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Spray Foam vs. Blown-in attic insulation

Currently, I have blown-in fiberglass insulation in my attic which is not nearly what is recommended. So I need to add insulation. Recently I have read several articles promoting the use of expandable spray foam insulation. This foam is applied directly to the underside of the roof deck and to the gable ends. All attic vents(soffit and gable), are sealed and covered with foam. This process, in effect seals or "conditions" the attic. The proponents of this method say that it is superior to having the attic floor insulated as in conventional insulation methods.

Does anyone have any experience with this method of insulation? What are the pros and cons?


Timothy Miller
Re: Spray Foam vs. Blown-in attic insulation

Howdy, one draw back is a leaky roof and the foam not allowing the water to drip threw into the attic and onto a ceiling- alerting you to the leak.
But this water trapped by the foam can mold and rot the decking and framing of the roof. The other issue i ponder is where does all the moisture go ( since the famed roof i assume will not be vented) that otherwise is vented out of the house?

Re: Spray Foam vs. Blown-in attic insulation

R_Rosebraugh50 ....

Yep --- the method you describe has definate advantages of providing better thermal regulatiion of the attic by providing a semi-conditioned space.
The reason it's a semi-conditioned space generally there is no connection to providing heat or A/C into this space ---- then would technically be a conditioned space.

By applying insualtion along the roof and all perimeter surfaces along with closing off all vents to the exterior maintains a controlled temperature within the attic. This results in a warmer attic during the winter which reduces heat loss ( reducing the heat load on the heating equipment ) . The opposite during the summer with a cooler attic reducing heat gain ( reducing the cooling load for the A/C )

Doing this will also help reduce vapour drive from the lower living area into an attic space that has signifigant temperature and pressure differentials noramally associated to traditonally insulated vented attics.

I'm not convinced that applying foam to the underside of the roof decking is much of an issue when it comes to roof leaks. However , I would be more inclinded to consider the foam applied directly to the underside of the roof deck being somewhat problematic in the event of repairs to the decking is needed.

An alternative would be apply a backer spaced 1 1/2 away from the deck along the entire distance of each cavity. This should eliminate any concerns regarding roof leaks , allowing repairs to decking , provide the venting to the underside of the roof that some feel is needed for shingles.

The downside to this with spray foam is cost. Consider the square foot coverage for the underside of the roof and the gable walls is considerably more than the floor surface area. Spray foam is at least 4 times the cost as conventional insulation products.

2 cents. :)

Re: Spray Foam vs. Blown-in attic insulation

I used spray foam insulation when I redid my house a few years ago and while it is more expensive if it is a house that is a long term investment it might be worth it. I had the open cell foam sprayed in all the walls and the underside of the roof. With the open cell foam water can pass through it so if you have a roof leak you should still notice it. As far as moisture problems in the attic this should not be a problem. You get condensation when warm moist air hits a cold surface, since the roof is now insulated you do not have a cold surface anymore and should not get any condensation. Oh and my house is a 1-1/2 story cape so the exterior walls on two sides are only about 5' tall therefore part of the roof line is visible. This surface does not get cold and there has been no moisture issues.

As a side note, I also had the garage walls and ceiling spray foamed at the same time. I'm in RI and I know it is not Canada but for about 3 weeks earlier this winter the day time temps never made it above 25 and nights were around 0. During this time period the garage temp never dropped below 37 degrees and the garage is not heated yet.


Re: Spray Foam vs. Blown-in attic insulation

As a spray foam contractor, I believe it is the best solution. The problem you are trying to solve is to keep heat from migrating into the attic. A lot of your heat loss is from hot air rising into the attic. Any strategy you adopt must include air sealing to be effective. If you do cellulose in the ceiling, you need to seal every penetration into the attic. That involves more typing to explain than I am willing to do. It is MUCH easier to spray the underside of the roof. You don't have to deal with any existing insulation.

If you are in a very cold climate, you should use closed cell foam as moisture can slowly migrate through open cell foam. Our building code in MA says to put a vapor barrier on open cell foam. Closed cell foam is its own vapor barrier.

I have seen a number of houses with no insulation that had rotted roof decks and no indication of a roof leak. Open cell foam can hold a lot of water. I wouldn't want to hope that open cell foam would give you warning that the roof needs attention because of a leak. I think closed cell foam would confine the leak to a small area and minimize the damage.

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