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sando777 10-28-2011 12:26 AM

Ins ext wall / BIFG, need vapor barrier?
Hi all,
I have a colonial style home with an attached 2 car garage which is under one bedroom of the second floor. My siding was damaged from a storm and am getting it replaced soon. The outside walls, 1 side wall and the front wall surrounding the door do not have insulation in them, but the builder did drywall, tape, mud and paint them. Previous owner paid builder to drywall, but skipped on the insulation (shake my head).

My contractor offered to open the side wall when he pulls the siding down so I can insulate it, then I can buy some new sheathing and he will put it up. This seems like my best option, but I have now looked at the blown in FG using the machine and have been considering blowing in more insulation in my attic anyway. I would be buying enough bags to get the machine for free, and am thinking about hitting this wall from inside the garage instead.

My concern is not having a vapor barrier facing the inside of the garage with the blown in. I have not read about this being an issue, but I have seen the thermal transfer during the winter on this wall and I can easily see the outline of the 2x4's when the cold soaks through. Should I go from the outside to get the barrier, or can get away with a little drywall repair by blowing it from the inside with no barrier?

FYI- The outside wall will be sheathed, wrapped and then sided with Structure EPS which I believe adds just under and R4 rating.


sando777 10-28-2011 12:39 AM

XPS under EPS Structure?
Hello everyone,

We are going to have our house resided with Mastic Structure EPS D-4 sometime in the near future. I read that we will gain roughly an R-4 with this product and have been looking into the idea of adding 1 inch XPS underneath to get us up to an increase of roughly R-9. So the wall will be layered with sheathing, wrap, 1" XPS and then the EPS siding. Is there any problem with XPS under EPS, any issues with fastening the siding the the sheathing through all this insulation?

I imagine many use the 1" XPS under regular siding so length of fastener I think would be the same, or is this just overkill?


Windows on Wash 10-28-2011 10:49 AM

There is not need to insulated a space that is not conditioned if I read your post correctly.

If you are saying that the outside wall of the bedroom is uninsulated, that is another issue and should be insulated.

The machines you get at HD for free blow insulation in a wall and doing so with fiberglass does not work properly. You could dense pack cellulose but that machine or any blowing machine from HD will not work.

You don't really need a vapor barrier in your region.

Structure EPS does not add an R-4 despite what the catalog quotes. It is more like an R-1 once you figure in the fact that it does not lay tight to the wall in all areas and the very optimistic R-value of EPS that they use for the brochure.

You have unique opportunity to get your home sealed up from the outside when doing siding.

Feel free to email me and I can walk you through what you need to do in order to accomplish that goal as well as getting that room above the garage tightened up and insulated properly. We don't serve Chicago but I don't want to type out the whole explanation.

sando777 10-28-2011 03:29 PM

I don't see your email listed, sorry if I missed it somewhere. I cannot PM, I'm guessing I do not have enough posts on here yet. Feel free to email me at with your information, I would really appreciate hearing your thoughts.

To clarify the wall location I am concerned with, the garage is attached with a bedroom above it. 2 walls are common to the home and 2 are outside walls. The front wall which has the 16' garage door on it and the west outside wall are not insulated, but they are dry walled and finished. That same west wall on the second story for the bedroom is insulated, that is the outside wall for the bedroom above the garage.

The garage is not climate controlled but, I may add a heater in a another project down the road. My concern with the vapor barrier was that since I am getting a frost silhouette of the 2x4's on the inside of the garage wall can or will some of that moisture in the garage seep past the drywall into the wall cavity, and if so, if it does not have a VP won't that be a problem?

I am skeptical on the R-4 value of the Structure EPS as well and that was one reason I ponder the idea of a 1" XPS layer first, if that itself does not cause any issues. I like the fullback or EPS idea for some added durablilty of the vinyl itself and hoping for some efficiency as well.


Gary in WA 10-29-2011 12:33 PM

The EPS will add less than that value though it will add stiffness against oil-canning:


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