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Rifraf
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# Posted: 28 Dec 2011 23:14 - Edited by: Rifraf
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Do you think this is a good way to do it, or should I just insulate with roll up insulation and forget the barrier?
http://www.youtube.com/watch?v=CjwZXoRzRiM
Im insulating a shed that is built to look like a cabin.
See here for my project:
http://www.small-cabin.com/forum/6_1646_0.html
another question, if it is such a great idea, shouldnt I do this vertical walls too ? if you can answer this please explain your answer, as im trying to learn all I can.
Thanks so much.
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MtnDon
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# Posted: 29 Dec 2011 00:23 - Edited by: MtnDon
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Well at least this guy admits there has to be a nominal 1" air space (3/4" actual) for a radiant barrier to work. I've seen some try to say an air space is not needed. Keep in mind the radiant barrier only reflects heat back into the direction of the air space, to the outside in this case. An R-value of 2.77 is assigned to such a radiant barrier. So, if you have R19 of some other thermal conductive insulation (fiberglass, cellulose, etc.) on the other side of the air space it's like having R19 + 2.7 = R21.7 against heat entering, but still only R19 against heat loss from the interior to the exterior. BUT that's not taking into account the R-value lost when the fiberglass used is compressed.
R19 fiberglass batts are rated R19 when installed to the manufacturer's rated thickness which is usually 6 1/4". Check the Owens Corning chart below. But 2x6's are 5 1/2, 3/4 of an inch less. According to Owens Corning that reduced the R19 to R18. If the fiberglass is compressed an addition 3/4 inch there will be more reduction in the R-value of the fiberglass. How much? Can't say for sure from the chart, but I'd say it's likely to lose another full R-value, maybe more than 1. So that means the fiberglass will lose maybe 2 points of it's insulating R-value against heat loss from the inside to the outside; coming in at maybe R17 inside to outside. ??? . The R-value in the direction outside to inside would likely be about R17 + 2.7 = R19.7. Perhaps this is worth all the effort as the R-value of 2.77 may not accurately reflect what actually happens on a day when the hot sun is beating down. I don't have the answer to that. That's a tad more than what you'd have without the radiant barrier.
Of course using the radiant barrier as was illustrated does provide some roof ventilation. (You need to add exit vents all along the ridge and air linlets in the lower edge of the roof at the wall top. Just stuffing R19 in between 2x6's will provide zero venting. Not good either. Note that a full one inch is recommended as a minimum air space and some building science experts believe 2 inches is even better.
I also wonder how well those three strips of foam will prevent the compressed fiberglass batts from squeezing that air space down to less than 3/4 inch?
There are also arguments between experts on how well the assigned R-value of 2.77 will hold up over time. Any dust settling on the surface exposed to the air side reduces the reflective ability and that will reduce the overall effectiveness of the foil.. This has been shown in lab tests. Oh, the R-value of 2.77 for the air space was attained with lab tests as well.
It's a dilemma. You should vent the roof and you'd like to have maximum insulation in the rafter bays. Vented can be omitted entirely by using a couple inches of spray on foam, but that can be pricey. Perhaps you could look into the DIY foam spray kits. I know of a couple guys who did successfully use it. It can be touchy; hard to get the amount right. Stop spraying for a few seconds too long and the applicator nozzle "freezes up" with setting foam.
Applying spray foam to the underside of the sheathing works by sealing the air and water vapor from direct contact with the cold roofing material. It changes the dew point to where condensation can not occur on the foam (if the spray foam is thick enough). Then the rest of the cavity is filled with f'glass or dense pack cellulose.
Cutting sheet foam panels to fit tightly against the underside of the roof sheathing works too. That must be completely edge sealed to keep air from getting through. If there are air leaks around the foam water vapor will find its way back up in there.
Doing the walls would be good but with the same trade offs in compromised R-values of the fiberglass.
Many new homes use the foil faced polyisocyanurate foam sheets on the exterior sheathing, with an air space and then the final exterior siding/stucco over that. It works well when the wall cavities are also filled with blown in dense pack cellulose.
There are some trade offs when you convert what was designed as a shed into living quarters.
I'm not sure what I'd do. I know what I'd like but the roof is already assembled and shingled.
G/L DonM
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jrbarnard
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# Posted: 29 Dec 2011 14:06
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If your place is already put together, as yours is and is mine, then the 1" thing is really a moot issue. The only place we would not have 1" available would be every 2 ft that we have a rafter. To me, what I have right now is 2" of insulation. The RB would "butt up" to that and there would not be any air..i.e. the 1" clearance/space.
The problem... I already am in an oven in SW Texas when the sun is up. The insulation is already absorbing or letting the heat pass into my cabin. So, by lining the interior walls with RB, I am trying to reflect even SOME of that heat back out of the cabin.
I can tell you that a 14,000 BTU portable A/C cannot keep up with the heat as it is. My hopes is that by lining the entire inside with RB, I will be able to cool the place (and I have pretty much the exact same thing you have right now.
Now, with the new cabin I build from scratch, that is a different story, but in your context.. don't worry about the 1" unless you plan on reconstructing the thing, which I doubt you do.
jmo
Russ
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Rifraf
Member
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# Posted: 29 Dec 2011 16:19
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Russ,
well, I could shim out the rafters easy enough by lining them with 2x1s or something like .. choices choices.
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jrbarnard
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# Posted: 29 Dec 2011 16:30
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naaahh.. I would not go to that much trouble.. I know "I" won't.. heh. The object is to keep as much heat as possible and improve an already constructed cabin for as little as possible.
Look at your cabin as it is. The sheets are the easiest way to do radiant barrier and it still will take about 100 of them to cover the entire inside.. that is, what.. $1000?
I know, for me, that means I am going to take 5-10 sheets out each time I go out there and I will eventually get it covered.. anything more than that is not going to be effective enough to worry about.. again.. imo.
Sure, you can build it so that it is compliant to reduce 100% of it's designed ability, but for me.. if I can reduce 80%.. hell.. I'm good! lol
Good luck amigo!
Russ
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Martian
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# Posted: 30 Dec 2011 08:47 - Edited by: Martian
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If you're buying the RB and insulation, then spending a lot of time installing them, I'd look long and hard at spraying a couple of inches of foam in the cavities. Otherwise, putting 2X2 strips on the roof 24" apart, placing 1 1/2" styrofoam between them, and covering with reflective metal roofing will help a lot.
Tom
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Rifraf
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# Posted: 30 Dec 2011 13:01
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Martian,
Thanks, in the end I may just have it sprayed throughout the cabin and not put any rolled type in there.. I have a question though.
In the rolled up type, the paperback acts like a moisture barrier when installed properly, if I have the foam sprayed into the cabin instead do I still need to get some paper cover to go over it all to act as a moisture barrier or does the foam do this on its own ?
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Martian
Member
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# Posted: 30 Dec 2011 13:20
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The foam does it all.
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Rifraf
Member
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# Posted: 30 Dec 2011 17:17
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Thanks Martian,
I found a reasonable quote for my cabin, 0.34 cents per board foot, meaning I can fill 3 inches in all cavities for about 900.00 USD.
I may spend a little more and have him spray under the cabin too.
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Martian
Member
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# Posted: 30 Dec 2011 18:12 - Edited by: Martian
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Is that $.34/bdft for closed cell foam.
Aren't you down in TX? I don't have my floors insulated (there is a skirt around the bottom), and they aren't cold up here in KS; so if you are thinking of spraying the floors to keep warm, I think you can save your money. Now if you are spraying to keep cool, it may help since cold settles to the floor. The more I think about it, if its gets 100 degrees in the shade, then it would be like having heated floors; not good when you're trying to cool the place. Spraying the floors would probably be a good thing.
Tom
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Rifraf
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# Posted: 30 Dec 2011 18:23
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Im in southern Missouri. I was considering spraying under the floors for to keep my hardwoods a bit warmer in the cold season,
I will have electric heat and ac, so wouldnt insulating the floor be a good idea ? The heat wouldnt be able to rise from under the piers to the interior as well, and my cold air wouldnt fall down there as easy either , at least that was my thinking.
i've mentioned a few times im learning as I go.
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Martian
Member
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# Posted: 30 Dec 2011 18:49 - Edited by: Martian
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Since you are using electricity for heating and cooling, I'd do everything I could to keep the air inside whatever temp I was trying to keep it. That means insulating everything.
A ceiling fan helps keep the air mixed up so you don't have hot/cold layers.
Tom
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