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Rifraf
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# Posted: 22 Feb 2012 08:22am - Edited by: Rifraf
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I'm considering this option.. I was wondering if simply drilling 1 inch holes at my roof crest from the inside out would be good enough ? I will also have to drill holes from under the roof overhang into the roof interior
If so how far apart should they be ? a few feet ?
A concern of mine regarding this though.
My shed was made with 2x4s.. even the ceiling trusses are all 2x4s, and I dont really have much space in my lofts to shim them out.. so would one inch air gap and two inches of rigid foam be a good enough insulator with enough airflow ?
Do they make any roof mounted fans to pull air through this system.. I was thinking a small solar unit could probably run on/off again and be sufficient.
And one other question.. does anyone you know of a kit to easily add the mini roof over the ridge holes ? I thought some time ago I
saw a stick on system that had anchor points every six feet or so.
Thanks for any insight you may have on this.
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Just
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# Posted: 22 Feb 2012 09:01am
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yes there are lots of different ridge vents mine is called ridgavent..that and some eve vents and some 2in.foam in each space and you should not need a fan. it will be a marked improvement over what you have now.
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PA_Bound
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# Posted: 22 Feb 2012 09:15am
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Rifraf... If I can ask, why don't you simply put a ridge vent on your cabin? I ordered my shed/cabin with a ridge vent pre-installed, but I've installed them previously and they are not difficult. You still have to drill the holes in the eves to alllow the air to flow into the roof (which I have to do also), but that would allow heated air to exit easily- likely without even the need for a fan.
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Rifraf
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# Posted: 22 Feb 2012 09:30am - Edited by: Rifraf
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PA_Bound,
well thats exactly what im trying to determine if my skill level will permit me to do  Sounds like it will be pretty easy I just watched a youtube video of the roll on ridge cap .. looks very easy but they sawed an opening the entire length of the roof almost. I was hoping to just pop holes up through it every X inches , do you think that would be good enough ? I was also going to do the same for the eves .
How do you feel about my air gap and insulation amount. ( 2 inches ridig foam and 1 inche air gap )
@ Just , thanks for your imput here, im starting to fee like I can make progress with this method. The cost of blown in cc foam was depressing me.
Thanks for the replies guys.
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PA_Bound
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# Posted: 22 Feb 2012 10:06am
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I would not let the cut along the entire roof scare you off- as I think that is one of the easier steps. After stripping the roof down to the sheathing, simply draw or snap a chalk line down most of the length of the roof and run a circular saw along it. It doesn't have to be pefectly straight (just close), and make sure you set saw depth for just slightly more than the sheathing width.
Regarding the air gap and insulation amount, I'm not experienced enough or qualified to answer this. It is my opinion that I would prefer a larger air gap, but if that's all you have... well that's all you have. Unlike you who, if I remember correctly, are planning to use your cabin full-time, I'm building my shed/cabin for 3-season use. But I am going to insulate it- to the degree ptactical. My roof has the same 2x4 trusses as yours. But instead of insulting between the trusses, I'm going to insulate with 4x8 sheets of rigid foam in front of them, then install my finished ceiling in front of the foam. That will leave the full 4" (well, 3.5" actually) air gap open for ventilation. But I was thinking of only using 1" foam (3-season, remember), so I'm not sure how well that would work with 2". But even I haven't yet finalized what width of foam I will install.
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Rifraf
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# Posted: 22 Feb 2012 03:40pm - Edited by: Rifraf
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PA_Bound,
Thanks, if I were not planning on using my lofts for the kids I would adopt that method. Do you think it would be best to use the foam with the reflective layer on it and have that facing the roof? And do you plan on adding any moisture barrier to the underside facing the living space before the final sheet rock (or other finish) is applied ?
EDIT POST :
I just read here on post #4
http://www.diychatroom.com/f9/drilling-holes-vent-soffits-94025/
That the ridge vent air flow should be equal to the soffit air intake. So by this I take it I could drill the same amount of holes in the ridge as in the soffits and should be ok with that technique or did I misunderstand.. So for example if I drill a 1 inch hole every 1 foot in the soffit area on both sides of the roof I could get away with a 1 inch hole every six inches in the ridge to get the same air intake ?
im sorry for being so ill informed, and asking so many strange questions but im much more confortable drilling holes then ripping a large portion of the roof off , if I can get away with it that is.
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Icebear
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# Posted: 22 Feb 2012 05:58pm
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Down in this part of the world the code says 40mm air gap above the foam in the roof cavity. I've never understood why they have specified that much gap - but it is what we do down here.
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Rifraf
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# Posted: 22 Feb 2012 06:11pm
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well, I could actually have about 1.5 inches of air space and 2 inches of rigid foam which is much closer to that number. I think that's all I can squeeze in there unless I remove the functional purpose of the lofts.
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Just
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# Posted: 22 Feb 2012 06:37pm
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you will want to protect the holes in the soffet from pest invasion.
they make round 2in.push in screens for that purpose.i would get a 2in bi-metal hole saw to cut both top and bottom holes.the ridge vent will have screen in the vent .
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Icebear
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# Posted: 22 Feb 2012 06:52pm
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I have 2" of foam inside a dressed 4x2 and its works nicely.
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Rifraf
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# Posted: 23 Feb 2012 10:18am - Edited by: Rifraf
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Will it be of any benefit to me to have a foil covered foam facing the roof or should I not bother with it.. just use normal 2" foam without the radiant foil.
and on the interior side, is there any need or reason to put up a moisture barrier before I apply the finishing wood planks ? like tar paper or something over the trusses&foam
@Just , is this the ridge vent you are using? Other than screening for my soffit holes , what more do I need to purchase for this system.. it seems I need about 8 - 9 of the product linked below, does this require any additional products for the end caps ??
Lowes Product, clickable link
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Just
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# Posted: 23 Feb 2012 10:49am
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looks the same, mine is metal comes in 10 ft lengths you will need two end caps ,
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MtnDon
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# Posted: 24 Feb 2012 07:03pm - Edited by: MtnDon
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There is a formula for venting. 1 sq ft per 150 sq ft of floor space. That's the minimum recommended ratio. If there is a an effective vapor retarder that can be changed to 1 sq ft per 300 sq ft floor space. But 1 to 150 is still preferred in areas of high humidity.
That 1 sq ft is the effective area... screening and louvers restricts area so vents you buy have an effective vent area stamped on them. I don't know what the percentage reduction would be for holes you add screen to yourself.
Note; a 1 inch diameter hole has an area of about .75 sq inch.
A 1 1/8" hole has an area of about 1 sq inch.
144 sq inches = 1 sq ft.
Then that total required vent area needs to be divided between ridge and soffit; 50-50 or some say make the intake (soffit) a bit larger in area, maybe 55 to 60%
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Rifraf
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# Posted: 28 Mar 2012 05:18pm - Edited by: Rifraf
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wow, my head is starting to hurt
MtnDon, any chance you can help me out here.( or anyone else that may have a nack for these calculations)
My roof is 32 feet long, my cabin counting the porch that has a loft above it is about 384 sq ft (12x32)
How wide should I cut the ridge vent opening if i cut the lenght of the roof minus 6 inches per end??
1inch total, (half inch per side of roof) 2 inch total ?
I dont think it could be 2 inches for my total sqft could it ?
Then of coarse my next challenge would be determining how many 2 inch soffit holes i need to match that ridge opening
If im asking too much of you , you can tell me to go take a hike, I wont be offended at all. believe it or not I took a crack at the math but am still not sure if my results could be right.
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Just
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# Posted: 28 Mar 2012 07:23pm
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the way i figger it's a slot 1in wide and 32 ft long.let s seewhat don comes up with??? my math could be way off!!!
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MtnDon
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# Posted: 28 Mar 2012 07:38pm
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If we take that 384 sq ft and use the 1:150 ratio let's see what comes out.
384 / 150 = 2.56 sq ft total vent area. Round that to 2.6 sq ft.
2.6 / 2 = 1.3 sq ft vent area total for soffit and 1.3 sq ft total for ridge vent.
32 feet long. I don't recall if the studs are 16" or 24" on center. I'm going to say 16". The studs/rafters will block a small amount (1.5") of clear space where the vent strip is cut or drilled. Over a 32 ft length that is actually about 36 inches, 3 feet, total that is blocked. So with leaving 6 inches uncut or undrilled at each end the 32 feet effectively becomes 28 feet available vent length.
We want 1.3 sq ft of ridge vent. The length is 28 feet.
It might be easier to think and work in inches and square inches.
1.0 sq ft = 144 sq inches, so 1.3 x 144 = 187.2 sq inches
Call that 188 sq inches.
28 feet = 336 inches length
"w" is the width of the slot
length x width = square inches area
336 inches X "w" = 188 sq inches
or (using some algebra)
188 / 336 = "w" = 0.559 inches
Call that 0.6 or 6/10 of an inch; almost 5/8 of an inch
So a slot cut 5/8 of an inch wide along the ridge (except for 6 inches from each end) would be sufficient ridge venting. I like to round up at a few spots as that helps make allowance for venting that would be blocked by insect screening.
How does this translate into a number of holes of any particular size?
IMO, there's no point in struggling with formulas to get the area of a circle when there are online calculators available.
Note the calculator uses the radius, not the diameter of the hole
With some trial and error we can avoid some intricate calculations.
Using the online calc we can determine that...
1 inch dia = 0.5 inch radius = 0.785 sq inches area
1.5 inch dia = 0.75 radius = 1.766 sq inches; call that 1.75 sq inches
2 inch dia = 1 inch radius = 3.14 sq inches; call that 3 sq inches.
With a 2 inch diameter hole we would need
188 sq inches / 3 = 62.66 holes; call that 63 or so.
Divide that in two, some for each side
63 / 2 = 31.5; call that 32 two inch dia holes per side.
I hope that helps.
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Rifraf
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# Posted: 28 Mar 2012 08:29pm - Edited by: Rifraf
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It does help alot, thanks so much.. The explanation really helped as much as the answer.
5/8 opening seems tiny.. Do you think it would either helpful or hurtful to round up to an inch and adjust soffits accordingly ? I dont doubt you that is the proper width for my square footage, I just dont know how accurate I can be on such a small cut.. that would be 2.5/8ths on each side of the roofline im not sure how thick my blade is, but its probably 1/8th at least
Oh, all the framing including trusses are 24 on center.
This board is lucky to you here
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MtnDon
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# Posted: 28 Mar 2012 09:51pm
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Thanks. I thought it was best to show how the answer was arrived at, rather than just give a number. That's the way my math teachers always beat it into me too. That way you can recalculate if there are other variables.
The ridge vent area should be equal to or up to 10% smaller than the total soffit area; never more ridge vent than soffit. Keep the proportion about equal or at most 40% ridge, 60% soffits.
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Rifraf
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# Posted: 30 Mar 2012 02:01pm
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Thanks again ! You're a life saver.
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