Unvented (Spray Foam) Attic and other questions

[Brian A. MacNeish]

Originally Posted by rcooke

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.

[Matthew Correia]

UPDATE:

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.

http://www.nrel.gov/docs/fy05osti/36643.pdf

http://www.buildingscience.com/docum...umid-climates/

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

[James H. Bushart]

Originally Posted by Matthew Correia

UPDATE:

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.

http://www.nrel.gov/docs/fy05osti/36643.pdf

http://www.buildingscience.com/docum...umid-climates/

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

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

[Matthew Correia]

James,

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.

Matt