|
| Author |
Message |
Asher
Member
|
# Posted: 14 Jul 2016 07:50
Reply
We had to completely scrap our original cabin plans. We had a cabin shell built and delivered on Tuesday. I knew how I was going to insulate the roof on my original design but this one is a little bit different.
So here is what I have... roof is metal, plywood with foil faced on the inside, and 2x6's, I know the foil needs 3/4" air gap to work properly.. I want (working from the foil to the interior) to tack on some 1" strips along the top sides of the rafters length wise edge for a spacer and then install 1/2" blue foam and spray foam/seal it in place, then install batted insulation then 3/8" plywood... I am concerned about venting and not having enough room to fit all the insulation I want to put in place and ending up with a good R rating... I even thought about just going with a 2" rigid foam board and sealing that in with spray foam, it only offers a R-10 rating but would give a lot of air space for venting. The builder installed some vents along the eves, but I'm not sure all the runs don't need to be opened..
I want to make sure I don't run into a moisture issue, this is a weekend place, but the wife and I both hate the sent of mildew...
So how would you do it if it was your place?
|
|
sparky30_06
Member
|
# Posted: 14 Jul 2016 08:37
Reply
how are you planning on finishing the ceiling inside?? I would just use R19 faced bats. R19 bats are about 6" thick and will end up settling. So you won't have your optimal 3/4" air gap to your foil but I would think the thicker insulation will make up for it.
|
|
ShabinNo5
Member
|
# Posted: 14 Jul 2016 08:50 - Edited by: ShabinNo5
Reply
We nailed 1x2 strips along the top of our rafters. Then filled the space with high density foam board. This left a 1.5 inch air space for venting. I layered 2" and 1.5" foam boards making sure to seal the edges and stagger the seams between layers. In your case you could fill with two layers of 2" (R-20). We also applied 1/2" foam board across like Sheetrock to add a bit more R-Value. Then we applied the finished ceiling. (Note, we also taped all foam seems)
|
|
Steve_S
Member
|
# Posted: 14 Jul 2016 08:53
Reply
It would be helpful if you could provide a photo or two of the connection between the wall, roof & eaves. Depending on how it was built, that will guide you in regards to insulation & venting. So you have a soffit all around ? Many of these "Shed Packages" have almost no soffit or eaves which changes the methods available to you.
One thing I would suggest strongly, is to consider using Roxul (Rock wool) instead of Fiberglass batts, the advantages are well worth it.
Depending on your heights (floor to rafter) you may have enough space to add a 1" sheet of Foil Face ISO (foil in-wards) before installing your wall ceiling (drywall or whatever). That would add R6 + reflectivity back inwards, with Roxul's R22 (5-1/2" thick batts) gives you R28+. That can be further be increased ~but~ that depends on the structure itself.
FYI: The foil faced Mylar on the inside of the roof sheathing effectively does nothing as it is in the wrong place to be effective... Even the Bubble Mylar with Foil has no real insulation value but it is a vapour barrier, of sorts... It's a good marketing "bling" just like a fancy Z22 sticker package on a cavalier, it's still just a cavalier, crappy as always but with fancy sticker's and $2000 more for it !
|
|
bldginsp
Member
|
# Posted: 14 Jul 2016 09:23
Reply
They make 4-1/2" rigid foam polyiso foam panels that will give you about R30. I would use those, hold them down an inch from the top deck, and make sure each bay is ventilated. Cut additional holes in the bird blocks if all bays don't have them, and install a ridge vent. The air must cross-ventilate, so for every bay it needs a lower vent to get in and an upper to get out.
|
|
littlesalmon4
Member
|
# Posted: 14 Jul 2016 11:27
Reply
install venting like this first.
http://www.homehardware.ca/en/rec/index.htm/Building-Supplies/Building-Materials/Insu lation-Acc/Attic-Vents/Accessories/11-3-4-x-48-x-1-2-Truevent-Insulated-Stop/_/N-2pqf Z67l/Ne-67n/Ntk-All_EN/R-I2718013?Ntt=truevent
Then install insulation.
|
|
bldginsp
Member
|
# Posted: 14 Jul 2016 12:44 - Edited by: bldginsp
Reply
The channels little salmon suggests are great when you use fiberglass bats. You could use R19 fiberglass with the channels, which will cost probably less than half what the 4-1/2" rigid costs. Depends how much insulation you want. The only way to get R30 in a 2x6 roof is with foam, either rigid or spray. The spray is even more expensive, but it provides a sure vapor barrier that, supposedly, will prevent any mold issues and requires no ventilation.
|
|
littlesalmon4
Member
|
# Posted: 14 Jul 2016 17:13
Reply
When we did our 2"x6"roof we did the following.
On top we strapped with 2"x4", 24" between strapping. Between the strapping we installed 1 1/2" Styrofoam, topped with tar paper then metal roofing.
Inside we installed truvent, then fiberglass insulation, 6 mil poly then strapped with 1"x4"
This seems to have corrected the venting issues we were having in our vaulted ceiling.
|
|
|
razmichael
Member
|
# Posted: 14 Jul 2016 17:55
Reply
Quoting: bldginsp The spray is even more expensive, but it provides a sure vapor barrier that, supposedly, will prevent any mold issues and requires no ventilation.
We went this route for ease of design and speed of install, Closed cell spray foam in roof (R30) and end wall of loft (R24). Obviously depends on your budget but in my case, I was back at work and so time was money and it worked out cheaper than any other method when I included material and my time. It also simplified the roof construction.
|
|
rockies
Member
|
# Posted: 14 Jul 2016 19:37 - Edited by: rockies
Reply
I'm guessing the roof is already built with the metal roofing in place? Why oh why do people think that a 2x6 rafter is strong enough? It leaves only 5 1/2 inches of actual wood to support the weight on top of it. You may not have any snow load to consider but someday someone will be walking around up there. What thickness is your plywood sheathing?
I was reading on the "Green Building Advisor" site that studies show that a 2 inch air gap above the roof insulation is inadequate and it should be 4 inches minimum.
I would suggest spray foaming the entire underside of the roof sheathing. Put stop-blocks between the rafters on top of the exterior walls. The spray foam will prevent the underside of the sheathing from allowing moisture to condense there and lead to mildew and rot. You can put Roxul insulation below the spray foam and then close in the interior ceiling. Then you won't need any venting or air gap in the rafter bays. "Tiger Foam" and others make kits that you can use yourself and then just return the empty canisters when you're done.
If the exterior roofing isn't on yet you could add 1x3 spacers on the outside of the roof sheathing and a second layer of plywood to create a vented "cold roof". More expensive but it will really help keep the interior cooler.
|
|
rockies
Member
|
# Posted: 14 Jul 2016 19:58
Reply
I found this on the Green Building Advisor website.
Radiant-barrier roof sheathing
The most common type of radiant barrier used in new-home construction is radiant-barrier roof sheathing — that is, plywood or OSB with a radiant barrier on one side of the panel. These panels are installed over unconditioned attics, with the shiny side facing down. Radiant-barrier roof sheathing only makes sense in hot-climate homes that have HVAC equipment or ductwork installed in an unconditioned attic.
Here's the logic: the builder knows that the HVAC equipment and ductwork will get very hot in the summer. The builder doesn't want to move the HVAC equipment and ductwork where they belong — inside the home's conditioned space — because it's cheaper to install everything in the hot attic. So the builder installs radiant-barrier sheathing to keep the attic a little cooler. At best, it's a halfway solution to a basic design problem.
That said, radiant-barrier roof sheathing is effective at lowering attic temperatures. Since it doesn't cost much more than ordinary roof sheathing, it makes sense to install it on new hot-climate homes.
New homes in a cold climate, on the other hand, shouldn't use radiant-barrier roof sheathing. Up north, a sun-warmed attic helps lower heating bills.
It would seem that having foil faced plywood is of no benefit in an enclosed ceiling space like yours (rather than an attic).
Here's the whole article.
http://www.greenbuildingadvisor.com/blogs/dept/musings/radiant-barriers-solution-sear ch-problem
|
|
bldginsp
Member
|
# Posted: 14 Jul 2016 22:07 - Edited by: bldginsp
Reply
For those of you new to this, when the roof plywood has a foil face (radiant barrier) that faces the open attic space, it reduces the amount of heat from a hot roof that reflects into the attic. Seems counterintuitive- but the foil has the physical property of allowing less heat from the hot plywood to radiate away than the bare plywood itself. Weird but true. Also, the foil reflects away any heat that comes at it from the air side.
The foil facing down in an insulated cathedral ceiling such as this should have some effect, how much I don't know. As in the scenario of an unconditioned attic it should reduce the amount of heat from the roof deck that radiates toward the house in summer, reducing how much the house heats up. And in winter, it should reflect back toward the insulation the heat that it is radiating. All the heat you put into a well-sealed house in winter escapes through windows and insulation. Insulation doesn't prevent heat transfer, it just slows it enough that if there is a constant heat source the contained area will hold the heat. The reflective foil will send some of that heat radiating off the insulation right back at it, reducing the rate at which heat transfers through the insulation.
Another thing to consider is a reflective foil surface on rigid foam insulation that is put in a cathedral ceiling, with the foil on top facing the roof deck. This will reflect heat from the roof deck in summer and send it back away from the insulation, hopefully keeping the house cooler.
So, what if you put reflective foil on the plywood facing down, and on the insulation facing up, both facing the air space? I don't know, might be a good place to roast weenies.
Maybe I'm just a crotchety old ****, but I am wary of spray foam. It is, supposedly, impervious to moisture transmission, but in reality no material is really impervious to moisture. It is supposed to seal out moisture from the inside, but then as well, it will seal in any moisture from the outside. I've been on top of a lot of torn off old skip sheathed roofs that had wood shakes on them, getting reroofed with comp or tile. The shake roofs were sealed against liquid moisture getting in, but were sieves with air. This was a good thing- they could air out between rain events. A roof with spray foam under the roof deck will have far less ability to disperse whatever moisture gets in there.
Secondly, the method described above using spray foam on the deck then other fluffy insulation (fiberglass or Rockwool) below the foam might be problematic. It is gaining in popularity because you get a lot of R value and don't have to ventilate, and you need less spray foam so it is more economical. But I think this method may have the following problem- the thinner layer of foam on the underside of the roof deck is not enough insulation to maintain the total temperature difference between conditioned space and outside. So, the underside of the foam will be colder than the conditioned space. The additional fluffy insulation makes up the R value but also is permeable to water. So moisture can make its way through the fluffy to the underside of the foam which is cold and then you have- condensation in an enclosed space, which is what we are trying to prevent in the first place. Mold loves to grow in fluffy insulation- it's a perfect medium for mold growth if allowed to stay moist.
The dynamics of moisture transmission, dew point condensation, insulation and moisture barriers are highly variable and dependent upon specific circumstances. It is very hard to predict where the problems will occur. One thing is for sure- if you have adequate ventilation any moisture that gets there- howsoever- will be able to get out. I vote for ventilated roofs, always. But as an inspector I accept what is approved.
If a 1 inch air space in a cathedral ceiling insulation setup isn't enough, that's a problem. I want to see the evidence and circumstances.
|
|
bldginsp
Member
|
# Posted: 14 Jul 2016 22:29
Reply
Quoting: rockies Why oh why do people think that a 2x6 rafter is strong enough?
???? 2x4 rafters sometimes calc out in snow loads, depending on design. I've walked on a lot of 2x4 roofs on 24" centers (not in snow country), I weigh over 200 lbs. Its bouncy but I ain't dead yet.
2x6 is commonly used for rafters of limited span. Problem isn't that it isn't strong enough, problem is that it's hard to insulate and ventilate a cathedral roof with 2x6s.
|
|
Asher
Member
|
# Posted: 14 Jul 2016 23:23 - Edited by: Asher
Reply
Thanks everyone... I think I will look into the plastic channels then just add insulation over that... I would rather have a lower R value then run the risk of getting a moisture problem.
I am not sure what the finished surface will be, but I planned on walling/ceiling the entire building with thin plywood and sealing all the gaps. That way any type of surface material I install down the road won't have to be nailed or screwed to studs, rafters, etc... I can't stand working with dry wall so I am doing my best to get creative with wall surfaces... I really liked the tin roof panels jjr used in his place... I did the same thing on the basement bathroom a couple years ago and really like the look and the light reflection...
|
|
sparky30_06
Member
|
# Posted: 15 Jul 2016 06:50
Reply
Quoting: bldginsp The channels little salmon suggests are great when you use fiberglass bats. You could use R19 fiberglass with the channels, which will cost probably less than half what the 4-1/2" rigid costs. Depends how much insulation you want. The only way to get R30 in a 2x6 roof is with foam, either rigid or spray. The spray is even more expensive, but it provides a sure vapor barrier that, supposedly, will prevent any mold issues and requires no ventilation.
Where does one find rigid foam 4 1/2" thick? Thickest I ever found was 2" and it didn't have a very high R valve. I'm looking for alternatives for my shack and spray foam is way out of the budget, even the do it your self kits.
|
|
Steve_S
Member
|
# Posted: 15 Jul 2016 07:36
Reply
@Sparky,
I bought all my ISO up to 4" thick in 4'x8' sheets from a commercial roofing company as "take offs" for CHEAP ! Another commercial roofer supplied my 2" & 4" XPS (Roofmate) in 2'x4' sheets @ $8 a sheet. Lastly my Fibre-EPS-Fibre (5-1/2" thick) came in 4'x4' sheets @ $10 ea which is what's in between my wall studs.
These insulations were used on my Pump/Power house & cabin no problem. The 4" XPS went under my slab foundation and that saved me large too (1/5th the cost).
CAVEAT: With Rigid foam you have to have a lot of patience and really plan out your cuts due to zero forgiveness. Best to lay in foam when actually building the walls, rafters to make all tight and before any rafter, stud or joist twists (they all waggle a tad after installation and humidity changes - it is wood after all)
*Commercial Roofers* These are the folks who do commercial buildings not residential... office towers, schools, malls etc all use really thick insulation on their roofs and when they have them redone it's a total strip off which results in a lot of "Good Take Off" insulation which can be bought cheaply from them before they ship it out or recycling. It is certainly worth calling around to various commercial roofers and asking them about "Take off insulation" which is usually ISO, XPS and EPS and a few variations depending on locality.
|
|
sparky30_06
Member
|
# Posted: 15 Jul 2016 10:58
Reply
Steve_S
Thanks Steve, after reading your post I remembered I did some pump repair work at a commercial insulated roof panel manufacturer up in Illinois 5 years ago that made the panel. Have to see if I still have a contact from them.
|
|
bldginsp
Member
|
# Posted: 15 Jul 2016 13:14
Reply
I got my 4-1/2" rigid insulation off craigslist cheap. Otherwise it's a special order item unless you live near a large commercial roofing supply.
Rigid polyiso foam gives you about R 6.5 per inch. So 4-1/2" gives you 30. I suspect that's why they make it that thickness. But you can use 2-2" thick.
|
|
Steve_S
Member
|
# Posted: 15 Jul 2016 18:36
Reply
Regarding buying the commercial foam panels... NOW is the time to go hunting as they ae doing the majority of this work in summer and from my understanding thanks to my local commercial supplier, they like to get the recyclable insulation shipped out in early fall. I noticed I forgot to mention, the ISO I got was all faced with a treated fibre-paper (fire, bug resistance).
Bldginsp is correct that you can order it but I hafta say, be prepared for "retail sticker shock" (gave me palpitations). If mem serves for 4" ISO Foil Faced was something crazy like $65 for a 4x8 sheet. (I needed 6 lifts of it ! NOT @ that price) and I believe it's gone up since then...
|
|
|
