Questions about adding insulation to Cape

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Old 12-22-09, 06:51 AM
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Questions about adding insulation to Cape

I have a cape style house in NH. It is 2 stories with the back having a full dormer. The front has a side attic (eves) that is accessible through a hatch in one of the bedroom closets. The front has the typical steep roof of a cape and you can see this slope as part of the ceiling/wall junction in the 2 front bedrooms on the second floor. When in the side attic you can see the back side of the roof decking and there is fiberglass batt insulation between the roof rafters with plastic poly over it for a vaper barrier. The knee walls are not insulated because the back side of the sloped roof is. There are air baffles under the insulation on the roof deck that extend up into the main attic. I have problems with ice dams on the back roof and with ice sheets and ice dams on the steep front roof. I think I have air leaking from the side attic (which is semi conditioned) up into the main attic. I want to seal this and provide more insulation for the back side of the steep front roof. My plans are as follows:

- Attach rigid foam board to the back side of the front steep roof over the fiberglass batt insulation. What type should I use? I read somewhere that the type with foil backing is a good choice. If I do this should I removed the plastic poly vapor barrier first? Can I attach some of the foam board to the top of the knee wall where it meets the roof to help seal between the side attic and main attic? The foam board from the back of the roof would meet a piece I attach to the top of the knee wall.

- I am also going to air seal all penetrations into the main attic and add more insulation. I have fiber glass batts between the joists and I can add more by going across these with unfaced batts.

My roof has vents in the gable ends. I had a ridge vent added a few years ago but it did not seem to help the ice dam problem on the full dormer back roof. Someone told me the gable end vents should be blocked off when adding a ridge vent to have it work properly. Should I block these off? I do have sofit vents.

Thanks.
 
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Old 12-22-09, 07:18 AM
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Hi fendesj, I'll be working in my side attics today, so your post sounded VERY familiar. Yours is insulating the rafters, I assume 2x6, which is one option. Air sealing at the eaves and up at the top of the kneewall is a challenge, detailed, but necessary. Currently you have in the neighborhood of R=12 up there now, and you want R=40, so add away. The PIC (polyisocyanurate) with the foil is fine and 4" would be great. I suspect that is more than you were considering, but install as much as you can, minimum or 2". The one inch usually costs almost as much. You will want to fill any gaps with can foam and foil tape all seams.

Now, the sloping ceiling where you can't add more insulation in the rafter cavities. Options are to remove the sheetrock and apply rigid over the bottom of the rafters, or apply the rigid over the existing sheetrock and then in both cases apply new sheetrock over everything. Messy and a project, but it will bring those slopes up to the r-value needed.

Yes, gable vents short circuit the ridge vents, so cover them, but be certain you have enough soffit venting.

Since the foil faced PIC will be your VB I would remove the plastic, but being in contact with the foil makes it optional.

One last area is where the rafters meet the top of the walls. That can be open to the insulation or have a 2x10 on the end of the floor/ceiling joists. If open, then an air block should be installed to be sure incoming cold air is not blowing into the ceiling cavity below.

Here is some reading: Welcome To Home Energy Magazine Online

Bud
 
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Old 12-22-09, 10:09 AM
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Bud, thanks for the reply. My sloping roof deck is exposed in the side attic except for about 2 feet of it at the top where it crosses the knee wall and reaches to the main attic. So if you are in the rooms upstairs you see about a 2 foot angled section that connects the wall to the ceiling. This is of course sheet rocked. I was not going to touch this part. I was going to add the foam board inside the side attic on the underside of the sloping roof from the eves/soffit area to as high as I can get it. This will leave about 2 feet at the top I can't do but I think this is still worth it. There is fiber glass batts in these 2x6 rafters. Not sure how easily I can air seal from in the side attic versus up in the main attic. I have to crawl around up there and have a look. The last time I was in the side attic I could feel cold air falling down on me which is what makes me think its not sealed well with the main attic.

As far as closing up the gable vents, how much soffit venting should I have? I know there are the styrofoam baffles that make air channels in the roof rafters from the eves on the steep front roof up into the attic. These are underneath the batt insulation. I don't remember how many of these there are. Should they be in all the rafters? On the back side the roof is much flatter because of the full dormer. There is soffit vent all along the back roof but I don't think the styrofoam channels are installed because I don't think they are needed like in the front because there shouldn't be anything to block the air flow.
 
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Old 12-22-09, 12:07 PM
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The baffles should be in every rafter bay to keep the insulation from blocking the air flow and to cool as much of the roof as possible.

I think we are on the same page with where you want to add insulation. I may be just describing it different, plus added the section of finished space that can be done later if needed.

Headed out, so post back if I missed something.
Bud
 
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Old 12-24-09, 07:43 AM
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OK, I went to Home Depot and they have Dow TUFF-R in 4'x8' sheets that is 1 inch and has foil on both sides. Its R6.5 for 1 inch. Dow's web site says to nail or stable this and they recommend installing it vertically instead of horizontally. In my case it would be easier to install horizontally because it would be less cuts and fewer seams. Is there any reason I shouldn't install horizontally? The back side of this sloped roof runs the entire length of the house but I only have access from the eaves to the top of the knee wall so I can't fit a 4'x8' sheet vertically. I would tape all seams of course.
 
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Old 12-24-09, 11:56 AM
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I see no reason for you not to install it horizontally. They may say to install it vertically because you don't want the seams of the foam board to line up with the seams of the plywood sheathing which is typically installed horizontally. If this happens in your situation, I wouldn't be too concerned about it.
 
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Old 01-01-10, 07:49 AM
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Well I went into the side-attic a couple of days ago. I have a hatch for access in the wall of one closet. I could not believe the amount of cold air I could feel flowing out of the hatch into the room. There is some rough flooring in there so we use it as storage. There are gaps between the plywood floor pieces and the air was flowing from under the plywood up through the gaps. I think the air is entering from the soffit vents. I have the styrofoam vents on the inside of the roof with fiberglass batts over them. These extend into the top attic. I am thinking there is nothing to block outside air from entering between the floor joists from the soffit vents. Some outside air goes into the styrofoam vents and up into the top attic I would think. Should there be some type of air block installed on top of the top plate of the outside wall and in between the joists to stop the soffit vent from feeding air into the side attic? In order to get a better look I am going to remove the stuff we have stored in the side-attic and pull up some of the plywood on the floor. Assuming I find what I suspect what do you think about using plastic garbage bags with fiber glass batts in them and stuffed into this area to seal it?
 
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Old 01-01-10, 08:20 AM
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What you are describing is wind washing, where the incoming cold air hits the end of the insulation and comes right through it. You will need to create a block to prevent this, as you describe or something rigid fitted in place. The only problem with the plastic bags filld with fiberglass is getting them to seal. I have looked at similar with the infrared camera and the air finds many ways around and into the space you are trying to isolate. As I stated above, sealing at the eaves is a challenge but necessary.
Here is another link with an example of wind washing.
http://www.efficiencyvermont.com/ste...ide_062507.pdf

Bud
 
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Old 01-01-10, 08:55 AM
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Bud, thanks for the reply. I just got out of the side attic. I pulled up one section of plywood floor near the edge of the roof line. What I found was surprising. On top of the floor joist where the roof passes over the outside wall there is what looks like a 1"x5" board across the joists. On top of this is a 2"x4". The roof rafters are notched and sit on top of this 2x4. This leaves a space between the top of this 2x4 and the back of the roof. The insulation on the back of the roof extends down on top of this 2x4 where it ends but just sits loosely on top of it. The cold air blows right past it. Its like having an open front door. So you are right about the wind washing. The insulation is dirty looking of course. I have a farmers porch on the front of the house. If I lift the ends of these insulation batts I can look right onto the topside of the porch ceiling. The porch has soffit vents so the air flows into those, enters the space between the porch ceiling and roof, and then flows into the main roof vents with some bypassing these and going into my side attic. Once in the side attic it can go right under my floors. I wonder how much this is impacting how much goes into the roof vents. Anyway, the question is how to seal it. I understand the plastic bags filled with fiberglass is not a perfect solution but it would be a big improvement. The problem with a rigid block is fitting it up against the back of the roof decking. There is fiberglass insulation and styrofoam vents it would press against. Any suggestions to deal with this? Another possible solution gets back to my original plan to attach PIC with foil on both sides to the back of the roof rafters within the side attic. The bottom edge of this would sit on top of this 2x4 (at an angle of course due to the slope of the roof. This would seem to provide an air block. I know I would need to foam or caulk along this edge. Do you think this would be a good solution? My only concern with this is that maybe air could get between the PIC and the insulation. Is this something I would need to worry about? Thanks.
 
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Old 01-01-10, 10:42 AM
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Another question in addition to my recent lengthy reply. I was planning on using PIC with foil on both sides (DOW TUFF-R). I have seen other rigid boards with foil on only one side and a blue plastic like finish on the other. Would this be better or worse than the PIC with foil on both. Consider one side will up against poly covered fiberglass.
 
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Old 01-01-10, 11:55 AM
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If you followed your second choice of carrying the PIC all the way down to the top of the 2x4 and sealing it, there might be some air movement through the fiberglass or between the glass and the PIC, but that air would be headed to the attic. As long as you seal off air from entering the side attic or floor/ceiling space, you should have a huge improvement. If the air entering is too much. it may render the glass ineffective, meaning thicker/more PIC or other rigid would be needed. A foil surface facing an air gap will add a bit of R-value, and an extra facing a solid surface doesn't help or hurt. Facing a mixed surface like stapled up batts, no harm, little help.

One fact about air leakage is that it needs two holes, an entrance and a place to go. Just keep that in mind as you seal thing up.

There are also batts of higher density material if you want to replace the glass right at the eaves, say 3 or 4 feet in. This is one, but I have seen other brands: Roxul ComfortBatt?

Bud
 
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Old 01-01-10, 02:15 PM
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Thanks again for the advice Bud. My rafters are 2x8 and there is the styrofoam vent plus insulation in there. The insulation comes right to the edge of the rafter so I assume its 6.25" and around R-19. The Roxul is R-23 at 5.5". Don't know if it would be worth the price but I don't know what it costs. Plus it might not fill the cavity at 5.5" so it might leave an air space. Wouldn't this be a problem?

I am thinking of using a 1" PIC. There is about 1" of the top of the 2x4 exposed where the roof rafter is notched and sits on top of it. The PIC will come down to this and it will be a little tricky getting canned spray foam in there I think. I think the edge of the 2x4 will meet with the bottom edge of the PIC due to the angle. I may end up spraying it along the edge of the 2x4 and hope it works up to the edge of the PIC. Any suggestions?
 
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Old 01-04-10, 05:37 PM
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Bud, I started fixing my huge side-attic air leak this past weekend. I had to clean it out first. I then installed 1" PIC (Dow Tuff-R foil on both sides) over the inside of the roof rafters. I had to cut the 4x8 sheets in half and make them 4x4 in order to fit them into the access hatch in the closet. So far I have gone the whole 36' length of the house and up 4' from the bottom. I then spray foamed the seam where the PIC sits on the 2x4 the rafters are on and the bottom of this 2x4 as well. It was a pain doing this because of the limited space in the side attic. Pretty much hands and knees the whole time. The space between many of the rafters had openings the size of softballs where the insulation didn't come to the top of the top plate. This was wide open to outside air coming in the soffits. I still have to do the other 3.5' up to where the inside kneewall meets the attic. But so far a huge difference on the second floor in how quickly it warms up and then stays warm. It seems like the first floor heat runs less often as well. I can't feel any air leaking in between the PIC and fiberglass batts. Thanks for the advice on this. I was wondering if you had a feel for how this type of problem would show up on an energy audit. Would a blower door or infrared camera reveal this or would inspection be required? Thanks again.
 
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Old 01-04-10, 06:43 PM
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It is hard to put numbers on actual savings even when doing an audit. The real proof will be your heating costs and comfort. The really good news is the work you can do will save a lot over the years and when done right add significantly to the value of your home.

As for the blower door and or IR camera spotting what you are dealing with, I would say definitely yes, but of course the IR inspection would involve a visual inspection as well.

Glad to hear you are getting results, let us know how it wraps up.

Bud
 
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