Poly/vapor barrier.

Behind the sheetrock.

What effect if any would it have on an IR scan for moisture?

What do you think?

…and I’ll tell you if your on course.

I think it could mess things up and not give good readings of what is going on in the wall cavity.

Carl what are we referencing here in relation to good readings are you talking strictly temp or just a thermal pattern

Finding true readings of the moisture in the wall cavity.

True readings of moisture in a wall cavity is not possible with thermal imaging regardless of vapor barrier presence.

The only way to determine what’s going on inside the wall cavity is to physically look inside of it.


I understand that all to well.

I just want to know if the poly would or could mess up the quality of the IR shots.

That depends…If there’s any air spaces between the barrier and the sheet rock it will make it difficult, if not impossible to determine whether or not the thermal signature is due to the air pockets/bubbles, or the actual signature of the surface of the sheetrock. Either way, even without the barrier, the signature would be of the surface of the sheetrock and not necessarily of what’s in the stud bay…further evaluation through invasive testing would be required.

What are your suspicions or what are you trying to determine?

Stucco house with moisture barrier on the exterior and poly on the interior.

Moisture intrusion from the outside via leaks.

The moisture in the wall cavity just bouncing back and forth rotting everything out.

And the poly making the IR not work at its best.

Polyethylene will most definately change your thermal signature. The real issue with polyethylene is its emissivity value. Poly has an emissivity value of only .10. Which means it will only transfer 10% of the thermal energy it absorbs.

The good news is, for example. If the barrier was consistant, but had a hole or tear in it, you could find a thermal difference in that area. The other good news is, even though it is only transfering 10% of the thermal energy it absorbs (once again if it is consistant) it would still show variance on one side or another. Bad news is, if it is not perfectly flat against the drywall or whatever else it is installed against, you could get a false reading. Remember, using IR is all about variances and image interpretation.

A solid, thermally sensitive camera really helps in situations like this as well.


Carl I shoot images of wet walls all of the time and I understand the reference to poly don’t know If I ever ran across a poly lined wall or not. My thinking would be if the poly is making contact with the dry wall it will make very little to no difference in the thermal transfer between the wet area and the exterior of the dry wall one is shooting but I would state that contact must be present. This kinda perks my intrest would be very easy to set up a demo mock wall and shoot varying degrees of images where one can control the ambient surrounding the mock up, might just do that time permitting

OK…If the interior poly barrier is in perfect contact with the sheetrock, the barrier will have no effect since it will be able to transfer its heat signaturte to the sheetrock.

If the barrier is loose with air pockets or bubbles, those voids will create an insulation effect and block the heat transfer from whatever is behind the barrier. This will likely appear as a typical moisture signature with mottled hot spots.

We have a new site about leaking houses and I think the IR is very much a needed tool and just curious about the poly and maybe even house wraps such as tyvek causing things to be missed or just just exposed fully.

Here is the new site


And the new message board



…even though poly may have low emmitence, this won’t be a factor since the camera is not looking at the poly. Conduction is a far greater factor and the poly will be able to transfer its heat to the sheetrock if its touching it.

I think everyone that is doing IR should check and see if the houses have poly on them.

Thanks for the replys.

Good night guys.

I would agree in theory with this statement

Lot of nice talk, but without tearing off sheetrock you don’t know if vapor barrier is there or not; OR if vapor barrier is present AND in perfect contact with sheetrock; OR if it has air bubbles or not, etc; OR if there is NONE there; OR if some genius DOUBLE wrapped it, etc.

So lets have the same answers to Carls questions without the part of your inspection which resorts to destruction, WHICH 95% of the time nobody will let you do. Now give us your real Big Boy thoughts…

There’s only one answer to his question…yes, the poly (or any other material in the stud bay) will affect the signature seen on the sheetrock.

The right Delta T and right camera span setting will minimize the effects. It doesn’t matter what’s in the wall…the camera can only see the surface.

How to interpret the image would also be a concern, I would think. If the poly is not contacting the wall completly at some point, is it the poly or moisture? With the emissivity of the poly being so low, and if it is not contacting 100% you could possibly get a reading that could be moisture. Just thought I would throw that in there, since your original concern was moisture. Of course a moisture meter can prove that true or false.