Insulating attic rafters, help-advice please


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Old 01-03-14, 05:35 PM
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Insulating attic rafters, help-advice please

I'm in Northern NJ. I would like to insulate my attic rafters to add more R value. What I have, attic floor is 2x6 already filled with fiberglass and 7/16 osb as a floor for storage. My rafters are 2x6 and the side walls are 2x4, roof is a cross gable. I have a ridge vent, electric vent, gable vents and sofit.

The sofit is only on one side of the roof as the back side of the house has a closed in porch with a flat roof that butts up to were the sofit used to be or is on the other side of the house.

I would like to use closed cell foam but that is way to much money approx. $5k for the space. I'm thinking R19 in the rafters but I see alot of talk about ventilation. Then I see you should add baffles but only for the first 4 feet and not all the way to the ridge. Then some people say no run the baffles all the way to the ridge, and others talk about fiberglass allowing plenty of air to flow through.

So now that I'm totally confused here is what I think I should do but would like some input. On the rafter side that has a sofit I would like to install 1.5 inch rigid foam with a r value of 9 and r13 on top of that. Total r22 and would take up 5 inches in my 2x6 rafter giving me about a half inch air gap all the way up to the ridge for ventilation.

On the side with no sofit, r19 filling the 2x6 cavity, no ventilation.
Close off the gable vent in the winter with insulation, remove in the summer.

Thoughts???
 
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Old 01-03-14, 06:02 PM
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My thought is, what you are wanting to do is probably pointless, could possibly create condensation problems, could possibly increase snow melt on the roof which could increase the chance of ice dams when that melt water reaches the colder uninsulated rafters below.

Depending on where you are located, R38 to R49 is suggested as being the level of insulation you should have in an attic in your area.

That doesn't mean half should be on the floor and half should be in the rafters. Hopefully Bud will be along to comment on this one.
 
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Old 01-03-14, 06:08 PM
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Also, faced or unfaced fiberglass in the above?

Thinking unfaced and then installing Radiantgaurd over the top to give me a nice finish
 
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Old 01-03-14, 07:00 PM
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Forgot to mention. I have my central air unit in the attic
 
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Old 01-03-14, 08:09 PM
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I get the feeling you want to go with your plan, but there are some basic issues you are overlooking. Bear with me despite this not agreeing with the current outline.

I know spray foam is expensive, but it or rigid insulation (less expensive) sealed in place have another advantage over batt insulation, they block any air from reaching the cold condensing surfaces.

When rigid is used along with batt insulation as you have suggested for the front, it generally should be 1/2 the total R-value. Again, this prevents air that will penetrate the batts from reaching a condensing surface. In NJ your r-9 and r-13 may be close enough to the 50/50 split, however the total is far from code which is considered a minimal value. You just cannot get enough insulation in a 6" rafter. Then, the 1/2" air gap will not provide enough air flow.

As X mentioned, you need all of your insulation in one place, not half high and half low.

I and many others are willing to help you work out a solution, if you want, but your current approach will have problems.

Bud
 
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Old 01-03-14, 09:18 PM
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Thanks for the reply. I'm open to suggestions.

What if i did 1 inch foam board from lowes, pactiv polystyrene r 5 between the rafters and r 13 batts on top. Total of 4.5 inches in the rafter, held away from the sheeting to give me a air gap from sofit to ridge?

Or 1 onch polystyrene polyshield with a foil outer layer installed same as above? In hopes that it reflects heat away in the summer and reduces heat loss in the winter.

Or just use the 1 inch polystyrene polyshield by itself on top of the rafters creating a wall from sofit to ridge and on the walls as well with the foil side facing out? Tape the joints and forget about the batt insulation in between the rafters. I know this is not going to achieve the r value reccomended for new construction but it's a 60 year old house.

Just install the foil reflective barrier?

The spray foam is looking better and better
 
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Old 01-03-14, 09:32 PM
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Since you are throwing all sorts of ideas around, how about this one. Add new 2x12 joists across the attic, rafter to rafter, fill the new floor with 11.25" of cellulose for an additional R-42, and add new osb sheathing to the top for storage. Enclose your AC unit and insulate it too, but make it accessible for repair. You might even be able to do it for under $5G.

The spray foam IS looking better and better.
 
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Old 01-04-14, 04:29 AM
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If you have a ridge vent you do not need all that other venting. In fact it's making the ridge useless.
The ridge vent will just use the other vents for make up air instead of drawing from the soffits like it's supposed to.
Insulating the rafters in an unheated attic just does not make since to me.
Need to be keeping the conditioned air in the building envelope.
 
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Old 01-04-14, 05:27 PM
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I was going to suggest putting rigid foam over the current floor to meet R value and put new subfloor on top of that for storage. You don't need to add R-38 there. 6" of Rigid foam will be more than enough. Stagger and seal all seams and make sure the areas where the rafters meet the floor joists is well detailed.
 
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Old 01-05-14, 05:14 PM
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Bud, what do you suggest? Maybe a low cost, mid and high end solution.
 
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Old 01-05-14, 10:25 PM
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I just spent an hour playing with your attic and did not come up with a good solution. Eyes are closing so will try again later. Like you, I'm caught between following code and finding a simple solution. Giving advice here on the forum often requires us to advise following code, but we have no control over what you decide to do. It looks like a sloped ceiling in your area requires R-30, R-38 if you have electric heat.

Later
Bud
 
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Old 01-06-14, 07:21 AM
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Bud, i appreciate your help. I have a gas fired hot water boiler with old school recessed radiators on the first floor and slant fin baseboard in the basement. Basement is finished and insulated with r13 and the basement is about 6 foot deep into the dirt.

First floor has about half new r13 since we remodeled the kitchen bathroom part of the living room wall and part of the bedroom wall. The rest is the old school rockwool (i guess black paper backed r13 if i'm lucky). Better than nothing. Probably more info than you need. One side of the house has a attached garage but with no direct entrance to the house and the back side has a enclosed porch almost the entire length of the house. I should probably take some pictures and post.

Anyway, back tot he attic. What im thinking so far is the following three solutions. I understand they may not be ideal, pass inspection for new construction and that their are better ways to do it. But, at what cost and what affect does it have on my storage.

1, I buy r13 and put in my 2x6 rafters giving me some thermal break and leaving me with 2 inches of air gap from sofit to ridge. I would also consider adding that foil stuff in between the rafters but not touching the insulation or wood before i add the r13. R13 on the side walls.

2, r13 on the floor on top of the old insulation and making my storage area smaller. R13 on the side walls. I like this because i could easily move the batts. I don't like this because I will not be able to insulate any more under my storage flooring and I'm nit raising it.

3, same as above but with cellulose building a perimeter dam and adding much more than r13, say 8 inches which is like r20 plus the r19 already there, but again the middle where my storage is will still be r19 only
 
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Old 01-06-14, 08:24 AM
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From your options, what I see missing is an understanding of moisture problems, you and 98% of home owners. Example, the storage flooring is forming a condensation surface for moisture the is leaking or diffusing up from below. How much, hard to determine without some effort, but that layer needs to be moisture permeable, more so than osb. This doesn't mean you have a problem, just a potential problem.

As for fiberglass insulation in the rafters, air movement above it filters through it and almost eliminates its r-value.

I'll attach a link on moisture, and if you search their collection of articles there is a ton of information.
BSD-012: Moisture Control for New Residential Buildings — Building Science Information

Bud
 
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Old 01-06-14, 07:46 PM
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Ok, i get it, don't insulate the rafters. I will look to reduce the storage floor space, add baffles at the soffits, blow in cellulose or add batt insulation and add batts on the side walls. Seal up the attic hatch and call it a day.
 
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Old 01-07-14, 09:31 AM
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Related question: If closed-cell foam is sprayed in the rafters, what happens down the road when the roof needs to be re-decked? Is it even possible to strip off sheeting that has a foot of spray foam stuck to the underside? I've always wondered but can't recall seeing the question here before.
 
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Old 01-07-14, 09:38 AM
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I've always wondered about that myself. Ideally of course the decking never needs replacement...but we know that isn't real life. I know how that foam sticks to everything and is darned strong.

You might want to Google "hot roof" just to confuse the issue even more.
 
 

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