The purpose of roof cavity/attic venting is not to lower the temperature of the sheathing or shingles but to allow moisture to escape.

In a very hot climate venting can lower the shingle temperature a bit but not enough to affect the life of the shingles. To lower the attic temperature use insulation between the rafters. To lower the roof temperature use light colored shingles.

Some shingle manufacturers have required venting as a condition of their warranty but most have removed that requirement as they slowly catch up with modern construction technology and practice. Never buy asphalt shingles without reading the warranty very carefully.

Thank you, thank you, thank you. Exactly the kind of info I'm looking for. In the same line of thinking about insulation at what point does it become a good idea to do fresh air intakes, hrv or the other? We plan on sealing ourselves but then what do we do? A blower test, seal some more until --x--? What exactly are we aiming for. Is it a whole house air turnover number less than x per hour during blower test? I know just enough that its a good idea but not enough to feel like I could ace putting it into practice. What is the end goal to test for.

Thanks again!

There are those who can advise you better than I. I am old and as furnishings outgas more chemicals I have come to appreciate the air leaks in my 122 year old house and just wear more clothes and light fires when guests arrive but I may have to move when the cost of energy exceeds my income. However, I'm not buying any new furnishings so it might be a good idea to tighten up the house a bit. It's unrealistic to think there's a way to win this game.

We are building...in Kentucky zone 7

Except that Kentucky isZone 4.

The minimum insulation levels and HVAC requirements for new homes built in Kentucky are set out in the International Energy Conservation Code (IECC) 2009.

Your designer/architect should specify sealing and insulation details to meet that minimum.

If you want a higher standard, specify IECC 2012.

If you have fewer than 5 air changes per hour when tested with a blower door at a pressure of 50Pa, you need whole house ventilation. Seven an hour is a pass in 2009 IECC; for IECC 2012, the max. in your zone is 3ACH.

This post was edited by worthy on Mon, Apr 29, 13 at 20:13

"We are building...in Kentucky zone 7

Except that Kentucky is Zone 4. "

Kentucky is zone 7 for plant hardiness..... most people on Garden Web refer to that when identifying where they live.

Not a gardener! It is inapplicable in respect to building.

from a construction, IECC and building code stand point, it is zone 4.

Most decent built homes these days can hit the 7, and even the 5 mark Worthy references above. IECC 2012 increases it to 3, which will take more care from you/your builder but certainly isn't hard with some attention to details. Foaming walls and roof will definitely aid in getting you there, however you really are not gaining anything but less money if you foam the upper walls instead of cellulose.

You would benefit the most from adding exterior insulation to your build. This will increase whole wall r value, help to decrease infiltration, and decrease thermal bridging. Chances are this upgrade will be cheaper than foaming your walls, and it will perform better than a spray foamed wall.

Adding additional insulation is typically pretty cheap on a build, and it can have cost tradeoffs such as reduced HVAC sizing, which in some cases can offset the insulation upgrades. But also be sure to check local utilities for rebates and incentives. They can be hundreds and up to thousands. Around here an Energy Star 3.0 certified house can get you 6700 in rebates. The lower energy usage is just side benefit!

Old timers still think air leaks are good to make a house breath ;) That isn't the case, hasn't been for some time now. Seal your house up and CONTROL what the air does inside your home. Give your house lungs install an ERV (or HRV).

Add the exterior insulation, get rid of the spray foam in the walls. Go with blown cellulose or blown fiberglass (avoid batts). Seal the sheathing plane up good and tight with caulks and tapes. Sometimes foam is the best option in vaults however, so I would recommend open cell there depending on the depth of your framing. Any areas that are not vault stick with standard vented truss roofs. Seal up the attic drywall layer around all penetrations and blow in a thick layer of fiberglass or cellulose. R50 and up is your target here. The cost to go from R38 to r50 is typically a few hundred bucks.

This is exactly what I need! You guys are great. Obviously I'm an avid Gardner and think in terms of that zone wise. It did seen strange to me that nc would be so different from our zone, they are not I was just wrong. So we are meeting code for insulation but I just think we could do better or at least I want to try for our best. The only upper walls are the gable ends, three dormers and the shed roof on back. I was reasoning spray foaming these areas because of making them air tight. By spraying the roof we are able to keep the hvac for second floor within the envelope. The entire first floor is surrounded by a wraparound porch and we feel comfortable doing additional insulating measures in that area. If you do the rigid foam board 1. I'm assuming it goes on prior to tyvec,is this correct? 2. How does it affect your windows and doors meeting the siding etc.
I'm going to hunt around for more info as well on building science and in some of your previous threads. I'm finding a wealth of info the more I dig.
Thanks again.

Have you thought about an ICF house?

Jennybc,

I suggest you post this query on the HVAC forum, and say in your post that it is also posted here. I think you will maximize you responses as well. You should also look at buildingscience.com. There is a plethora of information there.

A previous poster stated;

"You would benefit the most from adding exterior insulation to your build. This will increase whole wall r value, help to decrease infiltration, and decrease thermal bridging. Chances are this upgrade will be cheaper than foaming your walls, and it will perform better than a spray foamed wall"

This is creating a thermal break, or reduce thermal bridging. Consider this, a house elevation is 48 feet long, is built with tradition dimensional lumber 1.75 inches thick, and is insulated between the studs with any chosen insulation. This would calculate that within that single wall would be approximately 5.5 feet of dimensional lumber stacked side by side not including jack studs and headers for window and door openings. That is a considerable amount of virtually un insulated wall space in just 1 wall, the same holds true for roof trusses. The larger and more complicated the construction the greater the area without thermal break. So if you really want a tight building envelope create a full thermal break. I was told by a guy named Mike Holmes at a Home Depot conference a long ranch style home with R:6 on the exterior is equal to R:12 between interior studs. I can't confirm that, Mike isn't a scientist but I trust the guy.
Thermal break is a good keyword to google, all it means is the isolation of exterior air into the building envelope.

Don't forget you can over insulate until you do not have return on investment. I have a pal who's son sprayed enough closed cell isostyrene insulation that his utility costs are a few dollars a day in central Ontario Canada. It s good to know he is the owner of a spray insulation dealership. If he was to do what he did in retail prices the ROI would be something like 60 years.

This post was edited by SouthernCanuck on Tue, Apr 30, 13 at 0:55

You might try the renewable energy forum as well...lots of people with knowledge of building green there...

To elaborate on thermal bridging since it was brought up again...

there are several ways to calculate it, but to keep it as simple as possible, here is how it affects your total wall's insulation. First thing, if you put in r20 rated insulation product, you are certainly not getting an r20 wall for cavity walls. Other products such as ICF and SIPs (some SIPs even have thermal bridges) are nearly thermal bridge free and have what is called continuous insulation. My home is an ICF house from the footings to the roof, and there are no thermal bridges anywhere until the roof line. This gives me an r28 continuous insulation. However with wood frame structures, you have a framing reduction to take into account.

Most new homes, studs, heads and plates take up anywhere from 18-25% of the wall area. (lets keep the numbers whole and rounded for easy math) Say you select blown fiberglass in a 2x6 wall, gives you roughly r20. You have 1000sqft of wall surface. That r20 is only for the stud bays, and every 16" roughly you only have an r1 due to the stud (framing factor) So you have 800 sqft of wall at r20 and 200 sqft of wall at r1. . Your "r20" wall is performing at an r16 on average with the studs acting like heat sinks. Add just 1" of XPS foam to the exterior (r5). Your r20 turns into r25, and your r1 turns into r6. Now 200 sqft of your wall is an r6 vs an r1 and reduces the heat sink effect.The materials to add that 1" XPS would be roughly 500-600 for that 1000 sqft wall. The upgrade from blown fiberglass to open cell foam is typically more than that.

For adding exterior foam, details are pretty simply up to about 1.5" thick. After that, the details change and become more costly. Luckily for you, in zone 4. 1.5 is about as thick as you need to go. 1" would be a good start, and your details really do not change. I have detailed 1" foam and windows 1 of 2 ways. Either install the sheathing and foam and then the window. Or with 1" foam, most window flanges are 1 1/8"+ wide, so you can install the window and then install the foam. This gives them about 1/8"+ lap for a caulk joint for trim to hide the foam. The trim stands proud of the window with this detail obviously. 1.5" adds another step. I like to detail a frame out around the windows and butt the foam into it. So frame the wall as normal, install sheathing, then rip a 2x4 in half to give them 1.5x1.75 furring to frame out around the window RO. Install the foam, butt into the furring around the window, and then set the window, nailing into the furring. Everything planes out, and quite simple. Obviously this can also be done with 1" foam too, its just not necessary.
There are lots of details floating around out there. Check out green building advisor as well.
You can also checkout my blog for more details and nerdy building information.

Again, a very untechincal explaination, but you get the idea hopefully.

Here is a link that might be useful: home building blog

Great advice so far. Sorting through the options, decisions and implications for cost-effective insulation and ventilation can be tough. If you want piece of mind and some excellent value regarding all of this get your home certified to Energy Star version 3 by a third party. Involve them as early as possible.

We are at the beginning of a massive shift in energy code and healthy ventilation policy. Your local codes will probably have you build to IECC 2009 but I recommend going with the 2012 version as a minimum. The fast approaching 2015 will be even better and I personally feel that this 2015 version will have a great balance of cost-effective energy improvement (and ventilation) minimums.

The question now is do you really want to build the poorest performing home allowed by law? 2009 is outdated already and will very soon be eclipsed by much higher performing standards and minimums.

Insulation decisions are less important than blower door test results for reasons that go far beyond cost-effective energy improvements. For your blower door test minimums; ACH50 of 1.5 should be your goal. 2009 IECC wants ACH50 of 7 (very leaky). Energy Star wants ACH50 of 4. 2012 IECC wants ACH50 of 3 for climate zone 4.

Most high performance home builders are shooting for a minimum of 1.5 on the ACH50. I think this is a very cost-effective level to shoot for and think that this is where energy codes will be in 5 years.

Whether your home is airtight or not, using mechanical ventilation to introduce outdoor air for Indoor Air Quality is a good idea and is required by IECC 2012 which follows ASHRAE 62.2

For spray foam insulation at the roof deck, almost all contractors will spray below minimum code levels to appear competitive with traditional insulation methods. I would recommend ignoring contractors and raters recommendations and going with the code minimum levels (at least) which for zone 4 in the 2012 IECC is R49. Dont buy into "effective R value" or other similar talk. Open cell foam is 3.6R per inch. 49/3.6= 13.6" of foam minimum. This would be the poorest performing level allowed by international law in your zone.

Exterior insulative sheathing for wood framed homes is required by International Building Codes for zones 3 and above. We have always used SIPS for our above grade walls but we are about to try the new ZIP+R product which seems a very cost-effective way to eliminate thermal bridging, air infiltration, and the tricky trim attachment issues mentioned above.