Idea for attic ventilation

[Todd Burch]

Thanks for your comments everyone.

Spray foam might be best, but I don't need the best. I'm not so certain I would want spray foam regardless. I have the rigid foam, and have no other foreseeable use for it.

I'm trying to put this in perspective.

The house has survived with window A/C units, space heaters and no soffit ventilation, pathetic ceiling insulation and no wall insulation for 54 years. Granted, I haven't been living in it, but I would venture to say that 1/2 the houses in this town don't have a lick of wall insulation.

I gave an analogy to my wife this afternoon, comparing the inside of the roof deck to a pan that has been on the stove top. If you picked the pan up by the bottom, it would burn your hand bad. However, if you put on a cooking mitt, you can hold the pan just find. You have insulated yourself from the heat of the bottom of the pan.

Why wouldn't 2" of foam not have the same effect from the radiated heat of the shingles? Surely it has to have value, and I'm thinking significant value.

Why is rigid foam the go-to choice for commercial building roofs, and it's not appropriate for residential?

My buddy has spray foam in his new house, built by a high-end custom builder. It seemed awesome when the house was being built. When I walked into the house after it was insulated, it was like walking into an underground cave it was so cool - and the HVAC was not even on and it was the middle of summer. However, it also has a big drawback in his scenario. It's not his primary residence, so he doesn't run the A/C it all the time. If he does not keep the air on, or turn it on an hour before he arrives (via his phone), his house is hot. It takes forever (read, about an hour) to cool off. If doors are left open while you are there, again, it takes a long time to get the temp back to comfortable. How convenient is that? How is that the best system? Was his A/C undersized? No, it was sized for use with spray foam.

I don't have final numbers yet, but here is what I would anticipate:

Qty=60 sheets of 7/16" OSB @ above the polyiso would run just over $500
Qty=12 of 2X6X12' (ripped to two pieces of 2" widths) would provide a border around the roof (~200') and at the ridges (~80') @ $8 each = ~$100.
Qty=4 rolls of 3' X 150' tar paper for the extra layer @ $16 each = ~$70.
Qty=2,500 4.5" screws to go through the 7/16" OSB, through 2" foam and into the original deck. (They don't make 3.5", or else I could use those). (I took the 8d nailing schedule of 41 nails per sheet and used screws instead. But seriously, who installs OSB that uses a nail every 6" of the perimeter and 12" OC for the interior of the sheet? No one I have ever seen.). 2500 3.5" Headlok screws on Amazon would cost $800. Wow.
Labor - will find out tomorrow.

[Jim Matthews]

Your friends superinsulated house sounds like a refrigerator.

You still have to pump heat OUT, and that takes time.
Passive cooling systems will smooth out the variations,
but without a substantial heat sink, you will reach
the ambient air temperature.

Near Houston, that's pretty high on average.

I think you're on the right track to isolate the living spaces
and ventilate the roof for longest shingle life.

Is there anyone in your area that specializes in passive approaches?

A quick Google browse turned up these guys:
http://www.drenergysaverstx.com/insu...uperattic.html

Remember that this is not just an investment to reduce cooling costs, it's also an investment
in your comfort while you live in your home.

[Mark Bolton]

One thing you may want to inquire about is some issues with regards to ISO collapsing slight over time. It may result in screw heads popping through your shingles if the ISO and the sheathing shrunk (even very slightly). Again I have no idea but just a concern where your not going to have framing on even wide centers.

Another issue will be what will you do at the perimeter to cover the additional thickness? Wide drip edge? Aluminum fascia?

Your friends house is what I'm speaking of. When you encapsulate and get higher and higher efficiencies the entire structure becomes a system. Like a commercial building. You can't just walk away without leaving some systems to run periodically whether it's an hvac system, air to air heat exchanger, whatever. There are pros and cons but again, at least to me, you have to weight then out honestly with regards to what your intent is for the home rather than hooking on to the bad things in support of your original plan. Its very easy to lock down on the things that support our original ideas rather than looking at everything by the numbers.

But everyone does what they want..

[Pat Barry]

I think the typical approach for the insulation board on top of an existing roof is to provide nailers 16" on center, lined up over your existing rafters. Then simply affix the sheathing to the nailers. What I don't recall, but I'm sure could be easily found is the ventilation, if any, for this type of application. My cabin has a similar roof - they did it so the inside of the cabin wouldn't have the ceiling (pine boards, log rafters) obscured by the internal insulation approach. They didn't do any ventilation that I can see and I suspect they should have.

[Rick Fisher]

I have been in the supply business for 30 years. Sell millions of dollars in shingles each year and have been to a hundred claims in my life. Your roof would have no warranty in my area.

Air goes in the soffits, and out the ridge or roof vents. That is the deal. If you have sealed up soffits, and the shingles crack, degranulate, or buckle ( organic ) .. your out of luck.

Lots of weekend warriors like to argue and draw fancy pictures but your buddy the roofer is 100% ..

I should add.. 60% of the roof failures I have been too are due to exactly what you have .. the rest where .... sealing shingles on resawn boards, wind blow off, shingles nailed high and blowing off, or actual factory problems. The #1 source of claims however is ventilation .. exactly what you describe.

[Paul McGaha]

After the sun goes down powered or even turbine ventilation will do a lot for bringing the roof structure and attic space closer to ambient more quickly and most definitely helps eliminate heat from the attic/ceiling structure from continuing to radiate into the living space in the evening. But again, this venting does virtually nothing when the sun is shinning. The solar gain is far greater than can be dissipated in daylight hours. Taking a roof deck or attic space from 140 to 130 or even 120 will provide little comfort to the occupants though it will reduce the load on a cooling system slightly.

The real issue is if possible, had the same dollars that were spend on the powered venting, installation, and electricity to run (for a non solar vent), been invested in additional insulation the space would be far more comfortable than any powered venting would achieve because it wouldnt get hot in the first place. Of course many structures simply dont allow for additional insulation so venting can surely help to cool the structure more quickly after the sun goes down.

All due respect Mark I don't agree with your statement that reducing the attic temperature from 140* to 120* (with a roof fan) won't have an effect on the comfort level of the upstairs level of the home during the daytime. It most certainly will. And enough of an effect to easily justify the installation of a roof fan. Ivé seen numerous installations where it was the difference maker between the upstairs level being comfortable or not.

Roof fans aren't the only thing required to help keep your house cool. The attic has to be well insulated, Outside windows and doors need to be sealed properly, the air conditioning system needs to be in good working order, It's good to have a roof fan in the attic and ceiling fans in the bedrooms.

Get all those things done though and I've found that you can have a pretty comfortable upper level of your home with temps outside pushing 100*.

PHM

[Brian Elfert]

My buddy has spray foam in his new house, built by a high-end custom builder. It seemed awesome when the house was being built. When I walked into the house after it was insulated, it was like walking into an underground cave it was so cool - and the HVAC was not even on and it was the middle of summer. However, it also has a big drawback in his scenario. It's not his primary residence, so he doesn't run the A/C it all the time. If he does not keep the air on, or turn it on an hour before he arrives (via his phone), his house is hot. It takes forever (read, about an hour) to cool off. If doors are left open while you are there, again, it takes a long time to get the temp back to comfortable. How convenient is that? How is that the best system? Was his A/C undersized? No, it was sized for use with spray foam.

An hour to cool off a hot house doesn't seem unusual to me. I would put my heat at 55 degrees in the winter when I was gone for a few days. It would take the furnace 45 minutes or more to get the house back up to temp.

One problem with highly insulated houses is they don't cool off or heat up very quickly naturally with windows closed. If the temps drop to say 65 degrees overnight the temp in the house might still be 72 to 75 degrees. It can feel hot in the house because it is cooler outside and the A/C wouldn't be running.

[Todd Burch]

I decided to nix the 2" rigid foam - I don't have time to manage the changes. Took a framing hammer to the 1X4 blocking - came out pretty quick. PVC air vents were on clearance @ the BORG for $1.50 each. What you see is literally all I can install on the front of the house. However, the whole back of the house will get them. I think around 25 of them.

[Mark Bolton]

I have been in the supply business for 30 years. Sell millions of dollars in shingles each year and have been to a hundred claims in my life. Your roof would have no warranty in my area.

Air goes in the soffits, and out the ridge or roof vents. That is the deal. If you have sealed up soffits, and the shingles crack, degranulate, or buckle ( organic ) .. your out of luck.

Lots of weekend warriors like to argue and draw fancy pictures but your buddy the roofer is 100% ..

I should add.. 60% of the roof failures I have been too are due to exactly what you have .. the rest where .... sealing shingles on resawn boards, wind blow off, shingles nailed high and blowing off, or actual factory problems. The #1 source of claims however is ventilation .. exactly what you describe.

Not to drag this out, but the entire construction industry including the NRCA, IBC, IRC, as well as all the major asphalt manufacturers (GAF, Owens, Certainteed, IKO, and more than likely every other) addressed the issue of unvented roof decks some 15 years ago. Asphalt manufacturers and reps had long blamed shingle failures (of which few were likely the fault of the manufacturer in the first place) on heat. It was an easy scape goat and an easy way to get out of a claim. However when unveted roof structures due to ever more complex roof designs, log homes, cathedrals, and the onset of encapuslation became more prevalent they were faced with either #1 turning away all this potential business, or #2 finally coming clean that heat has absolutely zero to do with shingle failure.

The driving factor behind this was from builders, architects, and designers, who were faced with either no longer using asphalt on the vast majority of their work (read VERY large/complex multi million dollar homes) or expecting the asphalt manufactures to see the light. If you look at many of large homes today with very complex roof layouts full of dormers, valleys, hips, and so on, there is simply no soffit to vent in the fist place. There is no "ventable" edge and little ventable ridge. Then you consider log homes, SIPS structures, and the like, and your talking a major chunk of the asphalt shingle industries customer base going bye bye if they stuck to this concept. Not to mention the manufactured industry which is horrendously poor about venting roof structures in the first place.

Needless to say, 15 years ago, the shingle industry "conveneintly" said that running their product over unvented decks was suddenly OK. Their warranties allowing this speak only to things like adhering to the building code and installing insulation as per the manufacturers recommendations. All the majors warranties allow things like minimum roof sheathing thicknesses of 3/8" ply. This speaks specifically to the manufactured home industry. Their details with regards to unvented decks speak to one thing and one thing alone and thats maintaining a stable sheathing surface and nothing with regards to heat or heat dissipation. All they are aiming for is a dry underside of the deck to prevent the deck from degrading or curling/cupping and tearing the shingle layer.

The simple fact of the matter is you can lay out some shingles on your asphalt driveway, the hottest surface out there, with no effect.

None of this is to say that roof venting is not essential and absolutely wise. Its just to say that there is no form of reasonably implemented passive, or active, ventilation which can cool a roof deck in the sun by any useful amount. It has never been what roof ventilation is actually there for. Cooling your roof more quickly after the sun is gone, sure, but at noon, there is zero gain.

All this aside, I can honestly say that in 20+ in the trade I have never once been around, or even first hand, seen a shingle claim that was a manufacturing problem. On the other side of that I could easily imagine manufactures paying out on claims that really arent their fault far more often than they have a problem.

Nearly every single shingle failure I have ever seen or been a part of fixing could either be attributed to siting/location, or installation.

[Mark Bolton]

All due respect Mark I don't agree with your statement that reducing the attic temperature from 140* to 120* (with a roof fan) won't have an effect on the comfort level of the upstairs level of the home during the daytime. It most certainly will. And enough of an effect to easily justify the installation of a roof fan. Ivé seen numerous installations where it was the difference maker between the upstairs level being comfortable or not.

Roof fans aren't the only thing required to help keep your house cool. The attic has to be well insulated, Outside windows and doors need to be sealed properly, the air conditioning system needs to be in good working order, It's good to have a roof fan in the attic and ceiling fans in the bedrooms.

Get all those things done though and I've found that you can have a pretty comfortable upper level of your home with temps outside pushing 100*.

PHM

Well, it is what it is. The physics of the situation are just there. Having a ceiling surface that is radiating 115 degress of heat out of the attic/roof structure and into the living space as opposed to 135 degrees is still going to result in one outcome, occupants getting away from that heat source. Like I keep saying, there are most definitely some structures that are not feasible to insulate and venting may provide some modest relief but I will guarantee you that if you were to accurately measure actual temps in full sun, the fans would be doing little to nothing. The sun is a very powerful thing.. ;-)

[Brian Elfert]

All this aside, I can honestly say that in 20+ in the trade I have never once been around, or even first hand, seen a shingle claim that was a manufacturing problem. On the other side of that I could easily imagine manufactures paying out on claims that really arent their fault far more often than they have a problem.

What about organic asphalt shingles? They were used extensively here in Minnesota. I don't know if you call those a manufacturing issue, or a material issue. Roofer knew I had organic shingles just by talking to him on the phone and describing the problems. Some insurance will either cancel insurance for homes with organic shingles, or cancel the insurance on the roof until the organic shingles.