Sealed Attics

By adam
July 1, 2013   11 Comments

… spray foam as it pertains to our mixed-humid Raleigh, NC climate…

Foam is the best insulator, and it adds strength to everything it touches while is stops air movement.
Spray foam is possible due to specialized heating, pumping, mixing, and breathing equipment. Unlike the cans at the hardware store, to get the speed and consistency that you see in our photos here, it comes in two parts; two 55-gallon tanks mixed roughly half and half. It should be mixed and heated just right. The actual chemical ratios, formulas, and temperatures used may need to be adjusted for various reasons and this is done constantly throughout the day by our foam technicians.
The hardener, aka the “A-Side”  of the foam is pretty much liquid glue and always the same.
The bubbly part, the “B-Side”, is the expanding part
, and might have a small percentage of soy or other solids to enhance performance.

An average Raleigh, NC roof assembly of plywood, paper, and shingles will absorb and let pass, considerable amounts of vapor. Just a little sun-light turn liquids into vapors pushing moisture through bricks, wood, and lots of things within minutes. When the roof is wet and the sun is out, SOLAR VAPOR DRIVE in NC will force vapors all the way through the entire roof assembly, through the foam, and into the attic. It is important to realize that there is not force like solar-radiation to push the moisture within the roof decking or open-cell foam, so the moisture slowly dries to the inside of the attic, which is fine.

With Open-Cell Foam  the vapors pass straight through, which is GOOD, the moisture is dehumidified by the HVAC system.

Closed-Cell Foam. If the bubbly part doesn’t bubble too much and the walls of the bubbles are thick enough, and sticky enough, the Foam will stop water and vapors too. To get this vapor barrier, we simply swap out the B side for Closed Cell chemicals or “C” chemical. USING THE RIGHT FOAM Formula, AT THE RIGHT TEMPERATURE & PRESSURE IS KEY. Closed cell foam is usually installed to the density and thickness of a brownie. I wouldn’t recommend this approach in  NC attics because the roof might dry slower. I do not think you want a vapor retarder on one side of your roof-decking and a vapor barrier on the other, so either use open-cell foam to insulate your standard asphalt roof system, or eliminate the roof paper all-together and used closed-cell foam.

Open cell foam expands so much that the bubbles pop. There is no amount of realistic air-pressure that can force air through it, but vapors can pass through it evenly and slowly. Sometimes liquid water can literally drain right through it while it stops air and insulates. Open-Cell Foam expands faster and after it cures you can poke your finger through it. Spray foam is time proven. After more than 35 years since adoption of spray-foam into codes around the world, it is a trusted way to maximize savings and benefits and reduce construction times.

“Closed-Cell-Foam” is a vapor-barrier, it can be used to help water-proof a crawlspace or basement and boost energy savings. It is half glue and half insulation.

A SEALED ATTIC requires sealing off all venting and thinking about the consequences of closing the spaces where gas piping, gas water heaters and furnaces are located. Older homes might have “attic-vented-bathrooms” and new equipment is needed.

A VENTED ATTIC can be very problematic if it houses your HVAC system and is colder than the ambient temperature in winter and hotter than the ambient temperature in summer. Also, air-pressure, air-quality and moisture levels can be unfavorable.

In 1987 my father built a house for our growing family to live in. Both of my parents were working from home to a degree and we had a 3rd kid on the way, oops. My dad built what every builder wanted to build; A simple, 2-story box. However, instead of building a vented attic, he built a walk-up, third floor. The little spaces behind the knee-walls were finished, virtually eliminating all “attic space”. So I guess you could call that “sealed”, but the attic portion of the house was eliminated leaving tons of conditioned storage. The whole volume was put to excellent use by our family and had and will always have zero “exposed” insulation.

What we are talking about today is an alternative to a vented attic that saves energy and offers new benefits compared to even the best vented attic designs. It is an attempt to cooperate with the space between the ceiling and roof, when that space is problematic.  The key is air-sealing, and getting the ducts and/or air-handler “inside the envelope”, and if enough foam is sprayed to cover the joists and other perimeter supports, you can eliminate lots of “thermal bridging”. Spray foam is the only product that seals as it insulates.

A Sealed Attic can save money, by bringing the HVAC equipment inside the “envelope”, at which point you could say it is not leaking at all. The savings will, in time, pay for the upgrades, boost your resale value, and offer added storage.

In this post we will learn:

  • How to seal the attic
  • Which materials are used
  • How to get the critical details right
  • How to avoid mistakes


Where do I get foam? What type do I buy? How much do I need?

One great resource for energy conscious consumers is a non-profit named Energy Federation Incorporated

EFI sells all things energy efficiency, and many different spray-foam packages. They are extremely helpful and talkative. I recommend calling in. They walk you through the website, and send you or give you access to a current price list. If you decide to do this yourself, you’ll end up hauling tanks and hoses around your attic. The tanks resemble propane tanks like the ones for your grill, and weigh about 60lbs. Once the foam starts spraying it can be pretty fun. It feels so good to save energy. Protect your lungs to the best of your abilities.

Spray foam comes in open or closed cell formulations. And comes in either a can, a pair of hand-held tanks, or a truck full of equipment and barrels. When you are using foam to insulate as opposed to air-seal, it requires a seasoned foam contractor.


Open-Cell Foam

Open-cell products allow moisture, but not air, to permeate, which can be great, or problematic. If a building material temporarily stores some moisture, while moisture loads are high, and can dry out relatively quickly, you have durability. Open-cell foam is soft, and when trimmed, feels like a magic eraser sponge. It is generally bright white.  After it sets, and is completely permanent until you tear it out.

Closed-Cell Foam

Closed-cell foam can stop all types of moisture and is hard like rock. It is rigid and nothing compares. If you were to spray a one-inch layer on to a disc of card-board, it would make for a nearly indestructible toy of sorts, but a knee-high dog with healthy teeth could tear it to shreds. The color may start blue or white but will yellow with sun exposure. The surface is tough and bumpy yet smooth.

In “Mostly Heating Climates”…
Warm, moist air from indoors will want to travel towards the roof deck. (Warm air rises and carries things with it.) A closed-cell product against the roof deck, stops the humidity where the closed-cell insulation starts. If the moisture is stopped 5” or so from the frigid roof-deck, the surface with be warm enough to avoid condensation.  This same amount of moisture could permeate an open-cell product allowing condensation to gather against the cold roof-deck. So, in the North, choose Closed-Cell Foam for sealed-attics. And in cold winter climates it can be very dry, so retaining some moisture is a good thing.

In “Mixed-Humid Climates” … or Hot-Humid Climates,
Moisture will condense on the roof during the night, and as soon as the sun hits it in the morning it will be forced through your shingles, building paper, roof-deck, foam insulation, and eventually be dried by your HVAC system. This outcome is far better than trapping the vapor against the roof deck, because wood might rot if it can’t dry out fast enough.

In “Dry Climates” …
In dry climates, the extra cost of two-part, closed-cell foam might be wasted; however, a closed-cell product will keep moisture in. If you use a humidifier regularly, you should ask yourself if there could be any cost savings there. Also, closed-cell foam provides so much strength that it could extend the life of the roof deck itself. There is a lot to consider, so ask around if you have doubts about your plan.

Prepping the Attic
If the roof surface is in bad shape, or the decking is losing its strength, you should fix those things first. Asphalt roofs are vapor-permeable, metal roofs are not. Some roof materials reflect a majority of the sun’s energy, others absorb the majority. If you have damaged decking, foaming it means keeping it for good. If you are going to invest in a sealed attic, make sure that you are applying the new insulation to clean, durable surfaces, that will allow the product to perform really well for life. Neither type of foam repairs roof leaks, and covering the problems with foam only makes things worse.

Blocking and Backing is needed before foaming begins.

Large gaps should be treated with a “substrate” so that your very expensive foam goes only to beneficial uses. One example is the space between the top-plate of the wall and the roof-deck on a home with a vented soffit. These and other such penetrations can be sealed with a number of different backing materials, ranging from oriented strand board to fiberglass insulation to solid lumber; the foam is then sprayed over the backing.

Old insulation on the floor of the attic, especially “paper-faced batts”, should be removed to allow the living space below to “moisture-equalize” and “temperature-equalize” with the now “tempered” attic. This may prevent moisture problems, and allow for the friendly temperatures your attic HVAC system would like to operate in.  In general, when you are transitioning from a vented-attic to a sealed-attic strategy, removing the old insulation is a good idea. The new space will be cleaner; the visibility will be improved, which will reduce the chance of miss-stepping and falling through.

If you have complicated geometry, partitions may need to be built. The garage attic or porch attic won’t contribute to the cause, so build a partition to separate the attic into a sealed space over the house and a vented space over the garage or porch or carport, etc. Insulate the partition wall. In a few homes with car-ports where the attic ladder was located in the car-port, we simply sealed the entire attic and that included the car-port ceiling, so that it was easy and simple to access everything.

Code requirements also come into play during the prep stage. For instance, you can’t spray foam into the gap around a chimney or combustion exhaust flue; these need to be sealed with code-approved materials like sheet-metal and fire-rated caulk and maybe rock-wool insulation although a small gap in an otherwise completely sealed attic without a little foam is not a major issue.

Sealing the Ducts

If you are actually going to do this project, completely renovating your attic with spray foam, why not use the spray-foam to seal your ducts?! What I have done in the past is use a knife to cut the zip-ties or tape that is currently holding the old insulation on the old duct work. Next step; expose all the leaks. At this point I am imagining an attic with no insulation, or all the insulation has been raked into piles that don’t hide important stuff like duct-work and attic bypasses. Also, the duct insulation has been modified so that all the holes and seams and joints are exposed. Now you can use the foam to seal and insulate your ducts. Once again, if your system is really old and gnarly, it probably make more sense to replace it, but if it has some potential to last 5 or ten more years this could do it for you.

If correctly designed and installed, a duct system will pull the exact same amount of air that it supplies to each space. That way the home is compensating for the lack in balance.  A duct leak in the attic could put the downstairs into negative pressure. Areas under negative pressure are bad, but when that area hosts a combustion device, like a fireplace or water heater, the problems can be deadly. If you are in this realm of possibilities, contact a qualified consultant. Unfortunately, the average HVAC company isn’t qualified to help you with this analysis.


Design it right, so that your ducts and attic are going to be tight and insulated right. Use the right product to avoid moisture problems. Pay attention to the details of blocking and backing. While you are up there expose all the problems that might be present. You must be present to do this the way is deserves to be done. Read the directions.

Foam is weird; the temperature, humidity, and surface characteristics are important: Make sure that the tank temperature is above 70 degrees, the surface temperature is above 45 degrees, the air temperature is above 50 degrees, and the results are going to be great. You want to completely cover the rafters at least one inch to cut down on the thermal bridging, and if you are going to cover the foam with drywall or something else, you want the entire cavity to be filled, especially if snow could potentially fall on the opposite side.

Please don’t hesitate to call Stetten Home Services if you have questions about SEALED ATTICS or any other energy related question. If you think I missed something, let me know. Thanks for reading!