[rif]

I live in the northeast.  Assuming my attic is insulated and vented properly, it should be cold in the winter, right?

How about the summer when the AC is cooling the house?

[Peter J]

I live in the northeast.  Assuming my attic is insulated and vented properly, it should be cold in the winter, right?

How about the summer when the AC is cooling the house?

 Yes, assuming the insulation is sitting on the top side of a conventional flat ceiling. Theoretically the area between the top of insulation and bottom of roof deck will be close to outside ambient temperature. Same holds true in summer months. In practice, that's harder to accomplish, the scenario is typically a little warmer than ambient in winter because of air leakage and warmer than ambient in summer due to solar gain.

The above is a little over simplified. Perhaps it would help if you told us your specific concerns, rif.

[thunderbrick]

Yes, assuming the insulation is sitting on the top side of a conventional flat ceiling. Theoretically the area between the top of insulation and bottom of roof deck will be close to outside ambient temperature. Same holds true in summer months. In practice, that's harder to accomplish, the scenario is typically a little warmer than ambient in winter because of air leakage and warmer than ambient in summer due to solar gain.

+1!   

[rif]

Thanks, that's reassuring.  I had no specific concerns, I was just stuck in analysis paralysis.

 I do have a question about summer. I have a typical suburban 2 story house. In the summer, when the whole house AC is going, wouldn't it make sense to allow the heat on the 2nd floor to escape up into the attic? My thinking is that the attic is hotter the the 2nd floor ceiling, so why not let it vent up and out?

[mikeeastman]

Not a good idea, ac is designed not to be vented. Best thing in summer is install attic vent fan to keep attic temps down.

[macrojack]

Thanks, that's reassuring.  I had no specific concerns, I was just stuck in analysis paralysis.

 I do have a question about summer. I have a typical suburban 2 story house. In the summer, when the whole house AC is going, wouldn't it make sense to allow the heat on the 2nd floor to escape up into the attic? My thinking is that the attic is hotter the the 2nd floor ceiling, so why not let it vent up and out?

Just remove your insulation above the second floor every April and put it back in place in early October in time for the heating season. That way you won't need any vents.

I hope you realize I'm kidding. The best approach is to install a solar powered attic fan in your roof. It is most needed when the sun is shining bright - and that's when it operates at full power. Benefits include keeping humidity down in the attic space, prolonging the life of your shingles, and reducing the temperature disparity between your second floor and attic. If you have ducting running through the attic, it is best to heavily insulate it. Of course, you also need soffit vents to allow for circulation in the attic.

[rif]

Well I just got rid of the attic fan in favor of this configuration : soffit vents with those Styrofoam things along the slope directing the air up towards a ridge vent at the peak.  I had them seal up the gable vents to allow proper circulation.

I couldn't remove the insulation in the summer anyway - it's blown in insulation 

[ctviggen]

The best thing to do is (and do this now, while it's cold) find all of the holes and other areas (such as recessed lights), and seal them.  This is a good introduction:

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwiduLbi0-jJAhVBdj4KHY50BloQFgg7MAA&url=http%3A%2F%2Fwww.energystar.gov%2Fia%2Fpartners%2Fpublications%2Fpubdocs%2FDIY_Guide_May_2008.pdf&usg=AFQjCNEckBunET0QpLqrLoxV2GYmbfK_vw

Cap your stairs, too.  Once that's done, install blown-in insulation to a depth you can afford.  If you're paying someone to do this, have them ensure there's enough air intake on the sides of the roof (the soffit vents) and install a ridge vent if you don't have one. 

The attic vents are ok, but they can also suck air from conditioned spaces, through all the holes you haven't plugged yet. They also don't really fix the problem, which is air leakage and lack of proper insulation.   

If you have an AC/heater in the attic, that makes all of this more difficult.

[rif]

All the holes, etc should have been plugged up when I had the blown in cellulose installed.  They also, of course, took care of the fire stops.

There's also blown in cellulose in the exterior walls.

So basically the home's living space envelope us sealed up.

They did that test where they put a fan in the front door to see how leaky your whole house is.

And when I had the vinyl siding installed a few years later, I had them put in thick foam board insulation too. That made a huge difference in my energy bill, probably the combination of insulation and tyvek wrap.

The attic is empty - no hvac up there.

[JLM]

In winter, assuming an effective insulation barrier between highest ceiling and attic, the attic should be near exterior temperature (or warmer under sunny conditions).

In summer, assuming an effective insulation barrier between highest ceiling and attic, ventilation of the attic is critical for keeping the attic temperature as low as possible in order to extend the life of asphalt/fiberglass shingles and reducing air conditioning demands in the living space.  Air conditioning works primarily by dehumidification, so openings to the outside just makes the air conditioner work harder.

In summer houses without air conditioning can make use of a whole house fan (between the highest ceiling and attic) but can only be effective if sufficient ventilation from the attic to outside (which may require fan(s) between the attic and outside).

Recommend having infrared testing done to find all leaks (best done at night under cold conditions to provide contrasting conditions).  "Should have been plugged" is the operative thought here, but without testing its impossible to know.

[Peter J]

All the holes, etc should have been plugged up when I had the blown in cellulose installed.  They also, of course, took care of the fire stops.

There's also blown in cellulose in the exterior walls.

So basically the home's living space envelope us sealed up.

They did that test where they put a fan in the front door to see how leaky your whole house is.

And when I had the vinyl siding installed a few years later, I had them put in thick foam board insulation too. That made a huge difference in my energy bill, probably the combination of insulation and tyvek wrap.

The attic is empty - no hvac up there.

You're on the right track. Attic fans typically don't solve any problems (they treat symptoms) and in many cases create some. They are marketed well though and who wouldn't want to rid your attic of all that nasty hot air? A marketer's dream come true. 

Passive convective removal of attic air is what current building science suggests and I agree. What the blower door does is measure air leakage within the building envelope. Minimize that and you just make insulation perform at a higher level. Sounds like whoever's advising you has a good grasp on things.

There's snake oil in the home efficiency game...kinda like audio or a million other arenas.

[audioengr]

I just completed a 7 month remodel of a house in Denver.  It did not have central air.  The attic only had roof jacks at the top of the roof and no vents in the eaves.  The first summer like day when it got to 72-73 degrees I went up in the attic.  It was so hot, one could only be up there about 2 minutes, probably at least 120 degrees.  On hot days I can only imagine how hot it would get.  The upstairs was impossible to sleep in, too hot.

To remedy this, I installed 2 gable vents just above the attic insulation layer on the north end of the house, the cooler shadier end.  On the south end, I installed the best gable attic fan that Home Depot sells, about $120.  I put a switch in the closet below the fan that enables it to be thermostatically controlled in the spring fall and winter and on 100% of the time in the summer.  I left it on 24/7 in the summer.  The thermostat was set to 85 degrees F.  The vents alone did not do much BTW.

After installing this arrangement, the first story of the house never got above 74 degrees all summer, except for 2-3 days when it was really hot out.  For these, I installed a $130 Walmart window air conditioner in one bedroom so we could sleep.   The noise also helped with the barking dogs and noisy partying neighbors. I remembered again why I don't live in the burbs....

Moral of the story is that the RIGHT attic fan arrangement can be extremely effective.  The fan needs to be a good one with high flow rates, set to a fairly low temp, with a bypass switch to have it on 24/7 in the summer.  Combine with additional vents at the opposite end of the attic, and you have something very effective.

BTW, I also insulated the roof over the attached garage with R19 bats and put reflective stuff in the eastern facing windows.  All of this made a huge difference.

It's really a no-brainer to do insulation, reflective window coatings, fans and vents before you spend the big bux on central air or a swamp cooler.  The air conditioning when you do need it will cost a lot less.

Steve N.

[ctviggen]

Again, you have to be very careful with attic fans.  I installed one myself in my previous house.  It was electric with a thermostatic control.  When we had our roof done, I had it taken out.  I realized when doing electrical work in the same house that there was a lot of dirt in the electrical boxes.  Why?  Because the attic fan was sucking air from conditioned space into the attic.

For instance, see:

http://www.energyvanguard.com/blog-building-science-HERS-BPI/bid/38676/Don-t-Let-Your-Attic-Suck-Power-Attic-Ventilators-Are-a-Bad-Idea

I used to believe the same about attic fans, which is why I installed one myself.  However, I believe air sealing and more insulation, along with proper ventilation is better. 

[Peter J]

I just completed a 7 month remodel of a house in Denver.  It did not have central air.  The attic only had roof jacks at the top of the roof and no vents in the eaves.  The first summer like day when it got to 72-73 degrees I went up in the attic.  It was so hot, one could only be up there about 2 minutes, probably at least 120 degrees.  On hot days I can only imagine how hot it would get.  The upstairs was impossible to sleep in, too hot.

To remedy this, I installed 2 gable vents just above the attic insulation layer on the north end of the house, the cooler shadier end.  On the south end, I installed the best gable attic fan that Home Depot sells, about $120.  I put a switch in the closet below the fan that enables it to be thermostatically controlled in the spring fall and winter and on 100% of the time in the summer.  I left it on 24/7 in the summer.  The thermostat was set to 85 degrees F.  The vents alone did not do much BTW.

After installing this arrangement, the first story of the house never got above 74 degrees all summer, except for 2-3 days when it was really hot out.  For these, I installed a $130 Walmart window air conditioner in one bedroom so we could sleep.   The noise also helped with the barking dogs and noisy partying neighbors. I remembered again why I don't live in the burbs....

Moral of the story is that the RIGHT attic fan arrangement can be extremely effective.  The fan needs to be a good one with high flow rates, set to a fairly low temp, with a bypass switch to have it on 24/7 in the summer.  Combine with additional vents at the opposite end of the attic, and you have something very effective.

BTW, I also insulated the roof over the attached garage with R19 bats and put reflective stuff in the eastern facing windows.  All of this made a huge difference.

It's really a no-brainer to do insulation, reflective window coatings, fans and vents before you spend the big bux on central air or a swamp cooler.  The air conditioning when you do need it will cost a lot less.

Steve N.

There are examples such as yours that seem to support the active circulation model. And I don't dispute that ridding your attic of superheated air is valid and in some cases the only reasonable way to address the lack of passive convective circulation.

 I would maintain that it's treating symptoms and the root cause is actually a deficiency in original design.
You would rarely, if ever,  find attic fans incorporated into modern high efficiency design. Building science is an ever evolving story and was rarely considered in older structures.

My big beef with fans is that they are marketed as a blanket solution and that everybody needs one, which is just not true.