# Attic insulation and Venting question



## DeerManager (Oct 4, 2006)

I have a gabled roof with vents on each end and a vent on the peak of the roof line, do i need to install rafter vents when I add cellulose insulation later this year? 

From my understanding i "shouldn't" need to due to my roof venting though the vent on the peak and the intakes are on the sides of the house, am i correct?

I know i'm going to add about 5-6" on top if the 9" up there already, my heat bills are billing me this winter and it's not even oficially winter yet!

Thanks for any help 
Jay


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## HemlockNailer (Dec 22, 2000)

Yes you should add insulation baffles at the soffits. This will give you the desired chimney effect for air ciculation. Incoming air as low as possible, outgoing air as high as possible. Keep the attic as close to outside temp as possible.l


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## jondahighlander (Nov 10, 2008)

hemlocknailer is right. One thing I was taught is you shouldn't have both gable vents and a vent on the peak..........they cancel out flow. The flow is suppesed to go from the soffits up and out. Personally i'd go 16-18" deep............it's not that much more if you're renting the blower from home depot anyhow.-------------jon


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## Splitshot (Nov 30, 2000)

No matter how you do it, adding insulation to 9 inches is going to save you very little in energy.


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## Big Reds (Oct 14, 2007)

Your best bet is to get an insulation contractor to look at your specific application. Most reputable contractors will have free estimates and be sure to ask plenty of questions. If that is the house you are referring to (behind the truck) you don't have the typical venting involved. It looks to me that some of the attic space is used for living area. This will give the home some non typical venting qualities. Make sure to keep us advised!


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## Splitshot (Nov 30, 2000)

The Department of Energy recommends 1 sq. ft. of free air ventilation in the attic for every 300 sq. feet of floor space if the ventilation is divided into high and low vents.

Ventilation in an attic serves two purposes. In winter ventilation is used to vent excess moisture out of the attic. Moisture easily migrates through most building materials. Glass and metal are absolute vapor barriers and roofing materials are close. Moisture from your house will migrate into the attic. Worst case the moisture can condense on roofing nails and drip into the insulation and finally leak through drywall.

For every one percent increase in moisture in your attic insulation it reduces the effectiviness by about 2%. Good ventilation will help keep the attic cold and dry.

In summer good ventilation will keep hot air out of the attic reducing air conditioning cost and help keep you more comfortable.

Insulation calculation based on standard furnace and natural gas at $1.17 per ccf.

The chart below will show you the diminishing returns for adding more insulation starting with an R-value of one. This is what it would cost you per square foot per year living in mid Michigan with a standard furnace.

R-1= $1.54 per sq. foot. R-1 is like a single pane of glass.
R-2= $0.77 per sq. foot R-2 is like a standard double window.
R-4= $0.38 per sq. foot
R-8= $0.19 per sq. foot per year
R-28= $.0549 per sq. foot per year
if you add 6 inches of insulation you will save approximately $0.0173 per sq. foot. Law of diminishing returns. If you doubled the insulation you would only save 1/2 of the 5.49 cents per square foot you are losing now.

If you have a basement that is uninsulated consider it is about R-2 and lots of money can be saved there even if you don't directly heat the basement.

As far as the ventilation, you can never have to much so the ridge vent and the gable vents all help keep out the the hot air in summer and the moisture in winter. By the way free air means without obstruction. A window screen for example on a window reduces the free air by half.

If your looking to save money, caulk around both sides of the trim on your windows and doors, get the little insulators for you outlets and switch plates and by all means caulk your sill plate if you have a basement or crawl space. 

It is common for people to think they can save lots of money by adding insulation to their attic, but it is usually wasted money especially if you have R-19 or above.


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## DeerManager (Oct 4, 2006)

Great info thus far guys, and no that house in the background is no longer my house, i think i'll take pics later this week and post them up.

Splitshot what did you mean by your post?


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## MSUICEMAN (Jan 9, 2002)

i REALLY need to insulate my attic-space and my finished portion of the attic needs to be insulated from the non-finished portion (knee roof areas on both sides). my attic had blown in but it all compacted. I am thinking about getting it taken out and replaced, as who knows what material it is as its so old.

I am also going to finish the basement which should help immensely I would imagine. After all that is done, my house should feel much more stable temperature-wise.

thats what ya get for having a house thats almost 60 years old. I will also go with retrofoam around the exterior walls eventually.


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## Splitshot (Nov 30, 2000)

DeerManager said:


> Great info thus far guys, and no that house in the background is no longer my house, i think i'll take pics later this week and post them up.
> 
> Splitshot what did you mean by your post?



You will have to be a little more specific DeerManager.

MSUICEMAN,

You are correct, fiberglass insulation saves much less energy once you crush it. That is because it is all the little air spaces in the insulation that makes it fuction well. If you replace it, consider wet blown celulose. It has a higher R-value per inch and it wicks the moisture better. Some of the new foams seal and insulate better, but if you use it in your attic, it acts like a vapor barrier and could hold to much moisture in the structure. Seeing it is an old house, that is not likely to happen, but it is important to keep humidity below 60% at all times.


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## jpollman (Jan 14, 2001)

Jondahighlander is correct.

You do NOT want to have a ridge vent AND gable vents. You should close off the gable vents and add rafter baffles at the soffit. The others are correct that you want a "cold roof" and you want the air coming in through the soffit and traveling up the under side of the deck and exiting through the ridge vent. If you have gable vents it disturbs this flow and does not work properly. 

When I resided the soffits and gables on my home about five or six year ago (I have a brick ranch) I cut the majority of the 3/8 ply soffit out and left it open. Then I installed the rafter baffles in EVERY bay through the opening. This is much easier than trying to install them from inside. Once all the baffles were in, I reinstalled the vinyl soffit which is continuously vented. I had a ridge vent installed when I had my roof done the year before. My attic breathes like nobodies business! You can get away with doing every other bay if you want but the baffles are cheap and _*you can't have too much ventilation in your attic*_!

John


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## Splitshot (Nov 30, 2000)

John,

You are correct that you can't have too much ventilation and that soffit and ridge vents are the best especially if they are continious.

I disagree with you about gable and ridge vents being bad. The D.O.E. states that if your ventilation is either all low or all high, you need twice as much than you would need if it were high and low.

If he can't install soffits, he would be better off making the gable vents bigger.


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## jpollman (Jan 14, 2001)

Splitshot said:


> John,
> 
> You are correct that you can't have too much ventilation and that soffit and ridge vents are the best especially if they are continious.
> 
> ...


I guess if he can't install soffit vents that may be OK. But I was assuming that he should be _able_ to do soffit venting but is just asking if he _should_. Ideally yes it would be best to have soffit and ridge venting if possible. But I was saying that if he does install the soffit venting that he should close off the gable vents. That's the best configuration with a ridge vent.

John


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## Splitshot (Nov 30, 2000)

In either case we both agreed that you can't have too much ventilation so in either case leave the gables, it will not disturb the movement of air.


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## DeerManager (Oct 4, 2006)

Splitshot said:


> In either case we both agreed that you can't have too much ventilation so in either case leave the gables, it will not disturb the movement of air.


HAHAHA Thanks guys,
but back to my question now, do i need to install rafter vents when i add more insualtion to my attic?

I'll take pics to clear things up asap.


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## outdoor junkie (Sep 16, 2003)

My recommendation would be to add soffit vents with baffles. And if your ridge-vent is installed properly(enough of the sheeting cut back on the peak to allow proper flow) you should block the gable end vents. You do not want ridge-vent and gable vents with soffit vents. Your soffit vents will be bypassed and all your air movement will stay up top betweeen your ridge and gable vents. If you leave all three open, your going to have problems.


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## jpollman (Jan 14, 2001)

Generally speaking when you add insulation you should add ventilation. Unless it is found that whatever existing ventilation you have id adequate. You should use the baffles because it keeps the insulation from coming in contact with the deck and blocking air flow. 

John


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## Splitshot (Nov 30, 2000)

outdoor junkie said:


> My recommendation would be to add soffit vents with baffles. And if your ridge-vent is installed properly(enough of the sheeting cut back on the peak to allow proper flow) you should block the gable end vents. You do not want ridge-vent and gable vents with soffit vents. Your soffit vents will be bypassed and all your air movement will stay up top betweeen your ridge and gable vents. If you leave all three open, your going to have problems.


Sorry man, but that is not accurate, however I would be willing to admit your right if you show me something to substantiate your claim.


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## Ausable Junkie (Oct 2, 2002)

To optimize air convection in a roof, the best case scenario is soffit venting with ridge venting. The greatest pressure exists at the very peak/ridge of the roof. The lowest pressure exists at the soffit level. Gable vents are almost never as high as the ridge, except the triangular ones. Venting at the point of greatest pressure and least pressure is the most efficient way. That's why Native Americans didn't get smoked out of their tepees. 

Also, gable vents require the air to change its direction of flow from vertical to horizontal to allow the warmer air to escape. Any time you change the direction of airflow (90 degrees in this case), you lose efficiency. With ridge and gable venting, eddy currents occur. This happens at the dividing line where pressure determines whether the air flows up to the ridge vents, or finds its way to a gable vent. At this "dividing line", air moving to the ridge will be moving faster (greater pressure) than the air bound for a gable vent (lower pressure). Eddy currents will also occur at the point where the low side of the horizontal flow to a gable vent is pushed from underneath by air moving up. Just like in a stream, differences in air speed or direction result in eddy currents. Lower efficiency is the result.

To make a long story short, (I know, too late) its better to add "x" more area of ridge vent by cutting more sheeting away, than adding the same "x" area later on in the form of gable venting.

Wow, I think I set a new personal record for # of edits for my post LOL.


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## HemlockNailer (Dec 22, 2000)

DeerMan, if you re-read SplitShot's first post it will answer your question. If you already have r-19 (6 in. of fiberglass ) in your attic the payback for adding additional is minimal. Check for heat loss around lights , fans etc. Make sure that you are insulated all the way to the exterior wall, hence the need for insulation baffles at least above the top of the insulation. You say that you already have 9 ins., that should do the job if installed properly. Listen to SpitShot he knows what he is talking about. If your soffits are not vented that is where I would spend my money.


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## Ausable Junkie (Oct 2, 2002)

If you do add more insulation, make sure its the stuff without paper on it or use blown-in.


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