Cape Cod Style house attic venting!

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Old 09-07-16, 02:53 PM
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Cape Cod Style house attic venting!

So I have old cape cod style house (1.5 story) with the kneewalls and slopes ceiling upstairs. I am doing insulating and air sealing project. I wanted to vent my attic better. Currently it only has 3 roof vents on one side of roof as exhaust (close to peak) and 2 small vents on the gables on the ends of the house as intake(Close to the bottom). I have no soffit venting due to no overhanging roof.
I wanted to use bigger gable vents as intake ( close to the bottom) and a ridge vent as exhaust . I would have to seal off the existing rectangle roof vents. Can I use these type of vents together? What do you all think? Thanks
 
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Old 09-07-16, 03:47 PM
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Welcome to the forums.

I'd be inclined to leave the roof vents for the moment but remove then at the next re-roof, since they are potential leak points.
 
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Old 09-07-16, 04:48 PM
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Ok. I will leave existing vents and add some to north side of roof. So it's probably ok to increase the size of the intake vents down at the gable ends?
 
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Old 09-07-16, 04:58 PM
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I assume that behind those kneewalls there are small (low) attic spaces. If so, how are they vented and how is the slope vented?

If no side attic spaces then the exterior walls must extend up that extra half story.

We will continue from there.

Bud
 
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Old 09-07-16, 05:15 PM
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Yes behind kneewalls there is vented attic spaces . They are vented by small vents down low used as intake vents( 2 on east end of house and 2 on west end of house) and a couple of rectangle vents up close to the peak used as exhaust vents. The slopes are vented by the air gap between insulation and roof sheathing .
 
  #6  
Old 09-07-16, 05:55 PM
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OK, that would be a normal cape. The suggested total vent area would be the footprint of the house below the roof, ie 1,000 ft. Divide that by 150 for (as an example) 6.7 ft. Half of that area would typically be high venting and half would be low venting. Convert to in you get 490 in. Divide that by a typical 18 in per linear foot of ridge vent and you would need 27' give or take.

For low venting, the 4 low vents would need to add up to the same 490 in of net free vent area. More if possible is good. Check the net free vent area of the vents you want to install, that information should be available. But each one should have about 125 in

Now, this is all hypothetical based upon a footprint of 1,000 ft. You can follow the same math for your footprint or I can help.

Bud
 
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Old 09-07-16, 10:24 PM
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Ok let's see here...... I have 2 attic spaces, 1 on each side of kneewalls. They measure approx 6ftx37ft which equals 222 sq ft. Each side . So divide 444 sq ft by 150 equals 2.96 sq ft. So that's 2.96 sq ft of vent area divided between the 2 sides which equals 1.48 sq ft of vent area on each side. Which then gets divided between lower and upper vents correct ?
That seems very little
 
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Old 09-08-16, 04:41 AM
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They usually refer to attic floor area because moisture passes through the drywall along with all of the leaks. But capes don't have a lot of attic floor yet are still subject to the same (or more) amount of moisture. The "more" refers to the slopes which actually provide increased area. To simplify the calculation I was using the footprint under the roof which would be the total of the upper attic, lower attics, and the area below the slopes, ignoring the slope.

Use the footprint of the house below the roof. Divide by the 150 (one ft vent area for every 150 ft of footprint). Then divide that, half high and half low.

Since you are using low vents located at each end of those side attics as opposed to the full length of the (missing) soffits you will not be getting the full benefit of having the vents directly below each rafter bay. But it will still work.

Capes are popular in the northeast which is snow country but you didn't list your climate zone. If you deal with cold weather then the moisture issue is relevant.

I'll add a reference link on capes as they have unique issues, I know because I'm living in one and currently doing a deep energy retrofit.
Two Ways to Insulate Attic Kneewalls

Bud
 
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Old 09-08-16, 10:05 AM
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I am in Oregon. We do get snow. I to am doing a a remodel upstairs. I currently have one of the rooms completely gutted. Unfortunately I have 2x4 framing so the R value I can achieve on the sloped ceiling between rafters is little. But I think if I do a good job air sealing and proper installation of insulation I can increase comfort.
Our house is pretty old, built around 1940, no insulation in walls, old windows and drafty. I almost just want smash it down and build a new one hehehe

So for venting area: Footprint below roof= 888 sq ft
888/150= 5.92 sq ft of venting
5.92/2= 2.96 sq ft for each upper and lower.

I think right now I have about 2 sq ft for lower and about 2 sq ft for upper. Looks like I could use more ventilation.
 
  #10  
Old 09-08-16, 11:08 AM
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Vents vary in NFA so you can guess at the reduction or use 50%. Or search for a similar new vent and see what it is rated at. The louvers and bug screens reduce the effective air flow and we have to increase the area to get what we want. New vents can vary from 20% to 90% NFA.

Consider a layer of rigid insulation before the drywall. Note the code requirements in the link below where they specify a cavity value and then a continuous layer of rigid. The rigid can be installed on the inside, but check local requirements. Also the link is from 09 and there are now 2012 and 2015 updates so see what your area is following.
https://energycode.pnl.gov/EnergyCodeReqs/?state=Oregon

Bud
 
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Old 09-08-16, 12:38 PM
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Great, Thanks alot! By the way, are you documenting your energy retrofit anywhere?
I would love to know what you are doing.
 
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Old 09-08-16, 01:54 PM
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I have taken a few pictures, but there are many (and will be many more) well documented retrofits. Mine needed new siding and even though the old windows were double pane, they were wood and a pain to open. So I pulled the siding and added 3.5" of rigid foam in two layers overlapped and sealed. Then another layer of sheathing, house wrap, and vinyl siding. Because I lost 4of overhang i extended the soffits to get back to a foot plus. The key to rigid on the outside is it covers from foundation to above the top plate.

In the basement I'm covering the top 4' of the foundation with 1" foil faced rigid insulation, plus air sealing the house to foundation.

I'm old and slow so still work in progress BUT the house did really well this last winter.

To be honest I would have difficulty recommending a home owner do all I'm undertaking, above plus a lot more, if they have to hire it all out. Way too expensive. For me, I'm an energy auditor and have done construction and contracting so it is a labor of love and it keeps me busy.

BTW, the wife loves the new windows. That's an energy auditors joke because very often the things that matter are not the dollars saved .

Bud
 
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Old 09-09-16, 09:39 AM
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One thing I am learning is happy wife = happy life
 
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Old 09-09-16, 09:50 AM
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So I need about 3 sq ft NFA for each lower and upper venting. Is there any sense in rounding that up to 4 sq ft NFA for each? Since I dont have soffit vents and I have to use gable/turtle vent combo, would it be beneficial to have more NFA?
 
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Old 09-09-16, 11:06 AM
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To make sure I have the picture correct, you have 4 lower vents, 2 on each side with one at each end of the side attic. If so, then the 3 ft gets divided by 4 and each low vent gets 0.75 ft.

As for bumping the numbers up, online calculators will use 150 or 300, the difference being the presence of a good vapor barrier in the ceiling, well air sealed, and lost of insulation will use the 300. You are probably somewhere in between but using the 150 gives you some extra ventilation. Thus no real need to go beyond the calculation. BTW, these guidelines (the 1/150 and 1/300) were a wild guess from back in 1942 and other than having been used for all this time, no one has decided to test them to see if they are correct.

Bud
 
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Old 09-09-16, 11:37 AM
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yes that picture is correct.

Ok, Sounds good.

Thanks!
 
  #17  
Old 09-14-16, 12:16 PM
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Bud9051,
What is your preferred method for plugging and sealing the floor joists right below the knewall? I read you can stuff insulation in a plastic bag and stuff it in floor joist and then air seal with spray foam .
 
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Old 09-14-16, 01:01 PM
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One use for the bag trick is when blowing in insulation it blocks the cavity so they only fill in the areas towards the exterior. In your case, i depends upon what you have for access. If easy access you can cut and fit anything rigid and seal it in place. The objective is air sealing. Getting a perfect long term air seal with a bag of insulation and spray foam is an unknown.

Without going back through the thread, what do you have for access? Does that area have plywood over the floor joists?

Bud
 
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Old 09-14-16, 02:58 PM
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No plywood over the floor joists on attic side of kneewalls. Just combo of batt and blown in insulation. On the living space side of kneewalls I do have plywood as the floor.
 
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Old 09-14-16, 03:42 PM
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Since the cavities are basically open, can you cut and fit some one or two inch rigid foam? If you install a small stop block to the left and right it helps to position the piece of rigid. bevel the edges of the rigid to give the caulking or can foam a place to fill. Plywood, osb, anything rigid works.

With the venting and those spaces not blocked (or just fiberglass) the wind blows in one side and out the other. That is making the floor upstairs cold as well as the ceiling below. Very good fix to get those air sealed.

Bud
 
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Old 09-14-16, 04:12 PM
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Awesome, I will go ahead and do that.
 
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Old 09-17-16, 04:59 PM
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Hey Bud,
Do you think I could just fill that cavity with can foam?... It's 2x4 construction so the cavity would only be 3.5"x24"
 
  #23  
Old 09-17-16, 05:16 PM
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I don't like the idea of that much can foam. Never heard of a specific problem, but too much at a time and it has trouble curing. I've never used their large gap but check the mfg's recommendation.

Do you have any rigid foam board to help out?

Whenever I accumulate too many scraps I put them in a box and mark it free out by the road. Doesn't take long and it's all gone. I sometimes forget other people don't have those scraps to use.

Bud
 
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Old 09-17-16, 05:37 PM
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Ya I got some rigid foam. I was just trying to get out of another "cut n cobble " task... 😄.
Ok I will use some rigid .
 
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Old 09-20-16, 09:29 PM
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Hey Bud,
I plan on blowing in about 18 inches of cellulose into the attic. Do I need the attic baffles down where the roof meets the attic floor even though I don't have soffit vents? If I don't use the baffles there will be insulation right up against the roof sheathing down in that area. What do you think?
 
  #26  
Old 09-21-16, 12:50 AM
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You do want to keep the insulation away from the bottom of the roof, especially in the areas over the living space and avoid filling the soffits. Besides trapping moisture you or someone may in the future need to add some vents. Just best practices.

Bud
 
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Old 09-21-16, 09:21 AM
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ok, cool. that sounds good. By the way while I was up there the other day sucking out old insulation, I found a big group of carpenter ants!
The joys of old houses.
 
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Old 09-21-16, 10:30 AM
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I've battled carpenter ants almost every year and come up with my own baiting solution. I use liquid Terro in a jar with a lid. Dilute with a touch of water so ants can crawl out of the solution. No water and they get stuck. Then, I catch any ants I find and and toss them into the jar. A little shake to give a good coating and I release them right where I found them. 2 or 3 days later, no ants. Getting that Terro back into the nest is the best solution. Stomping on the ones you find may feel gratifying, but it has no affect on the colony.

Bud
 
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Old 09-24-16, 08:32 AM
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Hey Bud,
I was planning on removing old insulation and blowing new stuff in. Is it even necessary to remove old insulation? Can I just blow new stuff over top?.
I think it would be pretty easy to find all the penetrations and air seal with the existing insulation, it's not very deep. I already removed 1/4 of it though. It's a huge pain in the ass.
 
  #30  
Old 09-24-16, 08:56 AM
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I think the major concern with leaving the old in place is what may be dead under it. Rodents and droppings are disgusting, but since you have already removed some you know if that is an issue or not. Total depth equals insulation value at about r-3.5 per inch, give or take based upon what is there and what you use.

Bud
 
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Old 09-30-16, 10:38 AM
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I sucked out all the insulation on one side. Yay me. I sucked it out with handy insulation vacuum I made from leaf blower.

Didnt find anything to scary. Just those ants and some rodent droppings. Everything else looked pretty good. Old but good.

I also got some vents ordered for top and bottom so my ventilation should be on point.

On to the other side for some more insulation sucking.
 
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Old 11-28-16, 03:26 PM
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hello again Bud!
I had a question for you. So as you know I have 1.5 story house that I am trying to redo the kneewall insulation and attic floor insulation.
What do you recommend for the sloped portions of the ceiling? Without tearing down the drywall is there anything that can be done to add insulation?
I have read some people just dense packing them cellulose. I know that ideally you want an air gap under the roof sheathing.
any ideas?
 
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Old 11-28-16, 04:25 PM
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I'm not a fan of dense packing cellulose in those cavities. I have read some articles where they say it is against building codes to place a fibrous insulation against the bottom of the roof deck. I have also read that some do it anyways and claim no problems. Here are my concerns.
1. The code issue is trying to avoid any warm air reaching the bottom of the cold roof.
2. By closing off this ventilation path you isolate the top attic from the bottom. You end up with about 1/3 of the ventilation pressure you would normally have from low to high. It is the difference in height from the low vent to the high vent that creates the ventilation pressure.

Unfortunately, removing the drywall, padding out the rafters to 2x8 or greater, and filling those cavities with vent channels and insulation, is what really needs to be done. I'm not sure what stage you are at as to whether this can be considered.

The big "however" is, how much difference would that major effort make?

You'll have to refresh my memory as to what stage you are at. Are these sloped ceilings covered with drywall?

Bud
 
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Old 11-28-16, 05:17 PM
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There is 2 rooms upstairs. Bedroom 1 I gutted completely since it had some old broken paneling stuff. So this room I am able to properly vent and insulate those sloped portions.
Bedroom 2 was already "renovated" meaning it already had new drywall, texture,paint.
It is this Bedroom 2 that I am scratching my head on how to deal with the sloped ceilings without gutting it.
But that might the only option if I want to add insulation.

Im wondering if properly insulated and airsealed kneewalls and attic floors, along with proper attic ventilation will make a difference? Maybe the sloped portions is negligible?
 
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Old 11-29-16, 05:01 AM
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@MrCarbon: "Im wondering if properly insulated and airsealed kneewalls and attic floors, along with proper attic ventilation will make a difference? Maybe the sloped portions is negligible?"
That is where I was headed.

First, are these 2x4 or 2x6 rafters? Probably stated in the thread by I'm lazy.

Given the total upgrades there is no question that the room will be warmer. If you can get 3.5" of high density insulation, HD fiberglass or Roxul, plus a vent channel into those slopes they will be just fine. I can go into the heat loss numbers but I already know the minimal savings for upgrading that sloped section will never pay for the time effort and materials to rip it out and do it over. The other bedroom that is already exposed can be upgraded since the timing is right.

I like Roxul as it has good r-value and is very dense. It can be sculpted with a long bread knife and provides a good fit.

Bud
 
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Old 11-29-16, 01:35 PM
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its all 2x4 rafters. I know, crappy.
 
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Old 11-29-16, 03:44 PM
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In the room that is already finished what is currently in there for insulation and ventilation?

Bud
 
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Old 11-29-16, 04:10 PM
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For sloped portion: its got old R7.5 faced fiberglass batts that are in really bad shape. probably like an 1 inch thick. For ventilation it has an air gap between the roof sheathing and the insulation due the insulation being so degraded..haha.

for the kneewall portion: I will be doing R15 fiberglass batts which will be sheathed with R7.5 pink insulation board. That should give me about R22 plus the thermal break due to the sheathing and plugged and sealed floor joists under kneewall.

For the higher attic space ( small triangular space): There is more of that old degraded R7.5 fiberglass batts.

My whole attic ventilation consists of 4 gable style vents actually used as intake vents down low near attic floor and rectangle roof vents up by the peak as exhaust vents.
 
  #39  
Old 11-29-16, 05:26 PM
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To clarify, are the 4 gable style vents located in the side attics created by the kneewall?

With 3.5" of rafter depth you should try to get something additional in there. I don't know how easily that space can be accessed but you could slide a piece of rigid above the existing fiberglass and add spacers on each side of it to create the air channel.

Jumping back to your original question about dense packing those cavities with cellulose I did some more searching and found a building science corp article. I do respect their advice and it looks like a good article. I'll attach it for you and read it later myself.
https://buildingscience.com/document...-climates/view

Using the dense pack approach would still a question about high and low venting, but that isn't a terrible problem.

Bud
 
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Old 11-29-16, 05:32 PM
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Yes they are in side attics. I have no roof over hang so i had to use these as intake vents
Name:  ventilation.jpg
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Kind of like this graphic.
 
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