Hi,

We are nearing occupancy on our new SIP home. When I went to it yesterday morning after a light snowfall I was able to see the seams of the roof panels because of the melting of the light snow.  The outside temperature was just 32 and the inside temperature was 61.  Our home is supposed to have R-27 walls with a R-40 roof.  I notified the builder and sent him photographs of the roof.

How can we determine the true R-values of our new home?

Thanks,
Manyvines

Adding pictures - grainy due to resizing.

Snow ghosting. The melted stripes are a perfect visual example of the dramatic effects of thermal bridging. Your "R40" roof may not be exactly accurate but its going to be a truer number than most conventional stick frames.

What type of panels at what thickness? You should be able to get the claimed R values of the panels from the panel maker's website.

OSB sandwich with 5 1/2 polyurethane foam.

There is one area where the shingles humped up about 4 inches when the outside temperature was 15 degrees and about 50 inside.

Are you saying I should not be concerned with the 'ghosting'?

As long as the seams were well detailed it should be fine. By well detailed I mean canned foam was used to seal the 2x framing members together between panels to prevent air leaks. This is basic SIP practice and would be surprised if they missed it unless they are unfamiliar with SIPS. Best practice also includes taping the seams.

What brand panels were used? Some companies require separate framing which supports the panels from below as opposed to having the framing embedded between seams.

I would be concerned with the hump. 4"? Are you saying it only shows up with the big temperature difference?

This article will explain what is happening:

BuildingScience

This is a problem that some SIP manufacturers refuse to address and will lead to rotting SIPs. The science explains what is happening but will the installers and manufacturers address the problem or just hope the homeowner doesn't notice it until the warranty expires?

Ghosting does not equal airflow or "complex 3d airflow networks" as Joe describes it. The seams can be airtight but the thermal bridge will still cause ghosting.

If there is airflow in the seams, its more of an installer mistake than a manufacturer's.

According to Martin Holladay of Green Building Advisor:

Many owners of SIP homes report ghost lines on their roofs when there is a thin layer of frost, indicating SIP seams. Seams will leak more heat than the center of the panel. You can ignore the issue, or handle it above the SIPs or below the SIPs.

It depends on the wood SIP manufacturer. Some have designs which are inherent to having problems at the seams. Others have issues due to thermal bridging at the seams if they use a wood seam connection area. From what I have read, ghost lines are caused by air infiltration and/or thermal bridging.

Eventually the problem can lead to SIP rot on the roof if not addressed during the construction phase.

manyvines,

a properly installed roof should not have lines,
however if your home was built some years ago with solid 2x splines, then the solid wood spline would cause thermal transfer

Posted By cmkavala on 24 Feb 2013 04:46 PM
manyvines,

a properly installed roof should not have lines,
however if your home was built some years ago with solid 2x splines, then the solid wood spline would cause thermal transfer

With your steel SIPs, they are friction wedged fit, right?



I've seen some pics with a horizontal slit in the SIP EPS, as if it was for some type of metal spline. Why are there two different types?

Posted By Lbear on 24 Feb 2013 05:53 PM

Posted By cmkavala on 24 Feb 2013 04:46 PM
manyvines,

a properly installed roof should not have lines,
however if your home was built some years ago with solid 2x splines, then the solid wood spline would cause thermal transfer

With your steel SIPs, they are friction wedged fit, right?



I've seen some pics with a horizontal slit in the SIP EPS, as if it was for some type of metal spline. Why are there two different types?

Peter,
the slit is for cold storage applications where a 3" x 24" x 10 ga. steel plate is inserted at the horizontal girt line and a screw/clip attachment is done on the interior side of panels

Panels can be ordered with or without groove

Posted By cmkavala on 24 Feb 2013 06:18 PM

Panels can be ordered with or without groove

Now I understand the difference. For a roof application the friction fit wedged SIP would be used. This design also eliminates any thermal bridging.

Thanks of the clarification.

Posted By cmkavala on 24 Feb 2013 04:46 PM
manyvines,

a properly installed roof should not have lines,
however if your home was built some years ago with solid 2x splines, then the solid wood spline would cause thermal transfer

This picture taken this morning depicts a solid covering of frost on SIP roof with no thermal transfer at joints

Really helpful article....thanks