Hi. I'm in the process of getting bids to tear-off and replace the 3-tab roof with architectural shingles (Certainteed Landmark Pro). The roof pitch is 4/12. There are currently no vented soffits or ridge vents.

Right now, i'm having trouble deciding on which intake venting solution I want to go with. My options are either Certainteed Intake Vent or Certainteed Invisivent 3 1/3" soffit vents.

My soffits are only 4" deep and I'm not sure if I'll get enough intake air through the Invisivent to properly balance the airflow in the attic. It would probably be a no brainer if my soffits where a lot bigger than 4". I know it's recommended to have more air coming in than going out through the ridge vents.

I may also consider putting in both the Intake Vent and Invisivent. Going this route should provide me with more intake, even in non-optimal conditions, than whats exiting at the ridge vent.

Any advice is greatly appreciated.



Intake Vent - 9 sq inches of venting per 1 linear ft
Invisivent - 10 sq inches of venting per 1 sq ft
RidgeVent 12" - 18 sq inches of venting per 1 linear ft

1/150

http://www.certainteed.com/resources...akeVentTDS.pdf
http://www.certainteed.com/resources/220.pdf
http://www.certainteed.com/resources/802.pdf

 

Hi bing and welcome to the forum.
A continuous 3" or 4" vent on front and back should be fine. Have you done the calculations for the necessary high and low NFA?

I much prefer soffit venting to edge venting as the area, even with limited soffits, is better protected.

In addition to adding the vents you will want to have baffles in each rafter bay with maximum insulation below those baffles. I like the units that also drop down over the end of the insulation to keep the incoming air away from that insulation.

As a note, the air coming in will always equal the air going out. The reasoning for more low intake area than high exhaust area is a bit exaggerated. As a wild guess, think in terms of saving $2 a year, doesn't justify much effort. I can run the numbers for you, but it is really a low priority. You are located in the middle so to speak, but the farther south you go they need more high venting than low, yet the guidance remains the same.

I'm not sure how difficult it would be to replace those small soffit panels with vented panels but worth checking out. If too difficult or expensive then one of the others should do. But way up north here I've never run into them.

Bud

 

Hi Bud,

Thanks for all your feedback in the forums. I think everyone appreciates the knowledge you dole out.

My house is a split level and the two separate attics are about the same size. I'm just going to go over the calculations for one of them since it's applicable to the other attic.

Roofing job is about $8100 for 22 sq, Certainteed Landmark Pro with 5 star warranty. Soffit work is $1500 and Intake Vent is $1300

High and Low NFA calculations

How much ventilation do i need: http://www.certainteed.com/resources/802.pdf

1) Determine total area of attic (sq. ft.) to be
ventilated.
28' x 19' = 532 sq ft

2) Total free area of ventilation required for attic
532 sq ft / 150 = 3.6 sq ft

3) Convert square feet into square inches (sq in)
3.6 sq ft x 144 = 519 sq in

4) Location of vents
50% at ridge = 519 x .50 = 260 sq in
50% at soffit / eave = 519 x .50 = 260 sq in

5) Total soffit ventilation area required.
Area of soffit available for ventilation
39 lineal ft x 4" soffit depth = 13 sq ft
39 x 12" x 4" / 144"

Ventilation area required per sq ft of soffit = 260 in/13 sq ft = 20 sq in/sq ft

So if my calculations are correct, for an attic that's 28 x 19 with 4" soffits, i'll need 20 sq in/sq ft of intake venting for proper ventilation.

Invisivent will only provide 50% of that number under optimal conditions.

Ridge venting is fine since i'm going with 12" ridge vents that has 18 sq inches of venting per 1 linear ft.

I've already notified my roofer to go ahead and schedule the soffit work first that way we can verify if there's enough space to make a continuous soffit .

Do you think 50% of the required intake is enough and that's with the Invisivent soffits under optimal conditions. Ideally I would like a balanced system and afraid that the ridge vent will try and pull air from the conditioned space.

 

Not ignoring you, just super busy. Did some reading at 2 am, helps me fall back asleep, and here are some thoughts.

1. Your basic math is fine.
2. That style vent does not have very good numbers, 7% NFA (10/144 = 7%). IMO, vinyl soffits leak that much air even without all of the holes.
3. Natural attic ventilation is always pulling air from your conditioned space when the attic is warmer than the outside (in summer it reverses a bit). Having a balance between upper and lower vent areas does not change that. When upper and lower are not equal the pressures in the attic (plus at top and negative at the bottom) adjust to ensure the infiltration matches the exfiltration. Less lower ventilation would result in a slightly more negative pressure at the attic floor. But the difference is very very small.
4. There are other options but it sounds too late to rethink that. The Invisivent will work with their designed NFA, plus the loose fit of the vinyl.

Bud

 

2. Are you referring to the invisivent?
4. What other options are you referring to.

Soffits and roofing work won't start for another 10-14 days.

 

Yes, the invisivent?
Here are a couple of options if they can be fitted into your work.
http://www.cor-a-vent.com/uploaded_f...?1429120749012
Air Vent Vented Drip Edge Vent Carton of 10 10ft. Pieces from BuyMBS.com
SmartVent by DCI - The #1 Choice for Attic Intake Ventilation

Bud

 

To get the recommended 20sq in / sq ft, I was going to use invisivent at the soffits and edge vent from certainteed, similar to dci smartvent but made by air vent.com. Under optimal conditions I'll get about 19 sq in / sq ft. Realistically I'm thinking that I'll get 15 sq in /sq ft or so...especially if they get clogged down the road.

 

I find it confusing to jump back and forth between in² per sq ft and per linear foot.
The Certainteed under shingle edge vent shows 9 in² per linear foot which doesn't easily convert for a square foot comparison. At 39 linear feet it would give you 350 in². If all you need is 260, doesn't that make it without the soffit vents?

I'm old and busy so corrections are welcome.

Bud

 

Bud, I think you're right. ventilation should be fine if i just went with the edge vent. i plugged the numbers in Ventilation Calculator Download - Air Vent, Inc. and for an attic that's 532sq ft, i'll need at least 15 ft of ridge vent and 29 ft of the Edge Vent. Attic dimensions are about 28 x 19(eaves/soffits run).

I'm probably going to have them install the soffits but maybe not make it continuous and the edge vent.
The edge vent and ridge vent should more than be enough to vent the attic using the 1/150 ratio and be balanced at the same time. The soffit work will just be insurance to have more intake than exhaust.

 

That sounds good. Be sure to review air sealing house to attic, insulation, and baffles. You may have these already covered just reviewing.

Bud