Insulation, Radiant and Vapor Barriers - Venting problems in new bedroom above garage

We are in the process of converting an attic space above garage and part of the loft above garage into a bedroom in Columbus, Ohio. It will be a cape cod style bedroom with 4 foot knee walls with storage behind it. The ceiling above is the roof. It is a side car load garage. The front of the room is the front of the house and that area is shaped like a triangle with the peak of the roof in the middle. There was one gable vent in there which is being replaced with a window. There are two soffit vents, one on each side of the bottom of triangle which would be on each side of front of house that faces the street (one is in the only overhang over the garage). The problem is that the sofit vents might get air to the first 2 to 3 bays that go up to the roof on each side of room and that is it. There is no ridge vent. There is no other air coming in.

We were going to insulate with R30 in ceilings and floors. I don't want to have to put in a ridge vent for three bays at most, maybe 3 feet. There is no where else to put soffit vents. What options to I have? The room is 10.8 to 4 foot knee walls and as you step out 1 foot, the pitch of ceiling goes up one foot. It is almost shaped like an upside down T. The front of house where the triangle shape roof is represents the bottom of T which is 10.8 wide 11.6 and if you follow up to top of T, to the left, I have small area 5.6 by 6 ft square where the roof comes down at a slant and the other side of T is a small closet.

My questions are:
1. Do I have to put in a ridge vent for only maybe three feet of ceiling rafters that are expose to the soffit vents?
2. Is there anything else I can do?
3. Do I have to have an airspace between insulation in roof sheeting if it's just dead air in most areas, no air coming in and none going out?
4. Do I have to have ventilation in there if the existing soffit vents are not providing much air?

Thanks for your help.

http://www.eere.energy.gov/buildings/info/documents/pdfs/26447.pdf

On page 4 it illustrates on how to insulate knee walls and to ventilate.

Air is considered a fluid in engineering, that is because air movement is very similar to fluid movement. For example, if you take a straw and submerge it in a fluid, put your finger at the top of the straw and remove the straw from the fluid, the fluid inside the straw remains. If you remove your finger from the top of the straw, the fluid drops out of the straw. This illustrates "Adequate Free Ventilation". Another example is the buoyancy of warm air and water within the same mass, as well as the falling characteristic of cooler or cold air and water. While pressure plays an important factor, it does not influence the function of "Adequate Free Ventilation".

1 & 2. Ridge vents would be my last choice. Gable vents would be my first choice, if not possible, roof vents would be by second choice.

3. Yes, the reason for it is "Capillary Suction Break". Different materials absorb and expel moisture differently. Furthermore, temperature of those objects (materials) influence the Relative Humidity (RH%) of those objects. The insulation will prohibit conductive heat loss, it does so by retaining heat. It's RH% will be low as a result of the temperature. The roof sheathing does not have the same ability as the insulation and as a result of being in close proximity to outside temperatures will be at a much lower temperature than the insulation. The result is the roof sheathing will probably have a high RH%. Since Equilibrium Relative Humidity (ErH%) is influenced by percentage of humidity and not temperature of objects and the rule is high to low, if the insulation is in contact with the roof sheathing, the insulation will absorb moisture from the roof sheathing. The probability of condensation forming where the two touch is quite high.

While it is true the same process applies to air within the dead air space, air's unique ability to absorb/expel moisture, coupled with adequate free ventilation, this moisture will follow the path of least resistance to the air outside of the structure.

4. Yes. The rule with free venting is 1 square foot of free venting for every 300 square feet of attic floor area, assuming you are installing insulation with a vapor barrier. While it is usually nearly impossible to get the entire roof sheathing adequate ventilation with this type of construction, (eg. dormers, skylights, blocking, etc.) providing some type of access to the roof sheathing to the ventilation, even if it is inadequate, reduces the probability of condensation forming through ErH%.

In other words when it comes to "Adequate Free Ventilation", some is better than none and more is even better if you can.

Thanks for your reply. There is an existing gable vent, but a window will be replacing that space. I looked at the URL you gave me, but we are coverting the space behind the knee walls to strorage for toys, clothes etc. The space will be insulated and drywalled and have a floor and carpet. The URL is for knee walls that giving you access to the attic. This space above the garage was behind my loft on the second floor and has no access to the attic. I have a 1 1/2 story house like a cape cod.

This space was above my garage. My loft, a small part of my bedroom and this space are above my garage. We are converting part of loft and the rest of area to a bedroom and leaving a 4x 16 space still as the loft outside the bedroom. The loft is a part of a vaulted family room with back stairs up to loft and maseter bedroom. Maybe seeing the pictures might help. Thank you.

I attached the url's to 4 pictures of the bedroom above garage. The shots that you see the gable vent covered in plast
where the window
is going to be are from the door going into bedroom. The shots where you see white banisters are from in the room looking outside to the existing loft. The knee walls are on each side of bedroom and the top of T is by the loft. The sofit vents are located on each side of gable vent behind the knee walls. The bottom of traingle.


There may be 3 bays that are exposed to light and air, so if you looked up at the rafters that we made 2X10 and followed those down behind knee wall to bottom where floor is going to be, that is where the soffit vent is located. That is the only area where there is air coming in. There are no ridge vents.

http://i30.photobucket.com/albums/c343/kaj3079/bedroomovergarage/extrabedrrom001.jpg

http://i30.photobucket.com/albums/c343/kaj3079/bedroomovergarage/extrabedrrom006.jpg

http://i30.photobucket.com/albums/c343/kaj3079/bedroomovergarage/extrabedrrom003.jpg

http://i30.photobucket.com/albums/c343/kaj3079/bedroomovergarage/extrabedrrom004.jpg

Use spray foam and you will not have to vent!

I would love to use the spray foam, but we got a quote and it was very expensive.



Here are some pictures of outside the house. The room is above side car garage. It is above the wreath. The gable vent that you see is where the window is going to be. The window on the side of garage is in my existing loft which is off bedroom. The two soffit vents that are part of that space are, #1 look at picture #4 where there is a small over hang over garage with the gutter. It is right there. The other one is located at the bottom of the triangle on the other side by the front porch where the gutter comes down. There are other vents that are over my front porch that should go up to the attic and on the other side of porch is another shaped triangle with another gable vent. There is a small space that runs from one side of house to the other over the front porch to the knee wall space in the room where are doing over. So there is a connecting walkway if you will that might be 12 inch by 12inch. Remember, the vents only vent air to maybe 2-3 rafters bays on each side of room, so I can't put a ridge vent acroos the whole roof of room if there is no air coming up from the other rafters, right? The storage area behind knee walls are going to be insulated and dry walled.




http://i30.photobucket.com/albums/c343/kaj3079/roomisinfrontofgouseabovewreath.jpg

http://i30.photobucket.com/albums/c343/kaj3079/outsidehousepictures005.jpg

http://i30.photobucket.com/albums/c343/kaj3079/outsidehousepictures002.jpg

http://i30.photobucket.com/albums/c343/kaj3079/lookatthispictoseeoverhangwithgutte.jpg

http://i30.photobucket.com/albums/c343/kaj3079/gableventwillbereaplcedwithwindow.jpg

After viewing the pictures you posted it is apparent that ventilation was not taken under consideration during the construction of the room. It going to be difficult to provide adequate free ventilation at this time without redoing the framing.

In my opinion I would drop the collar ties (framing that forms the ceiling) at least a foot. You do not need that much height in this room. Make sure the insulation stays at least an inch away from the roof sheathing and install a ridge vent. This is actually fairly easy to do. Understanding why you should do it this way is somewhat difficult.

I realize your concern with the Hip Rafter that runs diagonally to frame your gable dormer in front of your house and that a few cavities between the rafters will not be able to have access to any ventilation. A moisture problem occurs when "The introduction of moisture is greater than the ability of the objects within the structure to absorb/expel the moisture." Roofing materials by design prohibit moisture penetration. Yet all roofing and building materials have a "Perm Rating". This is because the water "vapor" molecule is so small that it will permeate all building materials and even roofing. The Rating is an indication of the time it takes for the material to have moisture permeate it. The lower the Perm Rating the longer it takes for moisture to permeate the material. Both vapor barriers and roofing materials have Perm Ratings less than 1. It is the low vapor permeability of roofing materials that gives purpose to attic ventilation. Keep in mind that attic ventilation was not needed when houses were built without insulation. This has to do with Relative Humidity (RH%).

Ok, let me make sure I understand.

Question# 1
Since maybe 2-3 rafter bays have air coming into them on the front of each side of room, I just put the ridge vent on the roof above these bays, correct? It might be 3-4 feet. I don't need to put a ridge vent all the way across the entire peak because there is no air coming up the other bays.

Question#2 Also, your saying that I need a 1 inch air space in all the rafter bays even if the soffit vents don't provide air those areas, correct? So it does not matter where I put insulation on this catherderal celing, if there's ventilation or not, I need a 1 inch space?


Since this is becoming a bedroom, we can't drop the ceiling down much further because we were told by HVAC that we should put a fan in there for circulation in the summer. I swear these will be my last questions on this subject.

I can't thank you enough for all your help.

http://www.smarthome.com/3016.html

The site above shows a products that will work better than a ceiling fan. Have the HVAC person install this fan near this room and wire it to a three way switch that is installed in the room. The setting are On/Off/Auto. During the winter you may not need the air circulation, set the switch to off. On moderate days during the summer or when you are not home, set the switch to auto. This fan only comes on when the cooling unit is on. On those days or for comfort reasons turn the switch to "ON". What this does is keep this fan on constantly and circulates the air out of the room. For example, if you blow air into a balloon, the balloon gets bigger and if you let air out of the balloon, the balloon gets smaller. This room cannot get bigger or smaller as air is either brought in or taken out. In other words, if this fan moves 650 cfm of air into the room, 650 cfm of must leave the room. This room temperature air mixes with the room temperature air of the rest of the house, thereby alleviating temperature differences between this room and the rest of the house. Unlike a ceiling fan that merely circulates the within the room.

I still recommend you drop the collar ties (ceiling). Installing the insulation at least an inch away from the roof sheathing. Install the ridge vent, even if it covers a few cavities between the rafters. Hopefully by dropping the collar ties the other cavities will have some access. Even if it doesn't the dropping of the collar ties will make it easier for the air above the collar ties to absorb the moisture in those cavities through ErH%. It's the volume of air above the collar ties is what you want.

Furthermore, the dropping of the collar ties will work better in alleviating radiant heat transfer. All this is, the hotter an object gets, the more heat the object radiates. You roof temperature can reach temperatures above 170 Fahrenheit during the summer. Attic ventilation can reduce the temperature of the roof. By reducing the temperature of the roof, you lessen the heat gain created through Radiant Heat Transfer.

I can go on and on with this subject. I have omitted other factors like the "Latent Energy of Vaporization" (the amount of Heat Energy released when vapor condenses); "Conductive Heat Transfer" (how temperature differences influences Heat Loss/Gain); "Convective Heat Transfer" (how air movement influences Heat Loss/Gain) and much more. However, my recommendation literally addresses all of it.