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I’m stuck ! I need to insulate 2″ x 6″ rafters in an attic remodel I will be doing. I have looked at a number of sources and they offer two options. 1.-Insulate leaving an air space 2.-Pack the space with insulation and do not leave an air space. They caution against not leaving enough air space as this will cause a problem, however, they do not advise as to how much space to leave. If I leave a space, I won’t have much room for insulation.
I thought of placing rigid foam insulation against the roof sheathing and fiberglas batts underneath the foam. I will vapor barrier on the inside and drywall.
Any one run into this before? I could sure use some good advice.
Thanks for your ideas and assistance.
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Mike, what is the code-required R-value for roofs where you are?
Leaving a space only does any good if you have soffit and ridge vents.
*Mike is right about the vents. If insulation is a priority, you should lose a little headroom. Either build down the rafters and fill the extra space with fiberglass or cellulose or add rigid foam to the bottom of the rafters and insulate above as well. Code is rarely a problem with this situation because it is an existing condition. You just have to decide what you want to do. It is always possible to add a vent space when you reroof the house giving yourself a nice "cold roof".
*The air space is important. Two inches minimum. Here's one method of achieving it. Install 3-1/2" unfaced batts between the joists, then apply foam board to the faces in sufficient thickness to reach the R-value you desire. Covering the joists with foam has the advantage of reducing the cold spots along each joist. What can happen sometimes with batts only is that the ceiling along the joists is colder than between the joists. This results in occasional slight condensation which attracts dirt, and over several years you can get a slight dirt line appearing along each joist.
*You could sister additional 2x6's onto existing ones, gain insulation/vent space without losing a lot of headroom. I've done it several times, most recently in a 1840 pioneer log cabin. This job almost broke my heart, covering those old-growth OAK timbers up with drywall. Some of them were quite serious, up to 16" x11" and hard as rock. Here's a picture showing the sistered rafters.
*You could rip 2x4's in halve and nail each half to the face of the existing rafters. This will give you additional room for ventilation and insulation. At this point, I'd recommend installing high density R-21 insulation. You'll notice the difference right away... it's worth spending the extra money up front... and you'll see the benefits each month went you receive your utility bill.Another option is to use rigid foam insulation--it's more expense and alot more labor intensive.What I've done in the past with this method is rip down 2x lumber into 1x1 strips and nail two strips for each bay to the bottom side of the skip sheathing [like a set of railroad tracks]. Divide the space in between the rafters by 3 and then install the two strips according to your measurements. This will give you the room needed for the ventilation and act as a stop when installing the insulation. In addition, be sure to install high and low vents. From there, install the rigid insulation right up to the strips. You could use either three layers of 2" rigid which would give you approximately R-30 or use one layer of of 4" and another of 2" which will give you the same R-valve.Foam insulation is very messy to work with and bad for the lungs. Other than that, spray foam could be another option.Good luck with your project.
*Mike:I really like Schelling's suggestion that you create your vent space when you next reroof. What shape is your roof in? If there's any likelihood that you'll br replacing your roof any time soon, just go ahead and fill the space between your rafters with solid foam or fiberglass. After you stripped off your shingles you can add another layer of foam insulation then plywood sheathing, 1x2 sleepers 16"oc running from eaves to ridge follwed by a second layer of plywood, felt and shingles. I should have asked you what climate zone you live in, Mike. What I've just described is appropriate to places with serious ice and snow during the winter. With the recent global warm trend, who knows? Good luck, Mike.
*Another option would be to purchase styrofoam spacers design for insulating vaulted ceilings. They staple to the bottom of the roof sheathing and create an air space for ventilation. They're quick and easy to install. However,they do require high and low venting. Then, it is a piece of cake installing the insulation. The insulation actual makes contact with the spacers. No need to worry about the size of the air space--it's already build in.
*I always follow threads regarding vaulted ceilings as they are a problem here in Severe Cold region near Canada border. Accepted methods are as described with some using "modified cold roof" (vent and insulation in the bay) and "cold roof" (venting in separate bay above insulated bay). No venting has been notorious for severe moisture damage. There are those at this site who swear by "dense packing" cellulose in the bay, without venting. Sure would like to hear their responses to this oft repeated question? Randy
*To all: Thanks for your help. I was looking at my home e-mail site and not seeing any replies. I finally decided to look back at this site and low and behold, I had some great suggestions !I decided to use a combination of a number of suggestions. I will be adding a 2 x 2 to the 2 x 6 rafters gaining some depth. I will be using the styrofoam trays for air space with R-10 rigid foam and R-13 batts between rafters. I will build a bulkhead across the peak of the cathedral ceiling lowering the peak. This will open up a space for air to travel from the open space behind the kneewall to the peak. The peak right now does not have a ridge vent. However, I will be moving the current gable vents up higher to access the space across where I lowered the peak opening up the space. This will allow me to insulate the lowered ceiling and still leave air space above. I think this is the best I can do with this situation. It should leave me with an R-23 in the ceiling sides and top with an air space. In a perfect situation for Wisconsin, I would have an R-38 in the ceiling. I can't gain this without blowing the budget or lowering the ceiling too far losing height. Thanks again. Hope I can return the favor !Mike Charron