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spencerin
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# Posted: 14 Nov 2019 11:25am - Edited by: spencerin
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What do you think about ventilation with this roofing system (superior, adequate, or inferior)?
Here's a cross section of the roof, from the outside in -
1. Black g-rib metal roof with a ridge vent and closure strips in the ridge only (none in the eaves)
2. Single-sided radiant bubble-wrap barrier with the reflective side facing up/out
3. Horizontal 2" x 4" furring strips (to attach the roof to the rafters)
4. 2" x 6" rafters
5. Pine t&g ceiling (vaulted ceiling, no attic)
6. R-26 Kraft-faced fiberglass batt insulation between the rafters
7. Climate zone 3
It appears to me to be sufficient to allow for adequate moisture control. Are there superior systems? Yes. But, I don't see any issues with this.
Your thoughts?
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sparky30_06
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# Posted: 14 Nov 2019 11:33am
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Add a moisture barrier between your fiberglass and your T&G.
Place the radiant bubble wrap between the rafters and the furring strips
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spencerin
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# Posted: 14 Nov 2019 11:52am
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I would disagree on both points -
1. The paper face of the insulation acts as a vapor barrier. Everything I've read says putting another vapor barrier over it is either not necessary or possibly even detrimental.
2. Putting the bubble-wrap barrier between the furring strips and the rafters exposes the furring strips to more moisture (and insects). I would think that would increase the likelihood of, or accelerate, rot.
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sparky30_06
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# Posted: 14 Nov 2019 01:53pm
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1: craft paper alone isn't enough on the ceiling, you will have major air leakage. I made this mistake.
2: creating a larger air space to dissipate the solar gain is beneficial when using foil bubble. Placing it directly against the metal tin is useless. Your furring strips are not exposed to the elements, you have a roof over them. You will have air movement so moisture isn't an issue.
https://youtu.be/H1Zmo7D6Qqs
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95XL883
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# Posted: 14 Nov 2019 02:28pm
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If you will pardon somebody with limited experience jumping in, how will there be any airflow by the furring strips if they are horizontal? To get airflow, don’t they have to go from eave to ridge?
Why no roof decking? Don’t you need that to “tighten†the roof?
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Brettny
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# Posted: 14 Nov 2019 02:51pm - Edited by: Brettny
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You really will have no ventilation for that roof with only closure strips missing. A 1x1in square every 7-8in prob ist going to be enough. Neither is r23 in zone 3. Have you looked up what the code is for attic insulation in your area/zone and have you checked into what the manufacture specs for eve vent size for your ridge vent?
Really the best way to insulate under a metal roof is to spray foam or use the wrap and strap method.
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sparky30_06
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# Posted: 14 Nov 2019 04:48pm
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Spray foam is the BOMB!!!
You need to have airflow from the bottom to the top. You can't seal the ridge off, you need ridge vents and soffit vents
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Steve_S
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# Posted: 14 Nov 2019 05:12pm
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 This allows reasonable airflow with convection, while providing a thermal break from both heat and cold allowing the actual roof to perform better. Additionally, it will allow any condensation a drain path preventing any wet spots.
The barrier between the roof "deck" and your metal is essential.
I used a variations of this on my home in Canada, it works amazingly well and have absolutely no regrets.
For more info see here:
http://www.houstoncoolmetalroofs.com/cool-roof-information/cool-roof-design-texas/
There's also video's and related articles for different climes.
Hope that Helps,
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ICC
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# Posted: 14 Nov 2019 06:35pm - Edited by: ICC
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So you basically want a cathedral type ceiling. They may be attractive on the room side, but a good cathedral ceiling that has sufficient insulation and is not a moisture trap is hard to achieve. Over the years we tore apart a few that were quite moldy in places. I believe the best ones are done in the manner of the "houston roofing" design and IF one must have a cathedral look then use spray foam on the underside thick enough to meet the climate zone requirements. OR build the roof deck with foam sheet panels and the "cool" metal, air channeled design on top.
Or simply use a thick layer of sparay foam under a good roof deck of OSB, plywood or ZIP panels and put metal panels directly on the deck if the work needed to do a cool roof is too much.
Forget the bubble wrap even if it is reflective.
Use roof sheathing panels. You may think not using panels saves money but not sheathing also weakens the roof assembly. The sheathing stiffens the roof, makes it more rack resistant. You can buy sheathing panels with a foil surface on one side. Face it up.
Horizontal battens, purlins or whatever you call them are counterproductive. You do need an air space both for the air flow that helps cool the roof as well as for the reflective surface to work. No air space = no reflection of heat.
Or build a "cool" roof design using scissors type trusses on raised heels. You have space for ample conductive insulation and the "cool" design uses the reflective panels for the advantages in hot weather.
Good roofing/insulation designs have been done by engineers. Why try to invent the roof all over again unless you have experience or education in that field?
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spencerin
Member
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# Posted: 14 Nov 2019 08:54pm
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Brettny, my thinking was that all the undulations in g-rib siding would allow adequate airflow from eave to ridge.
sparky30_06, the furring strips aren't directly exposed to the elements, no, but should any moisture get underneath the metal roof, whether from a leak or an accumulation of condensation, it would run down and pool up against, or get wicked up by, the wood.
Regardless, I like the feedback.
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KinAlberta
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# Posted: 14 Nov 2019 10:54pm - Edited by: KinAlberta
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The undulations are relatively few so I can’t see there being a lot of upward air movement. I figure the air would have to dump out the sides to create any substantial mount of clean airflow.
The standard recommendations I’ve read about regarding using furring strips have never mentioned upward air flow. They seem to be all about just creating an air space.
Moreover, I’ve read many many posts where people have long installed metal over asphalt shingles without problems. I suspect that the course texture and tiered effect of overlaying shingles creates a fair bit of air space but with many thin horizontal contact points along the high point of each course of shingles (bottom edge of each shingle).
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Brettny
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# Posted: 15 Nov 2019 07:46am
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I dont believe the ridges in the metal roofing is enough air flow. Also i believe tou can do no ventilation if you spray foam directly to the bottom of the roofing. No air gap means no moisture to condense on the bottom of the metal roof.
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creeky
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# Posted: 15 Nov 2019 09:30am
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The Texas cool roof is great. Just make sure you put squirrel / bat /insect blockers in that still breathe.
Radiant bubble wrap is a waste of money. It was explained to me as being designed for outer space. On planet earth there's something behind the barrier so the radiant energy can be absorbed by the material behind the barrier.
That's one reason foil covered polyiso does work. The absorbing material behind is insulation. But bubble wrap. No such luck.
I would use the texas cool roof. I've built 3 so far. And they all perform admirably. Really noticeably in my washroom. A building with metal roof that faces directly south. The air flow keeps that metal roof from overheating the building in the summer. Excellent!
Good luck. K
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Steve_S
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# Posted: 15 Nov 2019 09:55am
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As Creeky mentioned, Metal Mesh is important (chipmunks & squirrels find a way if you don't) and the "Foam Enclosure Strips" which do allow air to move through is also essential.
Most Metal Roof companies void warranty if anything is sprayed onto the metal / tin. The only exception seems to be products like "CondenStop" which prevents surface condensation and quietens the metal (less sound from rain piter pater).
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gcrank1
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# Posted: 8 Feb 2021 11:14am - Edited by: gcrank1
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I built an 8x8'ish passive, convection solar heat collector decades ago that I think may have been similar to your roofing idea.
A box frame with a plywood back
1"exp insul board in box
A reflective panel on the insul
A flat black painted piece of corrugated steel roofing
Patio storm door glazing.
Inlet cool air back side of the box / hot air outlet at the top side back. I pulled the cool in from the basement and hot went into the living room at floor level, both through insulated flex tubing.
I definitely good some good warm air on any sunny day so there was air flow, can only imagine how much it would have been on the length run of a roof.
More to your point, the back of the box never got hot.
But I was trying to capture the heat, not free it. Thinking on your notion I would want as much free flow (Im sure there are calculators for such) beneath that tin as possible to max the cooling flow, not slow it; get as much of the hot air out from beneath that roofing as quick as possible = GOOD (slower that air moves the more btu's it has time to accumulate and pass on to the structure beneath).
I Really Like the Steve_S pic! It tics ALL the boxes for me.
A wise man once told me, "Rather than attempt to re-invent the wheel use what has worked in the real world for others".
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ICC
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# Posted: 8 Feb 2021 03:19pm
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the post from keshawalsh appears to be spam.
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gcrank1
Member
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# Posted: 8 Feb 2021 03:51pm
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Yeah, but it is pretty slick for a come on.
Sure would be cool if somehow we could gen a backflow to let the magic smoke outta their computers.
POOF
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95XL883
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# Posted: 9 Feb 2021 06:16pm
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Hey George,
Re a “backflow†on spammers, it can be done but it takes technical knowledge way beyond my ability. My youngest is a computer engineer. Just as Covid was hitting he built a powerful pc and was working from home. He is usually pretty easy going but I could tell something was bothering him. When I asked he explained he was getting spammed. He wrote a program specific to that spammer and let it rip. As he monitored it he started laughing saying that guy has to hate him. He said the program was overwhelming the spammer. Not smoke coming out but it solved his problem with that one.
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95XL883
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# Posted: 9 Feb 2021 07:31pm
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I’m officially old. I can’t even remember accurately what happened less than a year ago. The spammer was calling my son. Apparently there are tools Ian iPhone that can be used to spam someone. So my son spammed the phone spammer. The downside to the tool is the first spammer then knows your phone number and will most like retaliate. You then have to add a lot of phone numbers to your blocked numbers list but it is fun to drive a spammer nuts.
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