[Greenbuilding] Steel Building Vapor Barrier Issue

[Michael O'Brien]

Hi, David—

It’s a challenging situation, for sure. I hope this group can offer some useful insights about preventing vapor from condensing on the metal shell.

One aspect not mentioned is controlling moisture generated inside the living space. This dwelling should have fresh air ventilation to help control interior humidity. Outdoor air will be very dry during cold weather, when brought into the house and warmed to room temperature it might be down around 10-20% RH so would have good drying potential (often Coloradans in leaky houses complain about them being too dry in winter). I’m guessing the choice of a metal building in that climate might have been to save costs, so they might not want to pay for heat recovery ventilation, but it is an option. Where there is a concentrated moisture source, like a bathroom or utility room, add spot exhaust ventilation and tie it into the light switch. I think ventilation could suffice to dry the interior down, but could also add a dehumidifier. If the house plan is open, it could be located anyplace where it could drain out, like over a laundry basin.

Very best wishes,

Mike O’Brien



On Dec 16, 2016, at 8:39 AM, David Wentling via Greenbuilding <greenbuilding at lists.bioenergylists.org> wrote:


From: David Wentling <dpwentling at ymail.com>
Subject: Steel Building Vapor Barrier Issue
Date: December 16, 2016 at 8:39:18 AM PST
To: "greenbuilding bioenergylist.org" <greenbuilding at lists.bioenergylists.org>



Reaching out to this list for assistance in a situation involving a homeowner constructing a 40’ x 60’ steel building on a slab in zone 7 of Colorado.  House’s long orientation is nearly due east-west.

Initially, they installed the standard “thin” fiberglass batt (~2”) with the vinyl vapor barrier inward prior to installing the walls and roof. As we all know the insulation value at the compression atop each rafter is zero. Thermal Bridge. The homeowner then proceeded to convert half of the steel building into a house and the other into a garage. 

In the house area, the contractor installed, perpendicular to the horizontal purlins, 2x4s flat on the inside, and stuffed R-30 un-faced fiberglass between the purlins. Batts were pressed against the vinyl facing of the original batts. He installed thiner amounts over the purlins, between the 2x4s.  Drywall was installed over the 2x4s and painted on part of the house section. Weather has dropped temperature to 0-30F for the past several days so they installed the installation batts in the unfinished half of the house to continue working.

Now they are seeing some water dripping from the low area of the finished ceiling. Reaching up behind the unfinished knee walls you can feel the moisture in the insulation above the ceiling. Since the ceiling framing is not all air sealed, and the steel horizontal beams create a 'chill beam', because they extend out beyond the short walls to form a soffit, interior moisture is condensing on the cold purlins, beams and the interior of the vinyl barrier of the original insulation. They noticed that the insulation installed in the unfinished area was wet behind the insulation and the condensate had frozen to the vinyl and the batt. This is only evident on the cold north facing side. The southern metal roof stays warm enough to not show the same wet/frozen conditions. 

I was asked to look at the project to make recommendations since the insulation company will not return calls and they are looking for advice. Nothing like coming to the project late and realize that they missed opportunities early on.

Ideally if cost was not an option, I would say take the roof/wall off and install minimum 6” foam board with OSB & peal & stick, then reinstall the metal roof/wall. This would increase the insulation on the exterior of the existing vinyl vapors barrier and reduce thermal bridging from the steel structure. I would cut off the extended steel soffit framing and insulate on the exterior to eliminate the ‘chill beam’ effect. Based on the final insulation levels, possibly adding fiberglass insulation on the interior of the vinyl vapor barrier to ensure 2/3 of the wall/ceiling R-value is on the exterior. Current design has the vapor barrier with 9/10 R-value on the interior!!

Contractor is wondering if he removes the fiberglass batts, drys out the condensate on the vinyl barrier, then, on a dry warm day, reinstall the fiberglass batts and installs the drywall to seal up the cavity quickly, if he can reduce trapping moisture and not have interior rain showers down the road. We have discuss air sealing pathways to prevent exfiltration laden moisture from entering the insulated area. Unfortunately the vinyl vapor barrier will create problem for any moisture from the inside. No way to make the vapor barrier warm easily? Is there any benefit to slashing the vapor barrier before adding the un-faced batts? The exterior metal siding now becomes the condensation surface and would theoretically drain down to the exterior of the slab.  FYI - they did not insulate the slab edge! Did I tell you they installed radiant heating on top of the slab in the living area? They loose heat out the slab edge and the slab surface in the unheated garage area. We are discussing edge insulation options and the drainage plane issues.

I have not asked how the building’s heating load was determined. I am sure they did not account for the thermal bridging of the steel! Based on available research, I understand the overall surface R-value could calculate to be nearly half of the infill insulation added, due to the thermal bridging of the steel framing.

The collective knowledge of this group is immense and I have been monitoring it for years. Mainly learning for those that do, and contributing what little I can. I understand the theory of the problem. Looking for real world solutions.

Ladies and Gentleman, your thoughts please. What would you suggest, as least cost, to move forward without internal rain.

Your time and experience is greatly appreciated.

David Wentling



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