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CraigK
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# Posted: 11 Feb 2017 11:26am
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I'm wondering if some builders can give a non-builder some advice? I'm interested in someday building and using 16 inch centers and insulating with R-19 insulation and Reflectix reflective insulation on top of that. My questions are: is R-19 overkill in a cold climate area? Is reflective insulation advisable? And finally, will I need to finish it all off with a vapor barrier? What do you advice? And thanks!
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bldginsp
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# Posted: 11 Feb 2017 03:42pm - Edited by: bldginsp
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Generally in colder climates using R-19 in the walls and floor, and R-30 in the ceiling/roof is a minimum these days. So this means using 2x6 walls and designing a ceiling/roof that you can get R-30 into.
It's important to understand how reflective surfaces work to get any advantage out of them. Plywood with a reflective surface does two things that can help us- it will reflect radiant heat that radiates toward the foil surface, and it will not radiate heat from the foil surface.
Often plywood with a reflective surface is put on rafters in an attic with the foil down. In winter, heat from the house radiating upward is reflected back down by the foil. But it will only do so if there is at least a one inch air space below the reflective foil, which is no problem in an attic but can be in a vaulted ceiling.
In summer, the roof heats up and the plywood heats up as well. Here's the neat trick that reflective surfaces provide in summer- the foil prevents or reduces the amount of heat in the plywood that will radiate into the attic, keeping the attic and house cooler.
As far as I know, reflective surfaces do nothing when sandwiched between other materials- they must have that air space to work.
Vapor barriers are risky and I advise against them. They can create more problems than they solve. Buildings need to breath to get rid of moisture, but not so much that drafts lose heat.
Hope this helps.
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CraigK
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# Posted: 11 Feb 2017 06:01pm
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Thank you so much!
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rockies
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# Posted: 11 Feb 2017 07:28pm
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I take it you are talking primarily about wall construction? One thing to keep in mind is moisture vapor getting inside the wall cavity and condensing on the cold inside surface of the exterior sheathing. The best method of preventing this is to fill the cavity with something like Roxul insulation and covering the exterior of the cabin in another product like Roxul Comfortboard.
http://www.roxul.com/products/residential/products/roxul+comfortboard+80
By putting insulation on the exterior you keep the sheathing warmer and also prevent thermal bridging through the studs. As to a vapor barrier, you should use a vapor permeable air barrier on the inside (under the drywall or interior finish). This will prevent air from entering the wall cavity but allow any moisture to get out.
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Jabberwocky
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# Posted: 16 Feb 2017 09:16am
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To the OP: There's a helpful article on www.greenbuildingadvisor.com about reflective insulation. You will have to search for it, but basically the conclusion given is that reflective foil insulation has a very negligible benefit in colder climates. The return on investment makes it almost useless, as it only benefits during warm summer days when it keeps radiant heat from entering the building. During the winter, it works against you by doing the same thing - not allowing any potential benefit from sun rays entering the building, thus keeping it colder when you actually want warmth. (This is a very rudimentary explanation of course, and only based on what I recall when I read it six months ago).
I live in a southern climate, and I lined the underside of my old barn with the stuff. I stapled it to the underside of the purlins, therefore it has an air gap the thickness of the purlins between the sheetmetal roof and the foil. It made a NOTICEABLE difference in room temperature immediately, as I installed it during the middle of summer.
This successful experiment encouraged me to try it on our future residence, which will be an apartment built inside a post-frame shop building with metal roof and siding. I did the same thing - staple the foil to the underside of the purlins. The foil I used is Reflectix brand, and it has perforated holes to allow moisture breathe ability. Underneath that we are currently in the middle of installing the main insulation. Reflectix is not all that cheap, and therefore I am not planning on doing it to the walls as well. The return on investment doesn't add up in that application.
One final note: There are two types of Reflectix styles - one is bubble wrap with foil facing, and the other is just paper-thin foil with perforated holes. According to the article I mentioned, the glorified bubble wrap is twice the cost and isn't worth it. The thin foil is more bang for the buck.
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creeky
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# Posted: 17 Feb 2017 12:17am
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The reflective insulations are proven to basically be useless.
One way they do work tho, is like Jabberwocky used it. You can bounce radiant heat back. And that heats up the air in between. If you have a way for the hot air to escape it can drive a solar cooling cycle as hot air escapes cool air is drawn in. This is the principal behind the "cool roof."
If you want to learn something about good wall building: go to buildingscience.com. These guys rule.
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Jabberwocky
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# Posted: 17 Feb 2017 10:46am
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^^^ Yep, I have a 1.5" air gap (the thickness of the purlins) between the radiant barrier and the roof underside. Despite the purlins running parallel to the building length, the roof is "vented" due to the inherent design of the sheetmetal. The raised ridges sitting above the purlins create additional airspace that vent through the top and bottom of the roof.
Underneath the radiant barrier we are installing one layer of 3" rigid poly-iso, cut and cobbled in-between the 2x6 rafters spaced 5' OC. The insulation layer is sealed with expanding foam and air tight aluminum backed weatherproofing tape. This will be followed by a 2nd layer of 2.5" poly-iso installed the same way, creating a sandwich of poly-iso 5.5" thick - the exact depth of the rafters. Lastly, the underside of the poly-iso assembly will be covered with a smart vapor retarder. The final product will roughly be around R-33, but more importantly, impervious to air leaking while still allowing drying capability should moisture accumulate.
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KinAlberta
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# Posted: 17 Feb 2017 05:01pm - Edited by: KinAlberta
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Vapour barriers and dew points is a fascinating topic:
bolding is mine
DISCUSSION HIRF – ATTACHED GARAGE INSULATION VALUES The amendments to the Alberta Building Code 2006 addressing high-intensity residential fires include a requirement to provide a drywall finish (or other similar performing material), along with insulation and vapour barrier to the interior of attached garages. The requirement for the interior finish was added to delay the spread of a fire originating in an attached garage and to give occupants extra time to evacuate the associated dwelling unit. The requirement for insulation and a vapour barrier was added to the amendments as a precautionary measure due to the presence of the interior finish. It was felt that homeowners who purchase a house with a finished garage may be unaware that there was no insulation in the walls. If that homeowner were to then provide heat to the garage, thinking that it was in fact insulated, condensation would form within the exterior wall assembly which could lead to deterioration of the garage structural supports and the potential for the formation of mould and mildew.... 9.25.4.1. Required Barrier to Vapour Diffusion 1) Thermally insulated wall, ceiling and floor assemblies shall be constructed with a vapour barrier so as to provide a barrier to diffusion of water vapour from the interior into wall spaces, floor spaces or attic or roof spaces. ... If the fire protection of the garage walls and ceiling is provided by a material meeting the criteria of Clause 9.35.4.1.(2)(c) and the material does not create a concealed space between the wall studs or roof trusses, the insulation and vapour barrier described in Article 9.35.4.4. would not be required. This INTERPRETATION is applicable throughout the province of Alberta. http://open.alberta.ca/dataset/ef6b49df-1f41-4363-be84-62b3899c9e40/resource/0319bebf -fcb4-4293-bc60-563b43097855/download/06BCI023.pdf
THE DIFFERENCE BETWEEN AIR BARRIERS AND VAPOUR BARRIERS Can a house be too airtight? No it cannot. The difference between air barriers and vapour barriers is often confused. Also confused, is which one is more important. The most important thing to realize is that there is no fixed rule regarding vapour barriers. Building practices should always be determined by the climate in which you are building. How water vapour travels: ... Where the problem arises:The dewpoint in a wall is the point where the drop in temperature causes air to contract, and water vapour turns to liquid. Since the warmer the air is the more moisture it can hold, where the dewpoint will be in your wall is determined by the difference in temperature from indoor to out, and the amount of moisture in the air (RH - Relative Humidity). The job of both air barriers and vapour barriers is to prevent moisture from forming at that critical point, they just do it in completely different ways. ... http://www.ecohome.net/guide/difference-between-air-barriers-vapour-barriers
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creeky
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# Posted: 20 Feb 2017 12:44am
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Jabberwocky. Go have a look at building sciences walls and in particular roofs. Nothing wrong with what you're doing. but you'll want to see what they do.
Also google "texas cool roof." It details what you did with the air gap and the reflective wrap. but has some finer details that might help.
I found data that showed tin foil covered polyiso, 1" was almost perfect for this application. I used it on my washroom roof. Even on the hottest sunniest days, my "tin roof" shed has never gone more than 1 degree over ambient. Despite having 1/2 the roof exposed directly south in full sun. I love the cool roof design as you might imagine.
Climate is such a ... Living in Ontario we have both very cold winters and very humid hot summers. So worst of both worlds. I tend to build my walls to accept some moisture and make sure it can be released. Gladly paying the price in a bit higher heating and cooling load.
I put in good vapour barrier wrapping. But I ensure there's good wall/insulation drainage.
I took some pictures of two wall designs the local building collage is doing. I'll do a separate post tho.
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Jabberwocky
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# Posted: 20 Feb 2017 09:12am
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Thanks creeky. Looks like an informative site. Most of the knowledge I have acquired about building science has come from www.greenbuildingadvisor.com which I also really like.
I have communicated via email with Martin Holiday at greenbuildingadvisor regarding my particular application. He's the one that suggested the smart vapor retarder on the bottom side as an added security precaution. I have no experience with them, but apparently it somehow allows water vapor to pass through it when the interior is dry, whereas it seals it off when the air is humid.
Over the weekend I've ran into a new hurdle - starting on my second layer of poly-iso, but finding that the two layers together, despite being mathematically correct for the depth of my rafters, actually protrude below the bottom of the rafters. I think I'm going to need to attach some spacers to the bottom of the rafters before I install the strapping for the finish ceiling. Anyways.. I'm taking photos and will probably document all of this in my own build thread one of these days, but I need to get further along first.
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creeky
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# Posted: 20 Feb 2017 10:29pm
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Greenbuilding does a great job. If you watch for a bit you'll see they derive "some" of their content from buildingscience.
they do often refer to buildingscience. Which I appreciate. I'm a big fan of giving thanks and praises. being an one time journalist.
a smart vapour retarder. now that is cool.
gotta love tech.
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benny8
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# Posted: 6 Mar 2017 11:53am
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A buddy of mine uses bubble foil over his floor joists and hooks his flooring down over it. No other insulation in his cabin floor. He says his floor is warm even in below zero weather. He heats his cabin with a small woodstove.
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1967seniorsgt
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# Posted: 8 Mar 2017 07:30am
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my cabin is in mi up so I went with spray foam insulation. I sit on piers and they did under the floor too. this past winter 3 below and I could walk on hardwood floor bare foot,and in the summer inside cabin was 15-20 degrees cooler even in the loft
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