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Old 01-27-12, 06:55 PM   #1
scottorious
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Default 1.5 story house insulation ideas

Hello everyone. It has been a while since ive been on the site. I finally closed on my house and the work has begun to tear apart the entire second floor. It is a 1 and a half story house so the entire second story is is knee walls and slanted wall/ceilings. I believe the trusses are only 2x4 and this house is fairly old and I dont believe there was any insulation put under the roofing. I am considering a couple options for doing the insulation. The major factor is that I have to be able to do it myself. I am considering doing the "mooney wall" with blown in insulation to cut down on thermal bridging. I am also considering doing the cross pieces of the mooney wall with using rigid foam board insulation. I dont know if anyone has experience with a similar set up.

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Old 01-27-12, 09:00 PM   #2
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...It is a 1 and a half story house so the entire second story is is knee walls and slanted wall/ceilings. I believe the trusses are only 2x4 and this house is fairly old and I dont believe there was any insulation put under the roofing...
Yep, sounds just like my house.

Mine was built in the late 1880s. 2x4 baloon walls, 2x4 in the roof, etc.

Mine had a ceiling that was about 8' or 9' wide. No insulation anywhere.

I started out putting in R-13 fiber glass, then R-15 when it came along. Better than nothing, but that's about it.

But fiber glass is not so good unless you absolutely prevent air from passing through it. And the R-value per inch is not so high... very important when you have so little space for insulation.

After several different shifts in method, I ended up tearing out the ceiling thing. I was told by an architect that I could remove every other stringer that tied the roof rafters together... which I did. I put in rigid foam that I cut to fit between the rafters. I cut the foam very carefully so that it just fit in the space. I eventually used a method where I cut the foam pieces 1/4" small all around and foamed the gaps. Much easier and much more air tight. My 2 x 4 were actually 2" x 4" (imagine that) so it worked out well for the foam which I bought in 2" thicknesses.

I started out using EPS foam but I have since gone to XPS foam, which has three advantages, it has higher R-value (5 instead of 4), it does not shrink over time, and it is quite easy to cut with a hand-held jig saw. All those advantages, and I am getting it at the same price.

It is to your advantage to lap the layers so that you don't have seams at the same spot. You might even want to use some kind of tape that is made for such a purpose. No, duct tape will not work.

I didn't know about thermal bridging when I did my roof... I nailed 2" strips to the rafters. Not so good.

You lose about 18% of your insulation value if you don't break the thermal bridges. Using perpendicular strips like the mooney wall would do it just fine.

I'm doing my kitchen right now, same method... when it was particularly cold, I read the temp of the foam wall and also read the temp of a 6" stud... the stud was about 5 degrees F colder. Thermal bridging is for real.

So my advice is to go for foam XPS or iso, foam the gaps, make sure each layer is AIR TIGHT before you proceed to the next layer.

You will curse me all the while you do the work, but after you're done, you'll thank me.

And when your comp roof goes to hell, you should plan on adding a couple more 2" layers and then a plywood nailing surface. and then the new roof.

Insulate, insulate, insulate...

-AC_Hacker

P.S. Don't forget to add modern wiring while you have the house torn apart.
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Old 01-27-12, 09:14 PM   #3
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how did it compare cost wise? I think it sounds like a great idea....did you leave any gap for ventilation?
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Old 01-27-12, 09:59 PM   #4
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This is great info- lets detail this some more! I'm on my 2nd 1.5 story house- and the 2nd floor is the hardest to control the climate in, and besides the efficiency of upgrading the insulation- you reap enormous benefits in the usability of the space with some upgrads.
In my previous house I had 2 sunroofs and the cavities were blown tight with dense pack cellulose. I only had 2x4 roof rafters and had condensation in the winter around the drywall joints in the sunroofs. It was freezing or steaming depending on the season. I was one of the first people who tried one of those asian mini-split heat pumps (see my post on ecomodder) and really enjoyed the extra heat and cool.

My current 1.5 story house is also 2x4 rafters with old mineral wool in the cavity. In the summer you can feel the radiant heat beat thru the south ceiling.
So far- I added significant cellulose blown into the the area behind the knee walls- added plastic shutes down to the eaves for ventilation and even added 4 ft of foil on the roof rafters to reflect out the summer heat. The main problem is the cathedral ceiling that only has a 2x4s depth of poor insulation in it. During the last cold spell I used a laser thermometer, and the cathedral ceiling was a good 8-10 degrees colder that the rest of the walls.
I guess I'll have to rip out the drywall and add foam. If its on the inside do I even need mooney wall 2x2's, or can I just put 4x8 sheets across the the rafters after filling the rafter chase and drywall over the top of the foam?
Also- I'm still not sure how to vent these things so they don't get moisture problems. The bottom roof- behind the knee walls- has many roof vents all around. The top of the roof has a small area above my ceiling that is also vented pretty well. I may get a new roof thanks to hail and tornado damage if my insurance company doesn't fight me too much- so I'll probably get a ridge vent. My only concern is that I either plug the cathedral part totally and rely on the lower and upper areas to vent separately, or I jury-rig some venting kit inside the 2x4 rafter and still insulate it with whatever small area is left. Seems like a poor solution either way. And no- I've asked every roofer around if they would add a couple inches of foam and they won't do it if the area below the roof is vented. That means major detailing around the eaves and everywhere if foam is added on top of the roof- which means I might as well remodel and add a whole new 2nd story.
I'm more curious about the ac-hacker method. seems like the only way you can get it done without permits and moisture problems.....
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Old 01-28-12, 02:01 AM   #5
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My house is similar to yours. Mine was built in 1951 and has 2x6 rafters. I put in 6" of iso-board in the sloped ceiling, R50 in the ceilings and R14 in the knee walls. In one of the other rooms upstairs, I had leftover R20 so it went into the knee wall. All of this insulation was Roxul. I sealed off the iso-board with low expansion spray foam.
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Old 01-28-12, 02:44 AM   #6
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how did it compare cost wise? I think it sounds like a great idea.
At the time I was beginning the serious rigid foam install, I started running some numbers...

It's been quite a while ago, but I invented a 'figure of merit' that to my knowledge didn't exist before, and the figure of merit was 'R-value/dollar in a 6" space'.

Iso gave me more R-value at a higher cost, cellulose gave me least expensive R-value but lower R-value in 6", but in the 6" space, EPS gave me be best R-value bang for the buck. Since then, XPS rigid board has become available locally that has better R-value, at about the same price as XPS. So right there I would have preferred it. As I have recently learned, it also doesn't shrink... and it cuts easily with a saw.

But if you're asking just about price, foam board is more expensive. But it is quite simple to calculate the price & R-value of a square foot of various 2" foams, a bit more involved to calculate the cost & R-value of 12" x 12" x 2" of cellulose... but it can be done.

But when you've done all that, it's not so easy to choose one over the other, and that's why I invented the figure of merit.

Rising energy prices favor higher R-value, too. I started this whole foam board insulation thing nearly 15 years ago. Foam prices have risen and so have energy costs... I'd say that energy costs have outpaced foam costs.

I read some things about Iso board being hygroscopic (gradually absorbs water) which caused it to lose R-value over time... as I recall, they referred to it as 'R-creep'. I had more confidance in EPS on that account... XPS is even more favorable.

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...did you leave any gap for ventilation?
I assume you mean a gap between the foam and the sheathing. Sounds like a good idea. In fact, a 2" gap running from soffit vent to ridge vent would be excellent, and would make a much cooler interior in the summer... but I didn't want to lose that 2" of insulation.

I left no gap.

Contact me in 20 to 30 years and I'll let you know if I did the right thing.

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Old 01-28-12, 08:07 AM   #7
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I'm not sure about where you're from be here that kind of house is called a "Cape". They are IMO the most frequently mis insulated houses on the planet. With the way people try to "insulate" them I'd rather live in a yurt.

Here's the catch: People insulate behind the kneewall. I know, the damn auditors have me do it all the time. And it's more or less a wast of my time and good cellulose. That approach is like putting lipstick on a pig. It just doesn't work. You can't remove a portion of the house from the thermal envelope without making it as tight as a freezer. And no one can possibly do that with a kneewall. The proper way to do it is to insulate under the roof. If I had a cape I would make it a hot roof. At least until the top of the slope. I'd seal the soffit up entirely, screw/strap polyiso under the roof and blow cellulose behind it. Also don't forget to do the same on the crawl space gable ends. A house needs to have the entire thermal boundary consistently in the same place. You can either use the ADA (airtight drywall approach) or make the sheathing air tight. Most people use a combination of the two and fail miserably. So you spent all afternoon building, insulating and air sealing your fancy new door to your crawl space.... but what about the transition between the kneewall and the floor? How are you going to make that air tight? You can't. Your crawl space is cold and all that cold air comes through your baseboards. You can't even fix this with spray foam. And let's not forget the floor/ceiling joists that are there. Your crawl space is cold and all that cold air moves through your first floor ceiling and under your second floor feet. What to do about that? Get on the phone with the high density spray foam people and then sell a kidney. The auditors have me do all kinds of stupid things there. Like jam a wad of fiber glass in and then try to dense pack the cavity. Not air barrier there. I'd rather have a tight house and open a window than have my feet freezing all winter. Bring the crawl space into the thermal envelope, have a more efficient house and gain conditioned space. Bargain. Oh, and it's a LOT! easier to do than screwing with trying to insulate knee walls.

Sorry about the rant but I see this all the time and I want to cry every time I'm told I have to insulate a knee wall. I'm dieing when I see that they want me to blow am impossible to air seal wall that's only 3.5' deep when I'm sitting right next to a rafter bay that's almost 8". You can get R-27 in there no sweat and then have R 12 or so of polyiso on top of it. Frik'n R 40 roof. It's no R 60 but it's better than R 13.

Edit:

AC_Hacker, about the foam board loosing R value, it will happen to all of them. R 4 is about as good as you can get with air. Anything over that is due to fancy gasses trapped in plastic. This is why polyiso always has a facing. It only really matters though for the outer edge as the foam is closed cell. But all of them are slowly making their way back to R 4 / inch as time goes on.

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Old 01-28-12, 10:27 AM   #8
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so let me make sure i understand you. You said you would suggest taking foam and laying it over the rafters and putting drywall on top of that? and blowing in cellulose behind it?
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Old 01-28-12, 11:06 AM   #9
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S-F's comments about problems encountered with what I'll call 'junction areas', where walls meet floors, where walls meet roof rafters, etc. is really important. In my early efforts I did not include that, and I didn't get all of the insulation effect I should have from the effort and money I put into insulating.

It would be a really good idea to get some visuals on this problem (photos, drawings, charts, etc.) and photos on proven remedies to the problem. This is an often overlooked insulation failure area that we can all benefit from knowing more about.

By the way, the current issue of Fine Homebuilding has a very interesting article on a building (that houses the office of Building Science Corporation) that was foam wrapped, developed some problems, and 15 years after it was constructed, was de-insulated, inspected, and re-insulated. Very interesting... worth buying the mag or a visit to the library, or checking out this link.

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Old 01-28-12, 12:04 PM   #10
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so let me make sure i understand you. You said you would suggest taking foam and laying it over the rafters and putting drywall on top of that? and blowing in cellulose behind it?
No. I was talking about in the crawl space. Doing what you said but in the crawl space. If you are going to have your sheet rock torn down (and if so I feel for you as it's messy. I'm doing that right now in my house) then I think strapping or something like that is a fine idea. I was only talking about the rafters in the crawl space. Seal the top plate with spray foam (1 part is fine) and run sheets of polyiso from the top plate to the top of the knee wall. Then dense pack. Tape all the seams with foil tape, if you are using foil faced boards. I would also air seal the roof deck from the underside with a vengeance first.
Thanks for the tip on the Fine Home Building article AC_Hacker. I imagine that there will be a BSC article about this soon.

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