I have a home with three unvented attic spaces with polyurethane closed cell foam sprayed to the underside of the roof sheathing.
I am concerned with the buildup of humidity within these attic spaces. At what humidity level should I be concerned in these spaces?
Also what is suggested approach for reducing the humidity in these spaces (adding supply and returns for A/C etc)?

Location - Northeast Water Front
Snap Shot of House Today
Outside Temp 81 RH 51%
Basement Temp 69 RH 49% - Dehumidifier Running
1st Floor Temp 68 RH 48%
Attic Space Temp 76 RH 66%

** The A/C has not been running at all today and rarely runs because of the closed cell insulation

Your suggestion is correct. Add some conditioned air into the attic space with returns. This should help stabliize the humdity levels to near the indoor levels.
I assume that your home is relatively well insulated and sealed.  That being the case, if your AC is not running at all, the indoor air could become stagnant.  If you do not have a HRV/ERV on you HVAC, I might look into one to maintain fresh air changes into your conditioned space.

The whole house is insulated with Polyurethane foam and lifebreath hrv's are installed. Should I be concerned with a RH of 66 or higher?

If your hrv is geared to run only when the HVAC is on, then it does little good when you have days where the HVAC is not running. Considering that the RH outside my house is over 80% right now, 66 doesn't sound so bad. I still think you need some air movement into and out of the attic spaces.

The tipping point where mold blooms on cellulosic materials (like timber framing or paper) explode is around a sustained 70% RH & up. Human comfort factor starts to suffer at 60% & up. Generally if the conditioned spaces are maintained at 60% or less, the semi-conditioned spaces will track that unless there's a humiidity source (like groundwater vapor in basements, etc.). If the attic kneewalls are permeable, the RH behind the kneewalls will pretty much track the adjacent same-floor spaces. If the attics are isolated, with the conditioned space only below, they'll run warmer, but should have LOWER RH, all else being equal. If the ceiling/attic floor is vapor-impermeable (impermeable paints, etc.) you may have a vapor trap.

But there could also be other issues: Bathroom/kitchen exhaust vented into a previously-vented attic, etc. Fixing those kinds of things should be the first priority. Otherwise, small dehumidifiers set up to drain into a legitimate drain (or daylighted out somewhere under the eaves far enough that it doesn't drip on the house) will see NOTHING like the duty-cycle of a typical basement dehumidifier, and will likely be both cheaper & more efficient than ducting the central AC to those spaces. Otherwise, wes' suggestion for a small return duct into those spaces should reliably bring down the RH by sipping cooler-drier fully-conditioned air up from the conditioned spaces. (It shouldn't take much flow- one air exchange per DAY is probably enough, unless you have an undetected humidity source.)

My immediate thought was you might have sand witched wood between two air tight coverings.  Remember the synthetic stucco fiasco?

The problem that you are having is most likely due to a common error that is made with tight thermal envelopes such as spray foam.  Oversizing of the HVAC system will prevent it from running enough.  A smaller system will run more and with a lower power consumption.

During more humid days, your HVAC system is like a large dehumidifying system.  When it is not running, it is not dehumidifying.  If you do not make changes to the HVAC sizing, you will most likely need to install a dehumidifier in the attic.  Let me know if I can help.

I am sorry to see that people continue to have this problem and it is sometimes viewed as a "black eye" for tight thermal envelopes.  EnergyWise can help those looking to prevent this problem.

Scott V.
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