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An Unvented Attic is The Best Option

jar546

jar546

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After renovating homes in Pennsylvania and crawling around in over 10,000 attics for both code and private inspections and doing work in attics in Florida, I have a very strong opinion.

It is my opinion that a vented attic is just never going to be as good as an unvented attic when you move the thermal envelope to the underside of the roof. Let me explain.

With all of the existing home inspections that I did on buildings anywhere from relatively new to over 100 years old, a high percentage of the time there were problems. Mold growth, black staining, rusted nails dripping onto the insulation and more. Accessing an attic like this is often difficult as you are stepping on insulation and all of the ducts are insulated, making the larger. When I built a new home in Florida 5 years ago the builder used spray foam on the underside of the roof framing. My attic was never super hot and when you went up there, it was easier to move around and work if you were going to do some wiring. You also don't have to deal with all of that fiberglass in the air when you disturb it or the mess you get with blown in insulation that gets everywhere when you open the door.

The wear and tear on an air handler / HVAC unit is less because it is located within the thermal envelope and just adding a little RValue to the duct work, works wonders even though none is required within the thermal envelope. Keeping the heat on the outside of the building makes energy efficiency that much better.

Now I've seen people try to use fiberglass batt insulation with baffles for the entire attic but that creates a whole other issue with the paper-faced insulation being exposed along with the fact that you are still relying on the airflow between the batts and the underside of the roof sheathing.

Anyway, I thought I would just throw this out there. If I ever build a house again, it will have an unvented attic for sure no matter what climate I live in. If you don't agree with me, you are wrong but feel free to post your opinion anyway.... :)
 
When we renovated our home we changed our addict being unvented. We had closed cell foam sprayed on the underside of the roof. We have 7" +/-on the entire underside of the roof. We didn't open up the 1st and 2nd floor walls but did going to the basement and spray the rim joists and any other open available areas. Most of her basement is open but we have a small section of crawlspace under part of the kitchen. In that area we sprayed the underside of the floor of the kitchen so it would feel warmer.

As a result of this project and the installation of new HF gas furnaces and SEER 16 HVAC units (one up, one down) we cut our monthly utility bills by over $100/month. In addition everything more comfortable temperature wise. Certainly something I wouldn't hesitate to do again if the opportunity arose.
 
I agree un-vented attics and crawlspaces are better when done correctly

Pros and cons to everything. When you re-roof, and you will, make sure the contractor does not trap moisture that may be exposed to the existing roof sheathing when applying the underlayment. That was a big issue with SIPS panel's in the NW, NE and Alaska. The roofing contractors where installing the roofing and trapping the moisture in the OSB panels and they would rot away in a few years.

Not to steal your post but did you add an ignition barrier to the foam or install a foam product that met R316.6
 
My opinion/observation is that insulated decks (spray foam, SIPs, etc.) are harder and way more expensive to replace, and shingles will not last as long with no air moving underneath what they're fastened to.

Completely anecdotal, but: I live in a ~140 y.o. farm house with 1x sheathing and a vented attic. 16 years ago a good friend of mine built a brand new SIP house 5 miles away (as a crow flies) at the same time I put a new roof on mine. I know they happened at the same time and that the shingles are identical because I worked on both projects. I also know it was 16 years ago because it was the winter/spring before my oldest was born. Anyway - right now, my roof looks just the same as it did when I put it on. No leaks, no floppy shingles, and most of the granules are still there. It's got another 16 years in it easy. My buddies roof? He can't keep the shingles on it, it's mildew-y and stained, and most of the granules are washed away down the downspouts. It's bad enough that he asked me the other day if I had time this coming summer to help him put standing seam on it.

So is this proof of anything? Certainly not. But it sure helps sway my opinion in one direction...
 
I do like foam, but the roof is a big thing. You get a leak and you will be replacing decking as well as cutting out the foam in that area and respraying. I have also noticed that several roof sealants are not compatible with different types of shingles. They are causing blistering or premature failure at flashing's on all roofs here, but the worst ones are with foam which in turn causes leaks. A lot of hot dry days in the summer speeds up the process.
 
I like foam also, but agree that it makes everything more difficult if plywood roof sheathing needs to be changed out in the future.

How about a system where plywood is attached to the underneath of the rafters and then that is sprayed with foam. The space on the back side of the plywood, between the rafters, could be ventilated with soffit and ridge vents as we normally do. Maybe shooting foam on plywood wouldn't work well, but I would like to explore the idea a bit more.
 
I'm a fan of closed cell foam too, or a hybrid of a couple inches of spray on the underside of the roof sheathing, then batt added to get to the required R value (a slightly cheaper option then full spray). But as JCraver pointed out, the advantages of the roofing material getting air under it prolongs its life...a lot of roofing products require a vented roof or the warranty is void.
 
A few years ago at least Santa Clara County had suspended use of it, as one inspector told me: "I don't allow foam, we have two houses here that it is raining inside." I even had some subcontractors refuse to bid a home unless I assured them before they came out that I wouldn't be using foam, when the lawsuits hit every subcontractor remotely close, like plasterers, get named and spend months in daily depositions to say nothing of increased insurance premiums. A main reason the way, I see it, is that we put so much steel in our homes (for seismic reasons) that condensation on the steel is causing the problems.
 
I'm going with a steel roof with no breaks for vents. All venting will be done through the end walls with screened and louvred vents. Close to the botton of the attic in front (but above the insulation) and close to the top in back. It's only about 500 sq ft.
 
conarb said:
A few years ago at least Santa Clara County had suspended use of it, as one inspector told me: "I don't allow foam, we have two houses here that it is raining inside."
Click to expand...

Water has to have a way out. If not, it will condense at "heat absorbing" points. Just like dew.

Absolute sealing is a bad idea (unless you want to run a dehumidifier most of the time). Foam in wall and floor and conventional (breathable) on top with venting should be OK though.
 
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