Vapor Barrier on new construction

Guest User
11/26/2002
I am in the process of building a steel frame home. I have had conflicting advice on putting some type of moisture barrier between the roofing and the purlins. A couple have recommended a 1/4 foam product with foil bonded to both sides (r-value of 7). Others have recommended just screwing the metal to the purlins, making sure that the attic is well vented. The house will be in Eastern Texas and condensation can be a real problem. I plan on putting R-25 in bates for insulation in the attic. What is the most cost effect way to go that will insure the desire results. Thanks Ron in TX
Allan Reid
Dura-Loc Roofing Systems, Inc.
11/26/2002
Steel frame or wood frame the situation is the same. You have two sources of condensation to deal with. First I appreciate the R25 however I do strongly reccommend an air barrier under the inside finish of a minimum 6 mil poly with all joints and penetrations sealed with caulking. This will keep the warm air inside in the winter escaping into the attic where it condensates either in the insulation if it is freezing or to the undeside of the roof deck. Next you need a moisture barrier under the roof covering. If you have solid sheathing you can use a 30 lb felt paper. If you do not intend to have sheathing then you need to put a reinforced underlayment down. Our firm has tested and recommends a product called TriFlex 30. Make sure you tape around all penetrations such as soil pipes. This will deal with condensation forming from the hot/cool cycles of the atmosphere. Last but not least is to ensure that you have adequate ventilation. The best is to have 50% at the soffit and 50% at the ridge totalling 1 sf for each 300sf of ceiling area. If you can try to have a 1" air space between the metal and the moisture barrier that is vented as this will provide additional energy savings and make the house more comfortable in summer. Build with these principles and you do not need the reflectance of the foil. Good luck.
Guest User
12/30/2002
We just had a large addition added to our home. We had a new metal roof installed over the entire home. The contractor had to install new trusses because our home went from 24 feet wide to 44 feet wide. When they installed the metal roof should they have installed a vapor barrier first, then the roof? The metal roof was screwed directly to the trusses. We usually research these things more thoroughly, but we trusted our contractor to know what he was doing. We now have a condensation problem with water dripping down in to the new addition. We are going to finish the inside of the addition ourselves and before we frame out the rooms and install the ceilings we want to take care of this problem. What are our options?
Allan Reid
Dura-Loc Roofing Systems, Inc.
12/30/2002
When you talk about condensation/ventilation we really need to know your location. Anyways it is not a vapour barrier bu a moisture barrier that should have been installed under you roof which is a requirement for all roofing materials unless you test otherwise. A vapour barrier does not allow air to pass through while a moisture barrier holds the moisture while breathing. Obviously you have heat loss and that is taking warm moist air out and condensating as it cools down on the underside of the metal roof. If you are going to have an unheated attic make sure you have adequate ridge and eaves ventilation, an air barrier to the inside of the ceiling and good insulation. If you are going to have a vaulted ceiling then I would recommend putting a thin layer styrofoam between the trusses probably to the underside of the strapping and then have the sprayed in place eurathane insulation installed. Do not cut the inside surface of the foam as it will then act as you air barrier. If you need more help contact me directly and I can provide you with some interim solutions given more information.
Guest User
1/8/2003
Sounds like you have an unfinished space and naturally you are getting a great deal of moist indoor air rising into the future attic space. Check out Al's response in the question titled "snow and condensation" . His answer probably fits your situation as well.
Guest User
2/19/2003
How can I prevent 29 gauge metal roofing from sweating?...How can I weld a two piece valley together?....I would like any info on these two questions that you may have to offer or any other helpful info on metal roofing installation... Thank you Michael Patton
Allan Reid
Dura-Loc Roofing Systems, Inc.
2/21/2003
Very popular question at this time of year. Metal on a roof condensates from two sources. First when the ambient air temperature swings above an below the dew point and second from heat loss in the building that condensates as it cools. Many people get in trouble installing a residential roof as they do a farm building which is designed to breathe. A proper house ceiling/roof assembly consists of of an interior "AIR" barrier which stops the air loss, insulation, vented attic, moisture barrier and the roof material. Each time you cut on any of these reguirements you increase the risk of condensation and ice damming. Make sure the attic is vented and use a quality moisture barrier under the metal. If there are previous issues with heat loss etc on the home, then try to install additional ventilation and set the metal on a strapping sytem that allows for a minimum of 1" of vented air flow. There are a number of vented closure strips available today. As to welding valley, that is not a solution, you need to improve on your valley detail. On slopes less than 6/12 I recommend ice and water shield up the valley. This will provide self sealing around the screws if you get too close to the valle with them. If you have unequal rafter lengths feeding a valley you can get wash back and you need to look to a larger W or a double W valley to stop the speed. Next consider a loose fold on the outside edges of the valley pan which will catch and water. There are butyl tapes that can be used to cement the valley sections together however you must remember you are building a Kinetic roof that relies on controlled directional water flow to shed it off. And the first thing one should always do is ask the manufacturer. He designed the system and should know more than you. Why risk it.
Guest User
11/26/2005
I am building our home in Texas 1 hour east of San Antonio with vaulted ceilings with: 2x6 rafters 16" on center, OSB solar board on top, covered with 30" felt, 3" air gap, tin on purlins, R-max foam (foil on one side and vapor barrier on the other) to be installed on the bottom of rafters and then covered with sheetrock. Insulation will be put between foam and solar board in rafter cavity. I am installing a full-ridge vent for air gap. Is this a better plan than to apply tin directly to 30# felt? Moisture and energy savings are a concern. Thanks
Allan Reid
Dura-Loc Roofing Systems, Inc.
11/28/2005
You have a lot happening in your assembly. You do have a hail and possibly a wind issue in your area to consider and the building code now calls for hail approved roofs I believe and if not the insurance companies certianly encourage a Class 4 UL hail rating where discounts apply. Check with the manufacturer to ensure that you comply with their requirements. As to the reflective barrier, most need to have a minimum of 1" air space to work best. The air space needs to have free vertical air flow so you will need to set the horizontal purlins up off the decking a minimum of 1" with a vaulted assembly. I would tend to fill the 2x6 rafter spacing then with insulation which coupled with the vented air space will provide the best energy savings. Also consider an energy star rated roof.
Guest User
11/28/2005
Allan, Thank you for your input. It was very helpful.
Todd Miller
Isaiah Industries, Inc.
12/4/2005
We're glad that we could help.
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