Unvented (Spray Foam) Attic and other questions

Hello All,

I live in Elmore County Alabama (DOE Climate Zone 3A) and I had some major refurbishments done, in order to improve my home.

First, I had my attic spray foam insulated (i.e. retrofit from vented to unvented attic) with closed-cell foam. The contractor has approx 8 years exp w spray foam, and 20+ years as an insulation contractor. I opted for the closed-cell foam, as recent code recommends impermeable barrier to the roof deck. We left the fiberglass blown in insulation in-place over the ceiling joists, and contractor stated that was O.K. (i.e. cause no harm … can be removed later).

Second, I replace the 3-1/2 ton air-source heat pump with a 2-ton ground source heat pump. The HVAC contractor did the Manual J and with the ducts in the attic, the spray foam attic retrofit enabled me to go w a smaller GSHP unit.

With that in mind, the attic is a lot cooler than unvented (i.e. 105F versus 140F on a 95F day), but not 10-15F different from house temp.

Took some temp/humidity readings today (20 Jul 11), they are Outside temp: 80F, Humidity: 84%; Indoor house temp: 78F, Humidity: 70%; Attic temp: 86F, Humidity: 63%.

My questions are:

  1. Should I remove the fiberglass blown in insulation from the ceiling joist? Is there any benefit?

  2. If I remove the fiberglass blown in insulation, should I use some of it to fill the remaining 2x6 roof deck space (i.e. spray foam filled 2 1/2", leaving 3"). I would use rigid foam board affixed to the bottom of roof decking 2x6, then fill from top.

  3. Are the indoor house and attic humidity readings reasonable for my area(CZ: 3A), at this time of year (Jul)?

Thanks for your assistance.

 1 no
 2 no
 3 no
 4 not my area

Open cell foam is recommended for these type of applications since, if the rook leaks, you want to know about it and you want to know where the leak is.






I agree with Kevin and James.

Clearly, HVAC and home air circulation is a science. New systems and applications should be designed. For sure you have too much humidity. Start looking for mold. How much air movement or ventilation do you have? Just filling spaces with insulation and not adding air circulation or some type of additional venting is not proper.

I see this often. Adding insulation and not doing anything else in an older home. Generally, start seeing mold in closets, and along windows. Air exchange is important.

Moisture in an unvented attic will enter through air bypasses and ceiling penetrations. Most of the moisture that enters it will be generated from within the living space, below.

do a blower door test to make sure you have proper ventilation and if attic is unvented then it should be conditioned

That may work in mixed season humid climates but not in cool/cold locations!!

July 2005 lead story in **Energy Design Update **is about an icynene foamed house in Vermont where no air/vapour barrier was used…“the foam will tighten the house” said the contractor. After a couple of years, water could be squeezed out of the icynene!! IMHO, should use 2 lb closed cell foam on the roof sheathing.

I would find out where the moisture is coming from and get that stopped .
I would be careful about adding attic ventilation this can just pull the moisture from the Home .
Bath ,Kitchen and laundry drying should all exit to the out side

Adding a dehumidifier in the Basement might be all that is needed .

I live up in the cold north place called Canada…The RH readings seem reasonable for your locale and the figures you mentioned. Do you use the AC much and was it on when these readings were taken?

RH is much too high. Look at the shell and find the air leaks which is drawing moisture in. Wall to foundation seal; vents/penetrations; range duct; dryer vent; unsealed electrical outlets; windows; door seals; uncaulked baseboards/molding.

I live in FL. today it is 93 with ambient RH of about 85. My home interior temp is 78 w/RH of 44. My foamed attic is temp 79 w/RH of 42.

My home is 2 story, 4600 under air.

CDC says above 50% humidity can cause mold growth. Which I agree, especially in hotter climates.

Thanks all for inputs.

I researched unvented attic before having ours retrofitted in Mar 2011, and ensured it was installed in compliance with IRC 2009, see link:http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_8_sec006_par003.htm

Applicable conditions were meet (i.e. items 1, 2, [3 & 4 do not apply] and 5.1)

Will check and seal windows/door frames (windows replaced in 2008, but frames were not spray foam sealed as requested :(), as this is probably the likely source. Recall having blower test done and inspector pointing out these areas, so must get on them.

Still, very pleased as GSHP and unvented attic has cut electricity bill by 1/3 with cooler home.

Will monitor for mold, but so far none forming.

Thanks again for all your help.


Be careful on spray foam around windows and doors. Read and follow the manufacturer directions. Most important use the right type spray foam. If you use wrong type you can hinder the operation of the windows and doors. Expansion will push against frames.

I have seen this condition many times.

I’m in the process of utilizing two different brands of spray foam; both for window and door application.

One is “Great Stuff” for Windows and Doors, the other is “DAP tex Plus” for Windows and Doors.

Both are ok, except GS requires acetone to clean off an area if you have overspray, drips, etc. DAP is water based and easy to wash off hands, trim, etc.

Nossle on DAP product is easier to control and foam expands at a slower rate than GS, so you can stop before the product overexpands from the gap.

I completed one large window (64"X64") this weekend. took two cans to do the job. Gaps were huge (1/2") around window. Will tackle other windows/doors soon.

FYI: Bought Acu-Rite indoor temp/humidity reader, and indoor RH reads no greater than 63% over 24 hours after spray foam the one large window. Original blower door test in March, (before unvented attic spray foam retrofit) indicated air infiltration of aprox .2655.

Will keep you posted on results.

Thanks Mathew all information appreciated .
I just removed my roof vents ( installing a 8,000 watt solar collector ) Do not expect to replace them .
Will be closing two Gable vents this week ., Attic Insulation 17 inches and have closed air leakage to Attic from Home … Roy… I wonder $3,500;00 to spray roof … Need to think about that

Although I am all for no venting in new truly airtight homes, I would keep these open or close them in a manner in which they may easlily opened in the winter, if necessary. An older house cannot me made as tight as a new one simply by retrofit airsealing techniques; some air leaks will still exist. There may be enough air leakage at soffit/fascia joints/cracks to take care of any small amounts of moisture that may still end up in the attic. Check the attic after a week or so of cold still nights.


I have filled some more window and door frame gaps (French doors), as well as employed a Whirlpool 45-pint dehumidifier I had in storage.

Humidity readings now range between 50-55% over 24 hour period. Dehumidifier is extracting approx 6 pints of water in 24 hour period.

Discovered several articles from Dept of Energy and Building Science Corp that indicate supplemental dehumidification may be necessary in Hot-Humid South (3A) zone.



As my home is a retrofit unvented attic, it may now fall into this catagory. The research seems to suggest a stand-alone dehumidifier providing supplemental dehumidification will keep the home within safe/comfortable indoor air quality criteria.

Has anyone seen these configurations during their inspections?


Matt, what is your theory as to how 6 pints of water per day is finding its way into your “sealed” attic?


To clarify, the 6 pints/24 hours and the 50-55% humidity readings refer to the living space, not the attic area. Sorry for any confusion.

As for a theory, the 6-8 pints of water vapor the dehumidifier is extracting from the living space is probably from normal day-to-day living and humid air entering the house from leaks/pressure effects for unbalanced air flow.

Original blower door test in March, (before unvented attic spray foam retrofit) indicated air infiltration of aprox .2655. The house is probably more air tight now then before. However, some outdoor humid air will enter, and will be an addition to the latent (moisture) cooling load, but not the sensible (temperature) cooling load.

Lowering the thermostat does have an effect, but I have to lower it to 72F. I’d rather be energy wise and have the thermostat set at 75F and operate supplemental dehumidification, if necessary.

The science seems to point to a need for supplemental dehumidification in my DOE climate region (3A), but wanted to hear for those in the profession, if they’ve seen this in practice or is this too new to be out in the field.

Thanks for your feedback.