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Kramer
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# Posted: 5 May 2010 20:58
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I'm finding it unclear if a vaulted ceiling actually has to be vented in all cases. Some say you can get by without venting it, providing the vapour barrier is of a high quality, others say as long as it is spray in foam (which acts as an impermeable vapour barrier) it will be okay, others say you must always vent it regardless of what you use.
What gives, and why?
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larry
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# Posted: 5 May 2010 21:08
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it has always been told to me that the reason you vent is to allow air flow to the sheeting and also help asphalt type roofing...never did make sence to me.
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Kramer
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# Posted: 5 May 2010 21:43
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Quoting: larry It has always been told to me that the reason you vent is to allow air flow to the sheeting and also help asphalt type roofing...never did make sence to me.
Doesn't really make sense to me either... You don't vent your walls. I can see with a regular trussed roof where you have a large air space that it makes sense.
Perhaps asphalt roofing is not as water tight as one would think? Do the recommendations differ with metal? (Might go the metal route)
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larry
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# Posted: 6 May 2010 07:43
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i think your right about the roofing. it only seem that recently you here talk of venting. is this because building suppliers want to sell us more stuff? as far as the steel roof is concerned, it is attached to furring strips that are nailed to the roof sheeting so it will have an air gap.
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Kramer
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# Posted: 6 May 2010 09:59
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Actually, not all metal roofing has to be attached like that. It can go right on top of sheathing, or an old asphalt roof (IIRC, but I'd be hard pressed to do that), depending on the manufacturer's recommendations. I don't know if the venting recommendations differ in that situation either.
I'm quite confused, and unfortunately all my official literature skips right over the topic of vaulted ceilings.
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SheldonDesigns
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# Posted: 17 May 2010 19:33
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Venting the roof is primarily for colder climates where moist interior air (from human occupation, cooking, breathing, etc.) reaches the underside of the roof deck, condenses and eventually creates mold or rot.
A good vapor barrier on the warm side helps but unless it is perfect, some moist air will get through to the deck. The common industry standard opinion now is that a cathedral roof needs 50% of the vent area at the ridge and 50% at the eave... unless one uses spray foam. The spray foam will adequately seal all paths of vapor migration to the roof deck.
In warmer climates, venting the under side of the roofing or roof deck will tend to keep the roof cooler and add to it's life.
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nyantler
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# Posted: 1 Aug 2010 20:02
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the reason you vent your roof is because heat rises.. and if not properly vented the roof will retain heat which conflicts with lower outdoor temps and causes condensation which cause mold and or rotting of lumber. This is especially true with metal roofing where condensation buildup will run down the underside of the metal like a faucet
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nyantler
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# Posted: 1 Aug 2010 20:04
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Also the best method for venting is a vented soffit with a ridge wentas the heat rises through the ridge vent it pulls in cooler outside air that helps equalize the temperature under the roof
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Anonymous
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# Posted: 4 Nov 2012 14:53
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So what are you saying? If you have vaulted ceilings, you should NOT go to a metal roof??? We have a long ranch that has approx. 85% vaulted ceilings. The house was built in the early 1970's and only has 2"x6" joists. We have had to replace our asphalt roof every 11 years because it deteriorates so rapidly. We need a new roof again and we were thinking metal. I would truly appreciate some good advice. Thanks, Rosalie
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ShabinNo5
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# Posted: 4 Nov 2012 20:04
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Rosalie,
I am not sure where you are located, but my guess is that your house is in an area without much if any shade and high temps in the summer. This combination will tend to generate a very hot roof surface. This is likely the reason that your asphalt roof deteriorates so fast. I know from experiance that a hot asphalt roof becomes very soft. On the other hand a metal roof may get just as hot, but it will not get soft. So I would expect a metal roof to have a much longer life.
As for the venting issue. If the roof is properly vented it would not get has hot, allowing the asphalt roof to last longer. Additionally this would keep the inside of the building cooler. For locations with snow, a vented roof will keep the roof cool and help prevent ice dams.
With 2 X 6 rafters, to add venting will require a lot of work. Opening ceilings; replacing existing insulation; adding a ridge vent at the peak of the roof and soffit vents.
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TheCabinCalls
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# Posted: 5 Nov 2012 08:24
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You don't need to vent your roof if you use a thick enough layer of spray foam. It is now code to do so.
Also you can absolutely do metal. In fact metal does better then shingle in this application.
Major reasons for venting:
- try to keep the roof as cool as possible in summer
- try to eliminate moisture buildup in attic/space
- try to allow the heat to escape so it doesn't radiate into house
- to keep the attic space the same temp as outside temp
I will track down the links/research I found.
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Rosalie
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# Posted: 6 Nov 2012 07:35
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Hello ShabinNo5 and TheCabinCalls ,
I forgot to tell you that our roof has a 15 degree pitch. We are located on the Mass/New Hampshire border.
You are correct in saying the roof has too much sun; we have no trees in the front to shade the house in the heat of the day. This past summer made the roof much worse.
I can't thank you enough for your help; as we have been in this dilemma since midsummer trying to research the correct way to deal with the problem.
Rosalie
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TheCabinCalls
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# Posted: 6 Nov 2012 07:59
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If you can afford spray foam or a layer of rigid foam on top do that you won't be sorry.
We have a 13/12 pitch roof and we opted for spray foam. The long part of the house is south facing. In the summer with no A/C the temp upstairs was the same as downstairs. No noticeable difference. I think this is largely due to having spray foam instead of batts.
We have batts and blown insulation in the city and there are hot/cool spots all over the house. I can take my temp gun and shoot it across the ceiling and swing 15 degrees. Foam will tighten the cabin and make for a more even temp.
If you go with batts make sure you vent with the air baffles they make and make sure you have a solid vapour barrier. Use 6m plastic or the craft paper (tape cuts/tears).
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ShabinNo5
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# Posted: 6 Nov 2012 08:10
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Rosalie,
TheCabinCalls is right on with the major reasons. As for the 15 degree pitch, this is the same as the pitch on our building and for us a metal roof was preferred to allow snow to slide off (northern Minnesota). Also that shallow of a pitch is at the low end for using asphalt shingles.
If the only issue that you are trying to solve is to extend the life-span of the roofing material, then replacing with a metal roof is the way to go.
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TheCabinCalls
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# Posted: 6 Nov 2012 08:31
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Yes, metal is more reflective and will last much longer under your conditions.
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Rosalie
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# Posted: 6 Nov 2012 20:35
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Thank you for your feedback on the roof. We can't afford spray foam but we will install a good vapour barrier. I'm pleased to know that metal is the right direction, because the thought of putting on another roof again in 11 years was not only scary but too costly.
Thank you again, Rosalie
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Jacob Hartz
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# Posted: 18 Mar 2013 23:15
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When you say "vaulted ceiling" I assume you are referring to the roof which will have little or no attic space.
I have trouble wading through all the misleading reasoning in these posts.
First thing to consider is your LOCATION. Houses in southern Mexico are not built the same as in northern Canada.
One comment says "You don't vent your walls."
A wall is totally different thing: snow will not stay on the face of the wall, but it certainly will sit on a roof, and there's a world of difference. If your roof is allowed to get warm during the winter months, the snow will melt and the water will run as far as it can, until it reaches an area where the roof is cold and freezing. This will typically produce icicles at the eaves.
When the water runs, it will find its way into every small crevice it finds. When it freezes it expands. That expansion has considerable force and will lift flashings, break joints and ultimately cause the roof to leak. Over a period of a few years and a couple dozen cycles of melt freeze melt freeze, a roof can be seriously compromised.
So, in northern climes, a warm roof is a bad thing. Icicles hanging down from the eaves are indicators of a warm roof and trouble.
So the answer is a well-vented roof. During the winter any warmth from the house below is carried away and the roof surface is kept cold.
Of course you need a good vapor barrier to prevent moist air inside the house from finding its way to any place where it can freeze.
I would never want to be involved in an unvented roof.
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Paul
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# Posted: 19 Mar 2013 01:29 - Edited by: Paul
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ventilation is important! others have covered why for cold temps but it is just if not more important for hot climates. Your attic space gets very hot in the summer, this bakes the wood. if you have plywood it will heat up enough to breakdown the layers of glue that give plywood their strength. one by tends to lose its moisture content and get very brittle. I have seen 100+ year old houses with no ventilation but perfect wood. I have also seen new house (5 yr oldish)with ventilation that had heat damaged wood. Other things play a factor in this like shade and how extreme the weather is from one year to another. excessive heat will make your shingles blister and that exposes the fiberglass mat. google heat blistering to see pictures of this.
Quoting: Anonymous So what are you saying? If you have vaulted ceilings, you should NOT go to a metal roof??? We have a long ranch that has approx. 85% vaulted ceilings. The house was built in the early 1970's and only has 2"x6" joists. We have had to replace our asphalt roof every 11 years because it deteriorates so rapidly. We need a new roof again and we were thinking metal. I would truly appreciate some good advice. Thanks, Rosalie
Im sorry to say I think you might have a problem with poor workmanship, materials or a scammer. could you post some pics or give more details as to what makes your roof need replacing(does it leak,shingles missing,etc) You left out some important information(pitch,shade,location) but you are not getting even close to the full life out of your roof either way. Shingles have a prorated warranty of at least 20 up to 50 years. Now the minimum warranty is 30 years but 11 years ago it was twenty years. Assuming your problem isnt from improper installation, you might be able to recoup some of your expense for materials if you know who the manufacturer is. I'm willing to bet you have a 20 year organic asphalt 3 tab shingle on your house now. Here is my advice as a roofer without seeing your house lol
-up grade to architectural shingles they are the most cost effective roofing product for a typical house. go to the lumber yard and actually pick up and compare the different brands of shingles. look for the certainteed brand(landmark series) pick up, hold and compare the thickness of a certainteed architectural to every other brand you can find and you will see they are much thicker and better made. Certainteed also has a 130 mph wind warranty when you 6 nail them and nobody has anything close to this. Dont buy a 40 or 50 year shingle, it doesnt last any longer than a good 30 year shingle(landmark series) Also,energy star rated shingles HAD tax credits you could claim. I dont know if the fed govt is offering that this year with the sequester going on. Im not up to par with all the latest roofing news for 2013, my season hasnt started yet.
-you should add ridgevent even over the vaulted part. Your house is old enough the insulation in the vaulted part has settled slightly due to gravity so you should have a small air gap there already(depending on the type of insulation you have). use lomanco brand it is the best on the market. http://www.youtube.com/watch?v=vgMQnGOXOpA
-Before you think about putting a metal roof on, how long do you plan to own the house? you will need to live in it another 30 years to even come close to getting the value out of that metal roof. Metal can be anywhere from 3-5 times as much a a typical shingle roof. if you are moving soon you likely wont get that money back out of it or worse it might overvalue your house for the neighborhood. if you do go with metal you should still use ridgevent, all of the big makers off a ridgevent option i think
ps. I dont work or receive compensation from any of the products I mentioned that is just what I have found to work best on the 1000plus houses I have put roofs on.
Quoting: ShabinNo5 With 2 X 6 rafters, to add venting will require a lot of work. Opening ceilings; replacing existing insulation; adding a ridge vent at the peak of the roof and soffit vents.
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ICC
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# Posted: 19 Mar 2013 10:55
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interesting to see the varying opinions of so many experts. One group of experts I have a lot of faith in are at a site called buildingscience.com. Lots of info on best practises for different climates zones when it comes to roofs climate has a big influenece. here's a link to a great article on Understanding Attic Ventilation the conclusion of the research is
Summary
Both vented and unvented attic/roof designs can be used in all hygro-thermal regions. However, the designs need to be climate sensitive.
Control of ice dams, moisture accumulation and heat gain can be successfully addressed by both vented and unvented attic or roof design.
The choice of the venting approach is up to the designer.
Vented attic/roof designs have the advantage of a long, proven historical track-record. However, they work best with airtight ceiling/attic interfaces and where ductwork and air handlers are not located within attic spaces. The increase in the use of complex roof shapes and cathedral ceilings has resulted in problems with vented roofs.
Unvented attic/roof designs have the advantage of providing conditioned spaces for ductwork and air handlers. However, they require different approaches in different climate locations.
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Purplerules
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# Posted: 20 Mar 2013 09:36 - Edited by: Purplerules
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This is all so confusing to me. So I am just going to through this out there and hope someone has an answer.
We have a 16x24 interior with the gable roof (no attic). Our roof is asphalt shingles with ridge vents.
And we have insulated with batts not the spray foam. So can we just insulate over the ridge vents or do we need to leave a space for air flow? How to leave air space will be difficult since there is not a attic. The plans for the ceiling is the corregated metal. Our cabin is in the south of Texas were it is hot most of the year. But we do get below freezing a couple of times to a week a year.
Any ideas on how to proceed?
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toyota_mdt_tech
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# Posted: 20 Mar 2013 18:10
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Purplerules, I see your ceiling and insulation and I can still see raters hanging lower. Can you remove the insulation that is tucked up in between the rafters and move it down flush with the rafters on the bottom??? This would give you air space above the insulation. Then vent the area between the rafter tails (vented bird blocks) and again at the top in the form of a ridge vent. This will get you your insulation and still allow air flow to keep cooler and keep condensation out.
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bldginsp
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# Posted: 21 Mar 2013 21:12 - Edited by: bldginsp
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I'm going to attempt to post a PDF drawing of my vaulted ceiling section details to show how I've been taught to do it. Hope it works....
Whether or not your ceiling will have problems depends on a lot of factors. I've seen ceilings that should have had moisture problems that didn't, and those with seemingly lower probability that did. Because it's so hard to predict, best thing is to do as much as you can.
The simple answer is- always get air flow over the top of your insulation layer, as I did on the attached PDF (if it works...)
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Purplerules
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# Posted: 4 Apr 2013 18:51
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Thanks everyone for your responses. I think we have figured out that we should vent it some. Still on working on the how.
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Purplerules
Member
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# Posted: 22 Apr 2013 17:37 - Edited by: Purplerules
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Here is a pic of what we we finally did to vent roof.
Since we are only dealing with heat mostly and not snow.
We used the foam vent slots and the insulation still fit nicely between the rafters.
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Malamute
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# Posted: 22 Apr 2013 22:29 - Edited by: Malamute
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There is also "C" (for cathedral I believe) type batts, its rated the same R factor as standard batts, but is slightly compressed to give about 3/4"-1" of air space to vent. I've used them on cathedral ceilings, they seem to work fine.
After having dealt with a number of houses that leaked air into the living space when it's windy, I've quit using faced insulation. Its nearly impossible to keep the paper totally intact and keep from leaking air. I use unfaced, and cover with heavy mil plastic, taping the edges, and making any cuts tight to the electric boxes, windows etc. Insulators have told me they do the same thing, have mostly stopped using faced batts. I also am careful to get the walls sealed well, and edges taped to the ceiling and any penetrations. a realotr that has sold a couple of my hosues told me they were tighter than most other houses she'd sold, she said she can generally tell when she walks in them if they're tight.
Using plastic over faced batts can trap moisture, and is to be avoided from what I've been told by insulators. One can pull the paper off and plastic over it though. any stapler holes need to be taped, and edges/seams taped. Same for tyvek, tape all holes, edges etc.
One other difference between walls and roofs, walls breathe some, roofs dont. Tyvek allows minute amounts of moisture out, roofing doesn't.
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ICC
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# Posted: 22 Apr 2013 23:16
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if you do use foam against the outer wall or roof surface and then have fiberglass or cellulose you must use enough depth of foam for the climate zone. that keeps the foam surface warm enough to prevent condensing. use google there are a number of references some on the green building advisor website. if rigid sheet foam it must be sealed perfectly to avoid water vapor getting behind it against the cold wall or roof. spray foam self seals and closed cell is the only one that makes sense to use. using foam against the outside you do not want any interior vapor barrier, not even vinyl wallpaper.
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NotoriousAPP
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# Posted: 11 Aug 2013 16:44
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I have a similar question. I can't believe there are so many different opinions regarding insulating and venting a cathedral/gambrel roof.
I'm having a cabin built and there are some options which I can't change, specifically the roof rafters which will be 24" on center 2x6 boards. I really need some help deciding what to do with the roof venting and insulation. It seems like most opinions are in the context of colder climates like northern US. I don't get the same feedback from any of the builders or insulation companies down here. I've even had a spray foam installer (who is very reputable) tell me that he wouldn't recommend using spray foam (closed or open) since I won't be using the cabin that much, he was concerned about creating a sealed building
Here is the situation:
-VERY IMPORTANT: I live in central texas where we get 105 degree summer days and some winter nights in the mid teens. Most of the year it's warm here and I would be running the AC (window unit) in the cabin. It does not rain here often.
-VERY IMPORTANT: This is a weekend retreat cabin. I will only occupy it for 0-6 days per month in both the summer and winter. I don't plan on running any air conditioners or heaters while no one is in the unit.
-The roof will be the ceiling for the building and vice versa.
-The cabin is 16x40' with a gambrel roof which comes with a ridge vent installed but sealed off with black plastic (bridging the two sides of the roof on top of the OSB decking; I can simply cut the plastic open if I want to use the vent.
-There are no soffit vents installed anywhere and there is very little room to do so; the best I could probably do is to drill several 3" holes in between each rafter if I decide to use a soffit-ridge vent option
-I will be using R13 fiberglass in the walls. (I'm not sure yet whether they will be faced or unfaced; the cabin is wrapped in Tyvek.
-There is no insulation or vapor barrier below the foundation. The foundation will be on pier and beam 24" off the ground. I do plan on adding skirting around the building.
-The building does not require an inspection so I don't need to follow code but would like to get as close as possible.
-I don't want to say that money is not a concern, because it is, but I don't want to do something because it's cheap now and pay for it later.
Here are some scenarios which I've compiled from the 15 different opinions I received. My goal was primarily to make sure the cabin had no condensation or other moisture problems and secondly to provide adequate insulation considering the low number of days I'll be using the cabin. Each of the scenarios of from the interior of the building to the exterior.
Scenario 1: with soffet and ridge vent
Tongue and Groove Paneling
Unfaced R13 Fiberglass insulation
1" Foam board
1" air gap between foam board and roof decking (I would install 1" pieces of wood to provide stand off between the roof decking and the foam board)
Roof decking
30# felt paper
Corrugated metal roof
-I would add no additional ceiling vents.
Scenario 2: with no soffet and or ridge vent
Tongue and Groove Paneling
Unfaced R13 Fiberglass insulation
2" closed cell spray foam
Roof decking
30# felt paper
Corrugated metal roof
-Add 4 ceiling vents which would allow moisture to escape through the roof. They would always be left open when the cabin is vacant and likely open most of the time when the cabin is occupied
Any thoughts. Let's see if we can get more than one person to agree on a common method.
Thanks.
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PatrickH
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# Posted: 12 Aug 2013 19:40
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In our city of minnesota they do not allow spray foam claiming it creates a "hot Deck" and shotens the life of the roof we have stacked rigid foam in tight spaces leaving an air gap and foaming the edges to make it air tight seems to work well. notorious I would do scenario 1 on my own stuff.
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hct4all
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# Posted: 12 Aug 2013 23:07 - Edited by: hct4all
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Can you add venting after. I can get to my flat section at the top and where the soffit or facia vent should be. I was thinking about drilling a lot of holes in PVC pipe and putting it through every other section. Thoughts?
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MtnDon
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# Posted: 12 Aug 2013 23:56
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Sounds like that is the typical roof found on the variety of pre-built shed type structures we all see all over the place. That type of roof, with no overhang, is a problem to vent and to adequately insulate. The problems usually begin with the fact that the framing is often 2x4. Too small to get vent space and insulation.
If the roofing material has not yet been applied, IMO, the best bet is to do without any venting in the roof assembly. Have the rafters sheathed with OSB sheathing on the exterior. This supplies the rigidity for the roof framing. Then apply rigid sheet foam on top of that in two layers or more, seams staggered and taped. Then another layer of OSB sheathing with long screws through to the rafters. Weather resistant barrier on that and then the roofing material. Metal or shingles, your choice. In TX I'd use as much thickness of foam as I could afford with the idea to trying to keep the interior as cool as possible. Temper that with the degree of planned use.
Look at buildingscience.com for the best information you will see on insulation and venting. Lstiburek has spent decades working on climate and how to build the best structure for the climate.
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Quoting: PatrickH In our city of minnesota they do not allow spray foam claiming it creates a "hot Deck"
I would like to see the citation on that. My reason is twofold; (1) I live in the desert SW where we know what "Hot" means. Spray foam is used here when it is the best thing for the application. (2) Over the years I have heard a variety of things that are forced upon someone someplace or not permitted, and often it is a partial truth... in some case, for some reason, the item in question may be true. But often has been misinterpreted and passed along.
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Quoting: hct4all Can you add venting after.
What kind of a roof? Where? Retrofitting can be done but what to do varies with the structure that already exists, as well as the climate zone.
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