vapor barrier vs. the building inspector

[cariswork]

Hi,

We've got an unusual situation. We have a project in very hot and humid (during the summer) eastern North Carolina. It's also in the flood plain so the finished floor is about 3' above grade and the "crawl space" is protected by lattice work in between faux brick columns attached to perimeter piles.

The trouble came when the inspector noticed that the insulation installed was foil backed with the foil to the outside. He thinks that condensation will form in the floor structure and rot it out...of course, we're trying to close out the project and don't have the time nor the will to change it.

We think we're right....The inspector is standing behind his guns. I'd love to hear what all ya'll have to say.

This is what I think...it's the reverse of the typical cold climate vs. the warm humid condition inside the building in winter...The biggest humidity load there is on hot summer days in high 90's with 85%+ humidity...the AC is pumping like crazy managing a mild 76 degrees. Now remember this is on the bottom of the building...the cold dry air inside will tend to sink through any available crevice...but it doesn't have the water in it...the outside air does. My thought is that the predominant pressure is air flowing down out of the building. With a maximum temperature gradient of 18 or so degrees. Warm humid air outside will not be drawn into the building unless it is from above. Keep in mind that under the building the air will be cooler as it is in the shade and in contact with the ground.

The foil back on the insulation is broken every 2' o.c. by the 12" truss system (the insulation is installed a few inches from the bottom of the trusses). This implies that the floor system can "breathe" if it needs to.

This is no where near the extreme of 68 degree warm humid air inside slithering out through the walls and roof system encountering 20 below zero surfaces on the way out, no doubt causing condensation and major problems.

What do you think?

[cariswork]

I should also have mentioned that the North Carolina Building Code has recently been revised allowing for the ommision of the typical vapor barrier in the region. This is specifically due to the reversed humidity loading that buildings get in areas with more tropical climates.

Thanks for your interest in advance...

[gdavis]

I think your reasoning is flawed. It doesn't account for whether or not the HVAC system is creating a positive or negative pressure which affects the amount of infiltration of unconditioned air.

If I understood your correctly, your floor insulation is not hard up against the floor but is near the bottom of the floor trusses and there is an air space between the floor sheating and the insulation. If so, this pretty much causes the insulation to be completely ineffective. And, if this is the case, the foil backing on the insualtion is doing nothing but taking up space since it is not tightly sealed.

The end result is that you essentially have no floor insulation at all and that should be considered a code violation.

If you are using parallell chord trusses (with open webs), There is no good way to insulate the floor with batts. You need to use loose fill and hold it in place with something like netting.

If my understanding of your description is correct, I am guessing that you are likely sucking in crawl space air year round when the HVAC is running. This moisture (from the high humidity in the crawl space will be entering the interior of the house and will be worse in the summer. The insulation with the vapor barrier is probably totally ineffective. The moisture will also condence on the underside of the sheathing.

Also, you should note that not all NC counties are allowed to leave the vapor barrier off in the code. There is a chart of counties, which indicate which ones are allowed to leave off the vapor barrier.

You may have condensation on the underside of the floor in the summer and the existence of the insulation may slightly impede the drying however, the worse condition is that you have no effective insulation or vapor barrier and the insulation is required, and the vapor barrier may be required.

Since it is apparently so poorly installed, the fact that the vapor barrier is on the wrong side (according to the code) is a mute point.

I may have something twisted here but this is a good topic and worth discussing.

glenn

[traleigh]

Man,
When I first read your post, I was ready to start screaming, but when I read Glenn Davis' reply, I backed down to his more professional assessment as to why you are wrong. Please read his reponse two or three times if necessary and proceed accordingly.
A simple proverb:
When in doubt, read the directions.