Sealed Attics

By adam
July 1, 2013   11 Comments

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

Foam, the best insulator, adds strength to everything it touches while it 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 2 parts; two 55-gallon drums mixed roughly half and half and heated. The actual chemical ratios, formulas, and temperatures used need to be adjusted to compensate for changing surfaces/factors, this is done constantly throughout the day by the best foam technicians. It takes a 2-3 man crew to handle the work happening when a spray-foam rig is in action.
The hardener, aka the “A-Side”  of the foam is a “thin liquid glue”.
The bubbly part, the “B-Side”, is the expanding part
, and might have a small percentage of soy or other solids to enhance performance with respect to R-Value, Fire Resistance, Water-Proofness, Color…

A typical Raleigh, NC roof assembly of plywood, paper, and shingles will absorb considerable amounts of moisture. We rely on the tar-paper or underlayment, to seal and protect the decking.  Under enough vapor-pressure, tar-paper, and many underlayments let all vapors pass through eventually. Radiant heat from the sun causes solar vapor drive to occur pushing moisture through roofs and into attic spaces.

With respect to asphalt roofs and “sealed attics”, there is not much force like solar-radiation to push the moisture in reverse, back through roof assembly. We often rely on moisture slowly drying to the inside of the attic, where it is processed by the a/c or heat, which is fine, but with enough planning, and a little extra thought, the roof can be designed to resist moisture, limiting the latent load of rain-then-sun (or dew-then-sun), but also design with strategic “vapor-vents”. Foam is different in retrofit applications, because everything is done, so the insulator need to consider the vapor-permeability of the system.

With Open-Cell Foam  the vapors pass slowly 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 affect, the B side is swapped out for Closed Cell chemicals. 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 that has dried out or has been burnt. 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, and allow the now sealed gable vents or ridge vents to remain vapor-vent-able. So spray some foam on an adjacent surface and peel it off while it is still sticky and stick it over the gable vent screen or louvers to that the entire building envelope in that area that used to be the vent is now nothing but open-cell spray-foam, and if needed, TYVEK or some other weather-proof BREATHABLE barrier.

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. (like a “magic eraser” sponge) 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, as a roof surface, and to boost energy savings and building durability. 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

Amazon is a good place to buy tools and devices. Lowes and Home Depot now special order foam kits and air-sealing foam with the upgrade gun instead of the infuriating bendy-straw

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. and it yellows rapidly when exposes to sun-light or bright lights.

Closed-Cell Foam

Closed-cell foam can stop all types of moisture and is hard like badly burnt brownie. 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 will 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. I like the use of vented-roof-assemblies and metal roofs everywhere, and if I ever build a northern building I would build it with a metal roof over purlins, and spray open cell foam at a thickness of about 8″ and then a layer of closed cell foam about 2″ thick and only where the purlins cross the rafters would there be a thermal bridge. The closed cell foam, which is expensive would keep moisture from the getting inside the much cheaper closed cell foam while the metal roof takes solar vapor drive potential to zero.

In “Mixed-Humid Climates” or Hot-Humid Climates, with an existing asphalt roof…
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. Plan on allowing the disabled attic ventilation system to remain vapor permeable as much as possible.

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, and in areas where the gap is 8″ or less, often the technician can “build-up” the foam in multiple passes to create the height needed to seal off the soffit durably. 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 continuing the foam on the car-port ceiling, so that it was easy and simple to access everything in the attic from the same old access.

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. If your system is really old it probably makes 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 push-in and pull-out the exact same amount of air into each space, and that won’t be affected by door-closure. Many homes have the public areas negative, and the private areas positive and door closures greatly affect temperature balancing and air-flow. A duct leak in a vented or sealed attic could put the downstairs into negative pressure and the attic into positive pressure.  Areas under negative pressure are generally bad news, but when a negative 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. A worst case analysis can easily be done without equipment: crank up your central heat so that the fans are running on HIGH and the heat is on HIGH. Close every interior door. Turn on every bath fan. Run the stove exhaust. Run the dryer on high. Now, with the use of a single square of toilet paper you can go crack open doors, windows, or hold it under a door, to see which way the air is flowing and how fast. I like to crack a small window 1/4″ in the living room or kitchen and hold the toilet paper while it is blowing in hard, and then go and open every interior door, then turn things off one by one until everything is off but the hvac, and see what happens. It is a very telling experiment most of the time.


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!