Make sure the return air ductwork is properly sealed and that the ducts and supply pipes are properly insulated.

some open cell foams produce water as a byproduct of polymerization. If your attic is now well sealed it just may have had no place to go. Run a dehumidifier up there for a couple of weeks and perhaps it will go away and not come back.

I agree. New homes have a lot of moisture in them. Humidity should drop in a few weeks maybe even months. As Iam sure you know our region's above average rainfall isnt helping..

Condensation on outside of ducts (and refrigerant line insulation) suggests low or inadequate R values there. Still, the main problem is probably the high humidity. Hopefully your HVAC equipment is sized appropriately and not short cycling, which will mean improper dehumidification for the life of the system.

If problems persists and you arent short cycling it suggests that your slab vapor barrier is missing or poorly detailed and/or that you have excessively high air infiltration. Curious if you did a blower door test and know your ACH50? Iam also betting that you have only 7" of Open cell foam? You should have done (or been talked into) AT LEAST code minimum which is 11".

I am assuming your home is just being built/finishing up? If so, this can be fairly normal with tight shells. High humidity is a problem with tight, new homes. But tpyically for a short period of time. You have a lot of moisture inside the house from drywall drying, paint drying, even concrete. Even if it has been weeks or months since, products can still be curing out. Your humidity was probably always high, just did not notice it as much until you introduce a cold, condensing surface. The closed cell especially is a vapor barrier, so it will trap the moisture. You have a couple options to consider.
Your best option is to stick a free standing dehumidifier in the attic to pull the moisture out. The other option is to condition the attic to bring the humidity down.
With a tight, and assumingly high efficient home, you should experience reduced run cycles of your ac unit (good thing) but the trade off is shorter run times does not keep the humidity in check. Mechanical dehumidification will probably be needed in the hotter summer months or when outdoor humidity is typically the highest.

I have done a few high performance homes, and this is typically an issue at completion.

Brain
SC is zone 3, 7" of open cell is about right (tad low, should be 8") but certainly not 11, to hit IECC mins.

It's fairly normal for a new build to have very high humidity at HVAC startup. However, you should get with your HVAC specialist to make sure that your system has humidity control on it. Many times, newer homes are so tight that just the humans occupying them can create enough water vapor that the humidity will be high inside, even in the winter, which was a traditionally dry season. You want to be sure that you have a source for fresh air for the home as well.

Table 402.1.1 of the 2012 IECC calls for R38 in zones 2 and 3. If you want to build to the 2009 version and your inspection dept allows it, go right ahead. I think most experts would say the 2012 version is a much more cost-effective prescriptive requirement, thats why its now required in the new international energy code.

This isnt just about energy use. Its about maintaining comfort levels.

When you are going with such pathetically low insulation levels in such an important location the difference of 1" could be much bigger than you may think. 7" instead of 8" is roughly 13% lower than the poorest performance allowed by law in 2009.

Inspection departments and all those involved in building a new home should be building to the current international energy codes. At 7" the roof would be 37% below the MINIMUM current international building code requirements in zones 2 and 3. Arent we supposed to be building ABOVE what the minimum building codes require?

The IECC provides minimum energy conservation requirements but it is not intended to be a design manual. The code provides 2 methods of determining code compliance: (1) Prescriptive and (2) Performance.

The Prescriptive method [Section 402.1 General (Prescriptive)] is a rough "rule of thumb" method intended for use on simple buildings, additions and renovations with few energy variables. It relies on minimum R values for each major element of the building envelope as prescribed in Table 402.1.1 and does not consider the overall design of the project.

The Performance method [Section 405 Simulated Performance Alternative] is intended for use on new homes and considers not only building envelope insulation but HVAC efficiency, window area, etc. in order to allow balancing of energy conservation variables through trade-offs and arrive at an overall energy conservation rating. It relies on free RES Check software which can be used as a simple but effective design tool.

With either method projects must comply with the mandatory code provisions for air leakage, fenestration U-factor, controls, duct sealing, building cavities, and mech. systems insulation.

The energy code for South Carolina is the 2009 IECC.

Here is a link that might be useful: free RES Check software

This post was edited by Renovator8 on Thu, Aug 8, 13 at 9:42

oops, Brian you have 2012 IECC. We are still on 09, our state has not adopted 12 yet. My bad.

I see someone has been doing some reading..

Yeah lzearc, unfortunately I dont think any state has adopted it yet. Heck, most areas still dont enforce some of the 2006 requirements. NC has a hybrid that is mainly the 2009 versions with a few of the easy 2012 details.

Most of our homes do use the performance path, but all our homes use third party certifiers. The software can be used by anyone but the final result is best served by a third party professional.

The performance method is necessary to not stifle innovation. Its intended to allow freedom of design and is a great tool for achieving a balance for designs that may do something that doesnt quite meet the prescriptive guidelines but is still an efficient and comfortable design.

It is NOT intended as a way to circumvent the basics set forth in the prescriptive guidelines. The performance software is a great and amazingly accurate tool in the hands of the experienced and knowledgable. Its also easily manipulated and can lead to very bad design, performance and comfort by those unfamiliar with the many variables of the program.

The software is looking for the same R values as the prescriptive guidelines. Skimping in one area can be made up for by adding in another. If someone is making their home pass the 2012 IECC in RESFEN or other modeling software with only 7" of Open Cell foam at the roof deck, they must have some seriously efficient (and not cost effective) mechanical systems going on or are using the software wrong.

For the most part, following the prescriptive guidelines is the most cost-effective way to achieve the intended code which most experts feel is the sweet spot in balance of upfront costs and energy use/comfort.

Code inspectors generally dont enforce ANY energy codes. They usually just check to see that it has insulation and thats it. They generally dont care about your energy use or comfort although things are beginning to change.

Yeah SC is under 2009 IECC. One key requirement of this code that is arguably more important than the R value of the roof deck is passing a pathetically low blower door test of 7 at ACH50. GA has apparently begun to enforce this but not SC.

Its up to those building a home to see that it meets minimum energy codes. Or you can just do the minimum that your inspector will let you get away with. Your choice.