HVAC in unvented, spray foam insulated attic?

Is there a verdict? pros and cons / effects
of unvented attic, sealed with Icynene spray foam
on HVAC system and ductwork?
[Plan is for 2-story construction, Central Texas
(read: outdoor temps around 100 F, humidity around 40%…) ]
Anybody with experience with such a setup
under these conditions?

How about considerations for “make up” air for HVAC system
for this?

I would not be a fan of non-vented attics in Tx or Ok. due to moisture considerations. My thinking would be the ducts would have to be extra insulated to compensate for increased heat load.

Your use of the words make up air can be confusing, make up air is normally thought of as infiltration of normal use of external door use. Not to be confused with combustion air normally taken from an attic area. I would have concerns with combustion air on a totally sealed attic if truly sealed.

Larry…I inspected an icynene insulated house last summer just east of Pflugerville. I think it was around 4 years old or so and saw no issues with the ductwork in the attic. I’d have to go find the report and photos to refresh my memory but give me a call if you want some more information. Just visit my Homepage to get my number. Also see: http://nachi.org/forum/showthread.php?t=5062&highlight=icynene


Cen Tex temps in sealed attics, insulated w/ spray foam…
Isocynene ‘advertises’ attic temps reach only about 80 degrees
instead of 130 degree range…
Have you found this to be somewhat accurate? such
a considerable reduction in attic temps in Cen Tex?

We are moving toward spray foam insulated attics without ventilation in this area. I inspected one last week that was a few years old. The outside temperature was 80, and the temp in the attic was 76 in the mid afternoon sun. The attic I was in earlier in the day was well over 100 degrees with standard insulation and ventilation.

The utility bills on this home were also very low, comparatively.

I saw no issues in the attic or on the roof, however I understand that there is some concern that this type of insulation will cause a shorter service life on asphalt/fiberglass composition shingles.

I can not find it now but the article I read said there was not much difference in life and the Temp of shingles was only 5 degrees different .

. Sounds like a great way to go with all that I have read .


When this first started there was a big concern with the shingles and warranty issuse, but there are several of the big roofing manuf. that warrant this type of contruction. http://www.jlconline.com/cgi-local/view.pdf/1a8ee114cea4c996236a4c5459e3d431/www.jlconline.com/cgi-bin/jlconline.storefront/462e26c700291c2027177f000001053f

This method of insulating is being used in quite a few of the “Build America” homes with the HVAC unit/ducting in the attic. It’s a big energy saver since any heating/cooling loss of conditioned air is essentially into the conditioned space of the attic and the ducting does not “see” the extremes of freezing winter and boiling summer temps. It also saves airsealing all the wiring and other penetrations of the ceiling.

This was brought into the program by a Canadian ex-R2000 engineer, Joe Lstiburek, the principal at Building Science Corporation ( www.buildingscience.com ) He has also worked with the American Lung Association on their “Health House” program as co-chair of the technical committee. Believe it or Not!!! A healthy new home is airtight and heavily insulated.

Here is one from last year. Appeared to be working fine. I have some concerns about not being able to find a leaking roof until too late. Brought this concern to buyers attention, they were non-pulsed about it.

and another

I still have reservations about the life of roof coverings, but getting more convinced daily. One thing for sure: We’ll all have to get better at spotting leaks in their early stages, before obvious damage takes place, since water would most likely be wicked into sheathing material. I’d like to hear others thoughts on this.

It can out-perform ventilated attics, but there does need to be some way for moisture to escape from the home to maintain indoor air quality because the home will be more tightly sealed when the underside of the roof is foamed.

Joe Lstiburek from the Building Science Corporation puts it a lot better than I can. His articledoesn’t have all the figures included yet but when it come to building science… HE’S THE MAN!

Venting the attic only cools the roof by about 5%. Light colored roof-covering material can cool it by about 25% and that’s tops. (Building Science Corp. & DOE).

They recommend a low-permeability foam be used, so my question is more about the likelihood of roof sheathing decay if moisture gets trapped between the roof-covering material and the foam, especially if the roof membrane is also low-perm.
The one advantage I see to ventilated attics is that they allow the sheathing to dry to the attic side if the roof leaks.

My day job is working in the roofing industry at the manufacturing level, quality control lab. The issues about shortened life of roofing are not an issue do to the fact that the temperatures under the shingles (roof deck) are much lower than they would be in a vented attic. In Florida it’s not uncommon to have temps of over 140 degrees in vented attics. The temps in the attic with rigid spray foam are similar to the temperatures in the rest of the house. As to the issue of make up air, some type of energy recovery system is needed (air to air heat exchanger) this will bring in fresh air and allow stale moist air out, without this it would be like placing your house in a plastic bag. Bad.

Really informative thread, guys. I’m almost convinced! But I’m with Kenton concerning moisture trapped in the sheathing. Guess we’ll all find out eventually.

I’m new to Icynene, but I understand it’s an “open cell” product that allows moisture to slowly penetrate. I’m told that any roof leak would manifest itself as a slow leak through the icynene directly below the leak. Can anyone confirm?

I ran into an insulation contractor yesterday that was spraying the attic. He assured me that any moisture from a leak in the roof will travel through the foam insulation.

He joked that it will show up more readily because the sheetrock ceiling will have a stain on it real soon! :smiley:

None of the older attics or homes I’ve inspected with this type of insulation have had any moisture issue. Of course, we have no humidity near the coast here in FL…:smiley: :smiley: :smiley: