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Old 11-27-2017, 08:44 PM   #16
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Re: NJ - Attic Insulation


Although a few on each side is a lot better than none, the recommended is either a continuous strip of vent or some at each rafter bay. If the soffits are open between rafters then the few idea works kind of ok. The key is to air seal everywhere before new insulation is installed. Air infiltration carries a lot of the moisture. Some insulation installers will suggest that the cellulose acts as a blanket and serves to seal the entire attic, but not true. it is denser, but has high permeability. Even rigid foam board allows some moisture to pass so cellulose is virtually vapor open.

Good choice if it is a good audit company. Be sure they share all of the measurements taken and data derived for your records.

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Old 11-28-2017, 10:07 PM   #17
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Re: NJ - Attic Insulation


Hi Bud,

Not sure if you know the answer to this or anyone else. But I had a question about a comment that Nealtw made in this thread about "When in doubt more venting is better than less". Is that true? I remember seeing somewhere that if you have a lot of vents, it does not create a suction effect and you end up like sucking on a straw with holes in the middle (not sure if the analogy is correct in this case). Or at times, they start working against each other. Not sure if that will be the case in my scenario. Hence the question. In my case, I have gable vents and a ridge vent (notice I have it today hence why I didn't mentioned it before) but no soffit vents. Since both gable and ridge vents are consider high level vents, I'm thinking it might be okay to add the soffit vent as long as I stay with that 50-50 NFA you mentioned in this thread.

Last edited by Resnick98; 11-28-2017 at 10:16 PM.
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Old 11-28-2017, 10:34 PM   #18
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Re: NJ - Attic Insulation


The guidance for attic venting has a tolerance of minus 80% to plus 400%. I made that up to illustrate extremes, but the code required 1 sq ft NFA for every 300 sq ft of attic floor was a made up number as well. Here is a quote:
"The attic ventilation ratio “1/300” is an arbitrary number selected by the writers of FHA(1942) with no citations or references"
this is from this link: https://structuretech1.com/wp-conten...entilation.pdf

But there is a message in my exaggeration and that is, sometimes we need to adjust our ventilation, more or less, to deal with mother nature. Lots of ventilation facing a strong on shore wind from the ocean could (and is) be disastrous. On the other hand, a house nestled in among a dense thicket of evergreens may rarely experience much of a breeze. Without the wind the house would need to depend upon "attic stack effect" which is a very modest force.

Since the advice we offer (and the codes) require a short and simple explanation we settle for the 1/300 with the 50/50 distribution, and for most homes that will be fine. There are examples where too much vent area can cause problems, either snow, or when you introduce ventilation at many different elevations. But we don't have space here for all of that.

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