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-   -   vented vrs. unvented attic (http://www.diychatroom.com/f19/vented-vrs-unvented-attic-131239/)

rcassidy 01-24-2012 01:29 PM

vented vrs. unvented attic
 
My house is a 2 story colonial in Northern NJ with cold winters and hot humid summers. I've been having issues of condensation and mold growth in the attic on the roof sheathing. During the cold months, warm moist air is escaping the conditioned space of the house and entering the unconditioned space of the attic. The unconditioned attic has a passive vented roof with a ridge vent and soffit vents. The dryer and bathroom fans are vented to the exterior, and the central AC unit is located in the unconditioned attic space.

My goal is to resolve the mold issue and prevent it from coming back. An energy company suggested to seal off and spray foam the entire attic (soffit, ridge vents, gable walls & roof sheathing) with closed foam insulation so the attic space is part of the building envelope thus maintaining temperature within 10 degrees of the conditioned space. this contradicts what I've always heard that roofing materials (sheathing & shingles) and the attic always needed to be vented.

Does anyone have any experience or insight with vented vrs unvented attics?

md2lgyk 01-24-2012 02:08 PM

How much insulation in the attic? Where is it? My attic isn't truly a conditioned space, but the insulation is between the rafters rather than under the floor. Though it took a lot more insulation (I have a 12/12 pitch roof), I did it that way because my tankless water heater's up there.

Willie T 01-24-2012 02:27 PM

To be honest, I've never seen an unvented attic. Read code about them, but never experienced one.

shadetree 01-24-2012 02:28 PM

If you want to pay to condition your entire attic as well as your home, you are welcome to; however one usually wants to condition the smallest volume possible.

Not to mention the area of your attic floor is gong to be significantly smaller than the area of your roof, so to weather-seal & put down the same R value you'll need a whole lot less insulation sealing up your attic floor/house ceiling vs the roof.

Just wondering, but does the energy company have any relation to the company doing the spray foam?

rcassidy 01-24-2012 02:41 PM

Current insulation is fiberglass batt (R30), in fairly poor condition.

KennMacMoragh 01-24-2012 04:03 PM

Unvented = hot roof

Hot roofs are very uncommon, they can work if done right but are risky, if there's any air leakage then you will have problems. I've never seen a hot roof, only read and heard about them. I believe Sean AKA SLS-Construction has experience, I thought I remember him blogging about hot roofs. Not sure if he has a profile here though.

AGWhitehouse 01-24-2012 04:47 PM

Hot Roof? Roofs don't always need to be vented. It is generally believed to help shingle life, but most roofing manufacturer's do not require venting for their warranty periods. You will find arguments in every conceivable direction on this one, so I'd look at your other driving factors before you leave this as your sole deciding piece.

Spray foam the whole thing? Well, I personally don't recommend the stuff. Once it is in, it's in. Retrofitting stuff, renovating, repairs, etc. become a nightmare as it sticks to everything and requires chipping away at it to get it out. And if they install it wrong to begin with (not uncommon with closed cell foams) you're looking at a ridiculous and pricey process to remedy it.

Place the thermal envelope at the roof? Well there are two sides to your coin here. Shadetree brought up the first. You will be increasing your envelope surface area and there by increasing your heating/cooling loads. The second aspect is that your air handler is in your attic. By bringing that unit within the envelope you will be reducing the heating/cooling loss associated with it currently being outside your conditioned space. The real question is will the extra loss from additional surface area equal or surpass the gain from air handling efficiency?

The cheapest option? Analyze the soffit and ridge venting and ensure you have proper flow. 1:300 is the lowest ratio (1sq.ft. free-area venting for every 300sq.ft. of attic floor area). Remove all the old insulation, air seal everything, put in new batts to fit the joist bays. Cross lay un-faced R-30 batts over the top. Cover with tyvek or equal air barrier.

rcassidy 01-24-2012 08:59 PM

thank you everyone. I'm still evaluating but I'm leaning toward NOT spray foaming the entire roof and gable area. I aggree, once the foam is in, that's it, it is there for ever. Any modifications will be diffecult to impossible to repair.

md2lgyk 01-25-2012 09:42 AM

Quote:

Originally Posted by Willie T (Post 833372)
To be honest, I've never seen an unvented attic. Read code about them, but never experienced one.

I used to have one. The house is on Maryland's Eastern Shore and was built in 1959. Don't know what the style of roof is called, but it is very steeply pitched and has no soffits or eaves. No gable or roof vents either. I was in the attic a few times to install ceiling fans, and there was no evidence of mold or moisture damage whatsoever.

Maintenance 6 01-25-2012 01:29 PM

Make sure your soffit vents are not blocked off by mis-installed insulation etc. Make sure they are true vents that are open to the inside of the sttic space and not just vents installed over a solid soffit.
Make sure your ridge vent is open directly to the outside. The roof sheeting should have been cut back to allow air to penetrate the vent. Seal any air leaks from the living space that would allow air into the attic. Recessed lighting or leaks around a bathroom vent fan are notorious places for that to happen. Do you have a vapor barrier above your living space to prevent moisture from penetrating into the attic space? Do you have an attic access door that does not fit well or is not weathersealed? With working and proper soffit vents and a ridge vent, there is no reason you should have a moisture level high enough for condensation and mold to form, unless something is not working.


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