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Ellie
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# Posted: 22 Apr 2014 05:28pm
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Hi, I'm about to buy a 12x32 cabin and need to know how to insulate it without losing the rafters. The roof is vented and I'm located in s. Texas.
Grateful for any pointers.
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HoosierDad
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# Posted: 22 Apr 2014 09:06pm
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Short answer is you can't, if you want to insulate the roof. But it also depends - what dimension are the rafters? If they are big enough, you can partially fill the bays with insulation, and leave some of the rafters exposed. Of course, you want to leave a gap for airflow under the deck as well.
You can also build what is essentially a second roof over your existing roof and insulate that, but that's going to be too cost-prohibitive for most people.
In a hot climate like south Texas, though, a floating second roof can actually be a good idea. You might be able to skip the roof insulation then.
There are lots of options, but they all have a different cost.
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MtnDon
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# Posted: 22 Apr 2014 09:20pm
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Quoting: Ellie buy a 12x32 cabin
... as in a new pre-fab shed/cabin to be placed on the land, or as in an older pre-existing cabin someone else built previously???
Quoting: Ellie The roof is vented
... as in gable end vents or... ???
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Bevis
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# Posted: 22 Apr 2014 09:31pm
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Quoting: Ellie roof is vented
eaves and ridge vent or gable ends ???
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Ellie
Member
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# Posted: 22 Apr 2014 10:54pm
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Thanks HoosierDad. MtnDon, yes new to be placed and Bevis, ridge vent for sure, not sure about the eaves.
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MtnDon
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# Posted: 22 Apr 2014 11:11pm
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Quoting: Ellie yes new to be placed
Ask the builder if you can have one built using several inches of XPS foam on top of the standard roof sheathing, followed by another layer of sheathing and then the shingles or metal panel roofing. R5 per inch on XPS; south TX you would be well served with at least 6 inches of XPS. Or Polyiso for even more R-value per inch. That complicates the roof for the builder but beats anything you can do inside the framing on a pre-fab.
OR,
Ask the builder if he can deliver it without the shingles installed. Jusu the roof sheathing nailed in place and the shingless in bundles for you to install. Then you can do the foam on top, the final sheathing layer and then the shingles.
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IF you want to retain the exposed rafter appearance on the inside, and if you want insulation, there is only one place for insulation; outside.
OR you infill the rafter bays with insulation and sheath under that and then make false beams or rafters to achieve the appearance you desire. But then there is not likely sufficient depth in the rafter bays to insulate well.
Quoting: Ellie ridge vent for sure, not sure about the eaves.
If the exterior foam insulation route is followed there would be no ridge vent, no eve (soffit) vents and no gable end vents. An unvented but well insulated roof. Maybe that does not sound normal, but it can be done. Lots of different detailing than a regular vented attic, but workable.
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Just to make sure myself or someone else is not making false assumptions can you confirm the goal includes insulating the roof and having the exposed rafters inside for appearance, or correct if that is incorrect. I'm envisioning an interior space with an open cathedral-like ceiling. Maybe a loft, or not? The more details you can provide the better, more practical or more complete the ideas will be.
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Ellie
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# Posted: 23 Apr 2014 12:56am
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Thank you MtnDon, I appreciate you taking the time. I liked your idea of asking the builder to insulate the roof or delivering without the panels on.You are correct I do wish to insulate the roof and keep the rafters, I like the way they look. It is a lofted cabin. Sorry for the lack of details. I'd like to keep the studs on the walls too but I realize that's not possible, well it is but I'd be taking out a mortgage for the electricity bill, which kinda defeats the purpose. In my ignorance I was hoping I could get away with some kind of rigid insulation that wouldn't fill the depth of the space between studs/beams. I'm sure glad I asked! It would seem I need that chalk drawing guy! Thank you for your advice.
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creeky
Member
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# Posted: 23 Apr 2014 07:54pm
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You might want to look at the roof I did on my shizzer shack.
http://www.small-cabin.com/forum/6_4282_1.html
The drawing of the design is near the bottom of the page.
It's a simple matter to put purlins over the insulation.
* Use foil covered polyiso sheet insulation.
* Use a metal roof and ridge venting.
Basically the sun heats the metal, the air between the metal roof and the foil faced iso heats up. The foil reflects back even more energy.
What does warm air do? It rises. When it rises it pulls cooler air from the soffits closer to the ground and ejects hot air out the vents on the roof ridge.
You can also keep the studs on the walls by using foam insulation on the exterior of the building.
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toyota_mdt_tech
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# Posted: 24 Apr 2014 02:06am
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Quoting: Ellie Hi, I'm about to buy a 12x32 cabin and need to know how to insulate it without losing the rafters. The roof is vented and I'm located in s. Texas. Grateful for any pointers.
Lay studs on top of your roof sheeting, insulated, then sheet it over top, then felt and roof.
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Ellie
Member
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# Posted: 24 Apr 2014 08:07am
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Thank you Creeky! That's impressive, will have to get some more details. What size is the gap at the bottom?
Member, thank you.
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Ellie
Member
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# Posted: 24 Apr 2014 08:14am
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Creepy, will one inch purlins do it? And if I understand correctly, the polyiso goes foil side in, facing the roof?
Why will that not also work for the walls?
Thanks 😄
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TheCabinCalls
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# Posted: 24 Apr 2014 01:52pm
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If you want to leave the rafters exposed it is easy. Just insulate the outside and the rafter bays on top of the walls.
The best way to make it air tight is to use two layers and stagger the joints. And use tape to tape the seems. Then you can put a nice metal roof on top.
If you are leaving the walls exposed as well, then you can insulate the outside, but if not then you'll want to insulate on above the plate where the rafters meet the walls (interior).
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creeky
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# Posted: 5 May 2014 06:09pm
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Sorry ellie, I missed your messages. The foil faced polyiso is actually faced with foil on both sides. Even tho one side looks white, it's painted foil I think.
Some research I saw speculated it was the foil on the inside that reflected the radiant energy back to the roof, but I'm not an engineer...
Yes 1" purlins. And yes. It works on walls. Your 1" polyiso actually goes to r10 with horizontal purlins and siding on. See building science "Perfect wall" for a foam wall.
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MtnDon
Member
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# Posted: 5 May 2014 07:43pm - Edited by: MtnDon
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Quoting: creeky Your 1" polyiso actually goes to r10 with horizontal purlins and siding on.
I love polyiso insulation sheets and I embrace the idea of air spaces, but 2 things....just to be perfectly clear.
1. the purlins used for a wall with siding should be vertical to allow any moisture that gets behind the siding to drain. That also allows hot air to rise and vent out at the top of the wall when the wall is properly done with screen vents top and bottom. Ditto if purlins are used over rigid foam roof insulation; vertical to allow for water to drain freely down the slope of the roof..
2. R10 for an inch of polyiso with an air space is a bit of a stretch. ASHRAE assigns a value of 2.77 to an air space of 1 inch. There are more extravagant claims made but the official accepted value is 2.77 per inch. The 1" JM polyiso has a R of 6.5, though there are charts that say more. The foam insulations tend to drop some r value over time, the 6.5 value is adjusted for loss over. So 6.5 + 2.8 = 9.3 max. Most wall furring used is 3/4 inch, so the air space r-value is a less than 2.77. Bigger space does add a higher value.
And the shiny foil must face the air space , as the idea is to reflect radiant energy. Without the air space the energy transfers by conduction and all the foil in the world won't do a thing.
And just to be clear the air space value is only added once to the total foam thickness value.
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creeky
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# Posted: 6 May 2014 08:53am
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Ah mtndon. You are clearly erroneous.
The horizontal wall purlin is recommended, and the r value assigned, by dow. As noted, their researchers and engineers added the r value of the siding to the 9.3, which you have forgotten to include.
The foil is on both sides of the polyiso sheet. As stated. I'm not an engineer, but the study I saw clearly indicated that the radiant energy is reflected by the interior foil layer. Seems counter intuitive.
Again. This is a newer methodology ... but something of a Boone to those trying to offset the attic/interior heat created by metal roofing.
I'd like to see someone put a new metal roof over an older one and see if they get the heat removal benefit.
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MtnDon
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# Posted: 6 May 2014 10:43am - Edited by: MtnDon
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Could be... I was referring to what is known as a rain screen wall which also offers the benefits of air spaced insulation.
And I made an error by somehow drifting into thinking wall and not roof.
Links below for wall...
http://www2.buildinggreen.com/article/how-rainscreens-work
http://building.dow.com/na/en/dowknightsolutions/rain-screen/
http://building.dow.com/na/en/dowknightsolutions/rain-screen/benefits.htm
How about a link to the info you have seen with the horizontal ?
Could it be another case of different things for different climates ???
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Hunterda98
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# Posted: 17 May 2014 03:25pm
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I want to do the same thing on my cabin, insulate above the roof. Here is how Steve Maxwell recommends doing it. http://stevemaxwell.ca/insulating-a-cabin-roof-and-floor/
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