# Having blown in insulation added to my attic. Question about installing gable vent f



## Finch1985 (Apr 21, 2014)

So I've gotten a couple quotes about having some cellulose insulation blown into my attic. I have 2x4 truss construction and the existing blown in insulation is just about level with the 2x4s, so 3.5 or maybe 4" of insulation. 

Before they do this work, I'm wanting to add a gable vent to one end of the attic to help exhaust the hot air. Here's the kicker and I know it's not ideal. A few years back we had the roof replaced and they added those small circular soffit vents and a ridge vent. I never did get around to sealing off my gable vents. I've often wondered if the soffit vents are adequate enough to do their job. I feel that a gable vent fan would do a better job. Should I block off the ridge vent if I go with a gable vent fan? How would I?

Another thing, one guy included in the quote the installation of baffles to prevent covering the soffits. The other guy didn't mention them and when I did, he assured me his guy wouldn't cover up the vents. Well, I insisted he quote me with installing baffles. $455 for baffles alone, obviously priced as so I wouldn't go that route. That's not much less than he quoted me for the insulation. 

So in summary... if I decide to not have them (either company) to install baffles and the soffit vents get covered, is that okay as long as I installed the gable vent fan? Also, how do I go about covering or blocking the ridge vent? I almost feel baffles are needed to prevent covering my soffit vents no matter if I keep the ridge vent open or close it off and install a gable fan. I really want to install the gable fan because I feel it will do a better job than what I currently have. Thanks!


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## joecaption (Nov 30, 2011)

http://www.energystar.gov/index.cfm?c=home_sealing.hm_improvement_insulation_table
What type ridge vent did they install?
The roll type or plastic 4' long?
Reason I ask is the roll type just does not give enough air flow.
No way would I be blocking the ridge vents.
I'd be far more concerned with the possible lack of enough soffit venting.
Right now your about 9" short of the level of insulation needed.
Yes you need baffles.
Add that gable fan with the lack of soffit vents and you will be creating a negative pressure in the attic causing it to draw conditioned air into the attic.
Any of the quotes mention anything about air sealing before adding the insulation?
Air sealing is simply sealing up any place wiring, plumbing, ceiling mounted fixtures are.


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## Finch1985 (Apr 21, 2014)

It was the roll type. How do I figure if I have enough soffit vents? No one mentioned air sealing but I read that while searching the forums yesterday and plan on doing that myself. 

I really feel the gable fan would help keep my heat pump from running constantly but I don't want to do more harm than good either. What would you suggest?


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## Finch1985 (Apr 21, 2014)

Forgot to ask, even with two gable vents (one on each end of house), it would still create a negative pressure if I had a fan exhausting one side and left the other open?


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## Windows on Wash (Aug 30, 2011)

How many of the circular soffit vents are there? 

Ridge and soffit venting is ideal as compared to powered and gables. Now with the mix of ventings, you really need to have one done properly to work properly as compared to a couple of them working at half steam.


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## Finch1985 (Apr 21, 2014)

I'm at work at the moment but will verify when I get home. There is a small circular soffit vent per each truss bay (unsure of terminology). Basically one every 2 feet. I'll get a count when I get home


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## Windows on Wash (Aug 30, 2011)

Get an NFA rating on them, square footage footprint on the home, and we will go from there.


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## Yodaman (Mar 9, 2015)

Attic ventilation requires balance to work efficiently. When balanced properly between the
ridge and the soffits there is no need for a exhaust fan. The air flow must be unobstructed 
from the soffit to ridge, and thus the requirement for baffles. Manfactures use a 
term called Net Free Vent Area that specifies the amount in square inches per foot of
ventilation that you can expect from their products. So the ventilation at your ridge must 
equal the combined ventilation at your soffits directly below it. A typical rolled ridge vent
might have a NFVA of 12" per foot. So your soffits must have 6'' of NFVA per side to equalize 
with the ridge. Envision a slot 6" long x 1" wide and one slot placed every foot down the soffit run.
Some suggest that a 60% to 40% split between the soffit to ridge is better yet, with 60% coming
from your soffit. With the 12" NFVA ridge example, 18" of NFVA would be required from the soffits.

But if you have insuffecient vents at soffit the whole system beaks down. Now you have warm air wanting
to exhaust at the ridge and being starved at the soffit. This can cause negative pressure in the house as stated above.
So it pulls air from where ever it can get it, like your living space. Every crack, gab and seam thru-out your
house, basement or crawlspace will be the supply air. Also bringing with it contaminents,
moisture, unconditioned air and possibly raydon too. 

There are some other calculations regarding total minimum ventilation that say a older house
loosely insulated should have 144 sq in of NFVA per 300 sq ft of attic space and a newer tightly insulated
home should have 144 sq in of NFVA per 150 sq ft of attic space. 

In my opinion when the soffit and ridge vents are set up properly, gable vents, roof pans, and exhaust fans may
hinder the system.

Hope this helps


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## stick\shift (Mar 23, 2015)

With enough NFA on both sides, soffit and ridge vent are ideal and the gable vents should then be blocked.

Absolutely install baffles if you're blowing in more cellulose.


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## Finch1985 (Apr 21, 2014)

Guys, sorry for my delayed response and I appreciate the responses. 

My house is 26' X 46' but there is a 13'x16' section that is a cathedral ceiling so that doesn't really count. The soffit vents you see in the pic below is what my roofers installed. Like I said, they are about 2' apart. I have about 15 on each side of the house. 

I bought some baffles today... the kind that can be torn into. My trusses are 24" on center. My question is should I tear the baffles into or install them untorn? I assume I wouldn't push them all the way until they touch the soffit, maybe just beyond the top plate?


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## Finch1985 (Apr 21, 2014)




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## Yodaman (Mar 9, 2015)

Finch1985 said:


> The soffit vents you see in the pic below is what my roofers installed. Like I said, they are about 2' apart. I have about 15 on each side of the house.


Sorry, woefully inadequate. From my post above we should be looking for 6 sq. inches of NFVA per ft. minimum. These are giving us about 2 sq inches, so we would need 3 per foot. A continuous strip vent 1" wide will get us 12" per foot. (Just one option)

This 6" per ft is derived from my assumption that your ridge is giving you 12" per ft. It would be nice to know what it is difinitively. Either by asking the roofer what the product is or maybe a close up photo would help.

Regarding the baffles, leave them double wide for your 24 oc rafters. Yes only need to run them to the outer edge of your top plate. Be sure to run plenty high on the upper end, about a foot above the new insulation level.
It may take 2 lengths depending roof pitch.


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## Finch1985 (Apr 21, 2014)

I installed one baffle just to see how difficult access would be. Looks like I'll add another baffle to give me that foot difference for when the new insulation is blown. Still looks like insulation could be blown thru the gap the baffle leaves between the top plate though? 

It might be a while before I can add more soffits. Would you still recommend closing the gables if it will be a while? 

Also, I've included a pic of the ridge vent.


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## Yodaman (Mar 9, 2015)

your baffle looks good, looks high enough but can not really tell from the photo. Might want to check with whoever is doing insulation if they will be blocking the top end of the baffles to prevent filling with insulation. On the low end I would spray the gaps with spray foam to keep it out of your soffit cavity. The gable vents leave open until your soffits are upgraded. Your ridge vent actually looks like it might be a 4' length, the edge doesn't look like the roll type. Haven't found it yet. Maybe someone will recognize it. When we have it identified we can better calculate what the soffit NFVA per foot should be.


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## Finch1985 (Apr 21, 2014)

I seem to remember them unrolling it but I could be mistaken. 

I did the remaining baffles today. I actually did not staple the bottom so it closes that gap and I think would be enough resistance to not allow the insulation to go into the soffit. Good idea about the spray foam though.


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## Yodaman (Mar 9, 2015)

your call obviously, but nothing good can come of having cellulose in your soffit cavity. 
The cellulose goes in like wind driven snow and will most likely push your baffle tight to the deck (wear it should be) but gap at the bottom will be exposed. No time like the present to prevent it.


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## Yodaman (Mar 9, 2015)

http://www.homedepot.com/p/GAF-Cobr...Ridge-Exhaust-Vent-in-Black-2018000/202052410



I believe this is your ridge vent. This product has 18" of NFVA per foot. So you would be looking for 9" NFVA per ft. minimum on your 2 opposing soffits. I personally prefer a continuous soffit that will have a specified NFVA as opposed to individual vents that usually will require an educated guess. I used a 30% NFVA factor on your 3" rounds to come up with about 2" of NFVA. A more accurate way to know would be to measure the total length of slots and multiply by opening width. If individual vents are the plan we should be around 6 x 12" spaced every 2 ft in the center of your rafter span.


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## Gary in WA (Mar 11, 2009)

http://www.diychatroom.com/f103/gable-vents-good-bad-ridge-vent-254610/

Ideally; would warrant continuous venting, close to the fascia board for better air-flow pressure (less rain, etc.), pp. 617; http://books.google.com/books?id=Z8...page&q=attic airflow with gable vents&f=false

You need *5 *(five) of those circular vents PER FOOT of soffit;http://www.ventmastersstore.com/3-round-screened-vent-white/

The tape measure is covering the stated NFVA on the vent pictured....

Gary


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