Not Much This Week

The weather was the main deterrent to progress this week. The three inches of snow we received last weekend was followed by bitter cold and a couple of light snow falls through the course of the week. About the only achievement, and it was a significant one, was that the glass roof design was finally approved and is now going into fabrication. I spoke to Jim (glass roof) on Friday and he said he would know more about the timing of the fabrication when it actually gets into the schedule, but he was hopeful it would be only 3-4 weeks.

We still have not placed the order for the cupolas. I spoke to "Our" Mike about this Wednesday and told him I was uncomfortable with placing the order until we knew for sure how we were going to achieve the ventilation we wanted. I explained to him that, ideally, we would prefer to use this ventilation as the sole means of cooling the sunroom, without the need to run the air conditioner. In other words, this fan will actually create a breeze through the sunroom. That suggests to me that we need more capacity than what is normally sized to keep a room (or attic) ventilated. My previous on-line searches suggested that ventilation of a kitchen is considered to be one of the highest capacity requirements with 15 air turnovers per hour. Other types of rooms typically have fewer air turn overs. Based upon the 15 turnovers per hour, I made a preliminary estimate that we would need about 2,000 cfm in the sun room and 3,000 cfm in the garage. We left that conversation with the understanding that Mike would be evaluating the roof exhaust fan options. I am more and more convinced that, if we go with the 30" cupola on the sunroom, there will not be enough roof surface underneath the cupola to accommodate a conventional roof fan. I am also increasingly leaning toward the use a roof exhaust fan to relieve my concerns about rain infiltration.

Today, I consulted the Energy.gov website for whole house fan cooling. The recommended capacities on this web site ranged from 30 to 60 air turnovers per hour. I also took a closer look at the volume of both the sunroom and the garage.

Having a nice rectangular footprint, he garage is relatively straightforward. The floor area is 40' x 28' for 1,120 sq ft. The height to the trusses is 10' and the roof pitch is 9/12. This yields an overall volume of 17,080 cu ft. The sunroom is not a nice rectangular shape. Breaking it up into its component parts, I arrived at a total volume of 8,670 cu ft. Not too far off what I estimated previously.

Using these volumes, the following fan capacities would be required at various air turnover rates.

15 turnovers/hour  =  2,167 cfm
30 turnovers/hour  =  4,335 cfm
60 turnovers/hour  =  8,670 cfm

Mike had previously suggested we might be able to find a exhaust fan that would have a 1,300 cfm capacity and that we might be able to put two such fans under the sunroom cupola. I am increasingly skeptical that these could be situated underneath the cupola we are considering for the sunroom and even two of these fans may not create the amount of draft we are desiring. There is also the question of noise these fans will create. Of course, we want no noise at all.

Capacity based on air turnover rate is one thing. Creating a "breeze" is another. What constitutes a "breeze"? Intuitively, I guessed at least a 3 mph wind velocity. Seems the definition of "breeze" is a wind velocity between 4 mpg and 31 mph. If we open all the windows in the sunroom and both the french doors, the total surface area open to ventilation would be 307 sq ft. To achieve a wind velocity of just 2 mph, we would require a fan with a capacity of over 54,000 cfm. Obviously, we will not be installing that much capacity. The whole house attic fan we have in the main house is rated at only 11,790 cfm. In the garage, with the eight windows open and the garage doors closed, the ventilation area will be about 60 sq ft. To get a 2 mph wind velocity, we would need a fan capacity of about 11,000 cfm.

Another tidbit I found on the Energy.gov web site was the recommendation that, when installing a whole house attic fan, the roof ventilation should be about 1 sq ft per each 750 cfm of fan capacity. In the main house, with an attic fan rated at 11,790 cfm, we have only two 10" roof turbines, These provide less than 2 sq ft of roof ventilation. Per the guide lines, we should have about 15 sq ft. While we do not have ventilated soffits, there is some ventilation at the eves, so we really don't know how much ventilation are we really have. When we re-roof the main house, we will install vented ridges. We have about 250 feet of ridge. Using the value of 18 sq in per linear foot of vented ridge line, this will provide over 30 sq ft of roof vent. We might get more efficiency out of our existing attic fan.

Bottom line is that we are rethinking the entire approach to the sunroom ventilation. Right now, I am considering just installing an exhaust fan just to control humidity and air turnover in the sunroom. The fan used to create a breeze would be the main house attic fan. This approach would also minimize the noise in the sunroom. In the garage, I think a full attic fan in the garage would be appropriate where, with no other sources for cooling, we really might like to have a breeze wind velocity.

T has stated that if we do not have room in the sunroom cupola for the exhaust fan, and therefore have to mount it on the roof exterior to the cupola, she would like to have a decorative window cupola instead of the louvered design. She is OK with having a windowed cupola on the sunroom and a louvered cupola on the garage as long as the structure design is the same.