density fiberglass batts with R-21.

I myself would recommend the use of a high density batt product based upon the size of your studs without the use of foam board.

Should you plan on installing a heating system within the detached garage I would recommend air sealing the interior especially around any penetration such as windows and door, along with all intersecting corners as floor-to-wall, wall-to-wall, and wall-to-ceiling.

Hope this helps.

Good luck!

Windows on Wash 09-17-2012 08:46 AM


Great information. Asphalt facings on the ISO will have it as a Class II in most cases.

Gary in WA 09-18-2012 01:45 PM

Foam board on the exterior is best, but with your situation- - you can improve it greatly. Good that you are adding foam board on the interior to stop the thermal bridging of the studs/plates- R-19 reduced to R-13.7 whole wall R-value. Good for you for thinking correctly! And good you came here for confirmation. An un-faced f.b. (foam board) is best, either closed/open cell, mainly it needs to be thick enough to stop cavity condensation. This is just the opposite of cold climate, exterior f.b., yet both keep the cavity warm (by being thick enough), not to cause moisture deposits on the wood framing.

The moisture drive is from exterior to interior most of the year and with/if a fan-fold sheeting (1.3-1.7 perms, drying to the exterior the rest of year) is installed well, you shouldn’t have trouble. This will act as your WRB to deflect water down and out from the cavity. IF concerned by numerous tears/cuts/reverse laps, you could caulk/tape/mastic cavity pieces of housewrap at the studs exterior perimeters, with the bottom edge out past the bottom plate to drain. You might try tapping the bottom course of siding out gently for the room needed, as some face-nail it or to hold it tight to the starter strip (if used), and the bottom plate to stop bug entry there. Re-set any hidden nails with a flat bar and hammer, careful not to damage the cement board.

The corner sheathing is all that’s required to meet shear code for your location, no need to add more. If worried, add some flat “X” strapping anywhere inside for a little more shear resistance on the bare studs;
The existing sheathing is much better;

The EPS is about R-3.6 per inch and XPS is R-5, I’d use 2” of cc EPS (Type 1) or 1-1/2” of closed cell XPS. It depends on the Type/density of EPS, if comparing the higher perms for letting moisture through easier/quicker, or water absortion.

BSC has recommended foil-faced and later changed that to un-faced f.b. to let any moisture go through slowly to the inside room, letting the HVAC re-circulate the air and dry it;
1993; impermeable f.b.Figs.1, 2;

2009; semi-permeable f.b. Fig.3;

Did you notice both figures showed wood furring before the drywall?
Two reasons; keep an air space to dry any moisture on the paper-facing of drywall and to have the water vapor in a gaseous state after passing through the f.b.; second; stop drywall/foam in direct contact to limit conduction which would constantly cool it and compromise its purpose of preventing cavity condensation.
Canned foam/caulk the bottom edges of the foam board AND drywall for an air seal/capillary break, and the top edges against the new ceiling drywall for the same reasons. ADA the drywall, especially at the electrical boxes/switches, including the ceiling ones;
Remember, you are making the room perimeter your thermal/air barrier, take care to do it correctly. A 1/16” gap can compromise an air barrier, yet you can perforate a vapor barrier and not compromise it because it is surface measured.


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