I'm building a 2 story house w attached garage in Central Jersey.  Climate zone 4a about 35min. drive to zone 5a.  I think it's about 5500 heating degree days. All cooling comes from the attic.

Roof: Asphalt single with felt paper & plywood sheathing
Rear walls: vinyl siding, 1/2" foam insulation (r=2.5), felt paper, plywood sheathing, 2x4 studs
Front walls: brick, 1" gap, felt paper, plywood sheathing, 2x4 studs

My plan (subject to change)

Attic: spray foam attic roof line and gable ends to between R19-R25
Band Joists: spray foam
Walls: seal with caulk or foam base plate, t-walls, windows, top plate, penetrations, and possibly interior studs and gaps in sheathing, BAtt installation.
floors (non attic) R-19 batt unfaced for radiant heat above sub floor

My concerns:

Moisture, vapor, condensation concerns as a result of my insulation plan...Any advice?

Will open cell provide enough protection from vapor/humidity concerns or should I just do bite the bullet and do the closed cell?

My garage will be heated only enough to take the chill off 55 degrees +/-. Should I foam the common wall or batts will be enough

Above the garage is set up for future living space (at least 5 years out maybe much more).  When I insulate the garage should I spray foam (and what type)  to eliminate air/fume invasion into the space above? If batt, which way should the facing be directed (toward or away from the garage)?


Thanks for your advice and comments

With open cell on the gable ends you'd need an interior-side vapor retarder (vapor retardent primer on half-inch sheet rock is good enough) in your climate. On the underside of the roof deck 2" of closed cell, with the rest being spray-cellulose or high-density JM Spider works protects the roof deck (probably even 1" of ccSPF would be fine.) If 2lb polyurethane is too pricey for you, the newer 2lb R5/inch Icycnene is cheaper per unit R, with a permeability low enough to protect the roof deck from winter accumulation of moisture at depths of 2-3" (spray polyurethane is R6-6.5/inch).

It looks like 1" of closed cell + high density fiber or 2" of 2lb Icycnene + hd. fiber would make it in your climate:

http://www.buildingscience.com/documents/reports/rr-1001-moisture-safe-unvented-wood-roof-systems

Check the table on P13 for comparisons- you winter conditions are only somewhat warmer than maritime Boston, which is what I'd use as the closed comp. They're using 1.8lb Spider for the spray fiberglass in this sim, which is pretty comparable in many ways to 2-3lb cellulose (but doesn't buffer the moisture the way cellulose will.)

Unless your basement walls are insulated and well sealed from exterior air there is some potential for hitting the dew-point at times on the cold edge of the joists with R19 batts in the joist bays. Controlling the basement relative humitdy to 60% maximum will keep it from becoming a problem though.

For any wall system batts (of any type) are truly second or third rate compared to moderate-density blown or sprayed fiber, or sprayed foam. Medium density spray cellulose allows only 10% of the cavity convection of standard-density batts, even if perfectly installed. Dense-packed 3lb cellulose or 1.8lb Spider allows only ~2%- they are close to being an air barrier. (They are air barriers from a thermal point of view, but not from moisture-transport perspective.) Batts require perfect installation and seperate air-barrier systems to perform anything like their rated R across the full seasonal temperature swings you'll see in NJ. If you have open studs to work with, spray-cellulose at low density will usually be your best bang/buck, followed by dense-packed "blown in bag" cellulose. But compare prices with 1.5 SPF or 2lb Icyenene just to be sure.

In the event you have a roof leak, if using closed cell spray foam on the underside of the roof deck, the leak would be difficult to locate the location since the closed cell foam wouldn't allow the water to pass through the foam... this could also cause the roof deck to rot since it could be a long time before the leak is detected. Isn't this true? If using only 1" of closed cell spray foam, maybe this isn't the case? I have read to use open cell spray foam on the underside of roof deck.

Are you sure you have a 4a or even 5a climate zone near or in central NJ?

According to Figure 3: DOE climate zone map in the building science article "moisture-safe Unvented wood roof systems" I am in zone 4a just a hop skip and a jump from 5.

Is there a different map I should be referencing?

Thanks to those above for the info. What about the garage ceiling since it will have living space above eventually?

I see, looks like they are still in proposed mode. They need to address the issue that many people refer to the similar USDA zones as "climate zones". Use letter then number or something.

The use of closed cell spray foam on the underside of a roof will not cause a roof to rot. The myth about leaks in a roof being hard to detect with spray foam was created by the fiberglass industry as a deterrent from using a superior product. If concerns exist about foam on a roof deck, then a thermal infrared scan will quickly determine if there are any problems. The foam sticks so well to the substrate that even if there were a roof leak, the moisture is not going to be able to accumulate. Moisture would sit on top of the foam and quickly evaporate once the sun warms up the roof. Spray foam will actually help prevent a roof from leaking, it improves the structural strength of the roof, it bonds everything together so there is less movement of the roofing materials.

Posted By insulateright on 08 Aug 2010 09:51 PM
The use of closed cell spray foam on the underside of a roof will not cause a roof to rot. The myth about leaks in a roof being hard to detect with spray foam was created by the fiberglass industry as a deterrent from using a superior product. If concerns exist about foam on a roof deck, then a thermal infrared scan will quickly determine if there are any problems. The foam sticks so well to the substrate that even if there were a roof leak, the moisture is not going to be able to accumulate. Moisture would sit on top of the foam and quickly evaporate once the sun warms up the roof. Spray foam will actually help prevent a roof from leaking, it improves the structural strength of the roof, it bonds everything together so there is less movement of the roofing materials.

"...quickly evaporate once the sun warms up the roof" could be months in waiting in this climate zone!  Winter snow pack on the roofs can linger well into the spring rainy season- don't count on that drying mechanism to save you here.  In low/no snow zones, yes, it means something, but not as much as you might think.  Without a detectable drip minor roof leaks can go on for years, giving ample time for damage to occur through hundreds of wetting/drying cycles.

With closed cell fo roof leaks may be more difficult to detect, and roof deck damage can/will occur, but there's scant real-world evidence that roof deck damage from roof leaks is more frequent or extensive on roof decks insulated closed cell foam vs. open cell foam or even dense-packed cellulose cathedralized ceilings.

But there IS evidence that ventilated roof decks are inherently better protected.   When taking an unvented roof deck approach using spray foam you're balancing the potential cost of future repairs from leaks (which will tend to be very localized) against the cost of extra construction detailing to achieve roof deck ventilation.  Done from the interior that can be a LOT of detailing.  A cheaper/easier alternative opportunity happens we when it's time to re-roof (or in new construction):   Applying a secondary nailer deck mounted on purlins or furring for the new roofing can provides a ventilation gap above the structural roof.  Using a back vented nailer decking approach also keeps both the structural roof deck & the roofing material MUCH much drier, and the peak temps cooler, maximizing their life-cycle.

Even better (but definitely not cheaper) is to use panelized nailbase  iso or EPS insulation product (eg Hunter Panels, one of many vendors of similar product, some with back-vented nailer deck) installed on the exterior above the structural roof deck.  Adding insulation above keeps the structural roof & rafters warmer (lowering or eliminating condensation potential), and drier, so long as the interior closed-cell spray foam is limited to 2" or less to allow adequate drying toward the interior.

Posted By Dana1 on 09 Aug 2010 11:28 AM


But there IS evidence that ventilated roof decks are inherently better protected.   When taking an unvented roof deck approach using spray foam you're balancing the potential cost of future repairs from leaks (which will tend to be very localized) against the cost of extra construction detailing to achieve roof deck ventilation.  Done from the interior that can be a LOT of detailing.  A cheaper/easier alternative opportunity happens we when it's time to re-roof (or in new construction):   Applying a secondary nailer deck mounted on purlins or furring for the new roofing can provides a ventilation gap above the structural roof.  Using a back vented nailer decking approach also keeps both the structural roof deck & the roofing material MUCH much drier, and the peak temps cooler, maximizing their life-cycle.

Even better (but definitely not cheaper) is to use panelized nailbase  iso or EPS insulation product (eg Hunter Panels, one of many vendors of similar product, some with back-vented nailer deck) installed on the exterior above the structural roof deck.  Adding insulation above keeps the structural roof & rafters warmer (lowering or eliminating condensation potential), and drier, so long as the interior closed-cell spray foam is limited to 2" or less to allow adequate drying toward the interior.

What if you had energy baffles run all of the way, from the soffits to the ridge vent, then spray foamed on the underside of the baffles? This would allow for an air gap on the underside of the roof deck to allow it to stay cooler and dry.

With interior-only insulation, interior side air baffles would work, provided they were rigid enough to not be distorted/compressed by the SPF, with both soffit & ridge venting in place. If you can guarantee the ventilation gap stays free, a vented open-cell SPF would likely keep the roof deck driest. From a detailing point of view a double-deck roof with furring at every rafter is an easy more rigid & reliable ,moral-equivalent of an interior side baffle, eh?

But from a thermal performance POV it does nothing to provide a thermal break over the rafters, ergo my preference for unvented designs using exterior-side sheet goods, which puts all of the structural members at least partially inside the thermal boundary of the envelope. You get more real R out of your insulation that way. In your climate zone, with at least 35-40% of the R is on the exterior, you can use the much cheaper fiber-goods on the interior, with nothing more than standard latex paint for a vapor retarder. If you go with cellulose on the interior you can probably reduce that to 30% of the total R on the exterior since the cellulose can buffer substantial amounts of seasonal moisture without damage, wicking any incidental condensation away from structural wood.

A lot of attention is paid to vapor-retarders & condensation issues, but in the real world the much larger issue is controlling air-leakage, since air leaks move orders of magnitude larger amounts of moisture. By putting more R on the exterior, the assembly becomes air-leak tolerant, since the condensation potential at the outermost structural member drops dramatically. Conversely, making the interior highly vapor retardent dramatically reduces the ability of the assembly to dry (from incidental roof leaks, or seasonal condensation. Using exterior insulation (especially with ventilated nailer deck) makes the whole assembly much more resilient. SPF is great for sealing air leaks but it's a premium product at a premium price, and quite expensive when going high-R, with great potential for blocking drying- use appropriately & carefully.

Features like ventilated nailer decks for roofs or 3/8" rainscreen gaps between the siding & sheathing significantly enhance outward drying independently of snow loads, rain on shingles or rain-splash on siding. For instance, in your climate zone with a rainscreen ventilation gap you don't need anything more vapor retardent than standard latex (a class-III vapor retarder) on the interior of your wall stackup, due to the enhanced exterior drying aspect. See:

http://www.greenbuildingadvisor.com/sites/default/files/Table%20402.5.1.jpg

http://www.greenbuildingadvisor.com/blogs/dept/musings/vapor-retarders-and-vapor-barriers