Uvented Attic

Your thoughts? Have you seen any issues with foam insulation on romex,
condensation from humidity, shingles issues, energy issues, etc…?

I found this thread on the subject.

We have these on upper end homes where the insulation is sprayed onto the roof sheathing and no ventilation is installed. Since every house I have seen like this was less than a year old I did not see any issue with it.

The shingles may not last as long and any small roof leak will surely rot the sheathing for a long time before it is noticed. I doubt if they inspect the roof for leaks when the house is new before they spray it on there. The shingle warranty could be void too depending on the manuf and who they send out to evalaute a problem.

Since these attics are typically very large I wonder if the energy savings is really that great if any at all since the attic still gets into the 90’s during hot weather and the ceiling sheetrock is not insulated from all that warm surface area. As far as moisture goes, I doubt that is an issue with the large homes this system is installed on. I’ll take my R38 ceiling insulation over that system.

Good points.

My client has one of these ventless systems and the house does not seem to
be heating to the comfort they want. I know there are a lot of factors to look
at, but I was wondering what others have found when looking at these systems.


I have seen a couple of these houses and if I build myself another house it will have an unvented attic. I have been in a couple in the summertime and the temperature at 4pm was 82 degrees. By the way I would not foam the walls - blown cellulose or fiberglass works almost as well for not nearly the cost. Assuming the house was designed with cooperation between the insulation people and the HVAC people it should be very energy efficient. The problem I see is the house is seldom designed as a system with knowledgable subs involved early. I am going to be the phase inspector for a house here where the attic will be unvented.

I do not think the foam is causing the comfort problem. Is it a heat pump? Has the OAT been rather low?

I just recommended your IR class to a guy in Houston who was in my 5 day energy class I was teaching.

Blown close cell insulation is an excellent material and practice to overcome substandard insulation installation practices we see all too often (especially in complex roof designs/framing).

If they installed regular insulation properly in the first place, there would not be as great a need for this new material.

When I first came across this I contacted GIF and several HERS raters. My concern was increased roof temperature destroying the asphalt shingles. As with future roof leaks destroying the roof deck (which we really can’t work around).

Claims are that temperature rise of the shingles is only a few more degrees above normal (however this may push the temperature over the top and destroy the shingle). GIF stated that they would not void any warranties because insulation was installed below the roof deck, however in another breath they stated that excessive heat warranties would not be honored.

There are many variances that can ensure the shingles remain vented.
You can ventilate under the shingle.
You can use a hybrid approach and install ventilation baffles below the roof decking before applying the foam insulation.
If you read on a little deeper, you will start to notice that a different type of roofing is often recommended with this application. They are trying to get away from the asphalt shingles and go to more heat resilient roofing materials.

As far as the temperature in the attic goes, air leakage occurs through the ceiling tempering the air in the closed attic. At some point it will (should) reach equilibrium.
HVAC equipment (which leaks because of their own set of inadequate installation practices) helps heat and cool the closed attic space when located within the closed attic.
One source of heat transfer which increases heat in the attic is radiant heat emitting from the bottom of the roof decking. If the insulation keeps the bottom of the exposed roof deck at a lower temperature and emissivity of the insulation is lower than wood materials, the solar load will be less. Anyone know the emissivity of closed foam?

A very common practice in construction these days is to place the HVAC equipment in the unconditioned attic where in the south it is an extremely harsh environment, significantly reducing capacity and requiring increased equipment sizes. In my opinion the equipment should never be allowed in that location in the first place. Besides the equipment, the HVAC supply and return ducts are laying all over the attic space. The heat transfer rate through the air duct is a multiplier of the temperature difference above 50°. So insulating the roof provides a “buffer zone” where you can install HVAC equipment as well as lower the temperature difference between the interior of the house and a conventional attic.

These new installation techniques require a different perception of the process as does our perception of what insulation formally did when we installed it in the ceiling. You will lose in some areas but you may gain significantly in others (such as the HVAC efficiency). The SEER is not the same when you install the thing in a 150 degree attic.

Creating an airtight seal in the attic reduces stack effect. Therefore there is less air leakage into the finished space below.

The greatest number of leaks and penetrations in a house are located in the ceiling. If the ceiling is not against an exterior space there is no leak because there’s no pressure differential (even if there is a passage opening).

Air leakage is a greater energy loss than reduced R-Values.
Those of us with thermal cameras see those blue lines across the ceiling and wall juncture in every house. You cannot share the space at the soffit/eave with ventilation requirements and insulation that requires more space than you have in the first place. Not only is the R-value reduced, air infiltration is prevalent at these locations because of the complex framing at this location. Insulation reduces airflow, reduced insulation allows greater airflow. Loosefitting framing members allow air infiltration into the wall.

The biggest problem is when the old standards meet new technology.
Installers that continue to use the old standards by simply replacing fiberglass insulation with close cell foam and not adopting all of the applicable changes in insulation standards will produce a system just as inefficient, if not more than what we had before.

I had a very hard time swallowing this new technology that first but as I thought about it and realized how inefficient our existing installation practices are, I can see where this application has significant application under certain circumstances.

Foam insulation can be used in a hybrid approach in conjunction with past insulation practices and be extremely effective in improving efficiency. I don’t necessarily agree with foaming the entire house (the cost cannot always be justified).

Great info David… thanks.

It would seem that the entire application of spay foam is dependant on the need to
be air tight (especially in high humidity areas like my location in Texas). If there is a
transfer of humid air in any direction, this method could turn into a disaster of
condensation spots trapped in areas, some of which cannot be seen by the naked eye
(moisture can be conducive to mold, decay and wdi… etc…)