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How to design & vent deck-over-living-space?


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Greetings from the wet left coast of Canada (Metro Vancouver).

Working on the design for a new attached deck on the back side of my house. Deck will be enclosed underneath (and become my garage/shop).

Current burning question is: how do I design the deck to provide proper ventilation (along with insulation)?

The deck walking surface will probably be Duradek-type roll vinyl (which meets code around here for an exterior membrane over living space). The interior ceiling would be drywall, with surface mount lighting (not pot/recessed lighting). No ducting above the ceiling; just some electrical wiring.

I would like to insulate the ceiling, of course, and imagine that it would simply involve fiberglass insulation batts between the deck joists.

So, do I build an unvented, sealed space roof/deck, or should it be vented? If vented, then where/how, etc.?

Thanks for your ideas!

Randy

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Will the deck have overhangs? That is, will the joists overhang the walls at each end? They should have about 2 feet to keep your walls dry.

I often see perforated soffit covers installed under those overhangs. Good or bad, that is what we see here. A bit of air can flow through the joist cavities that way.

However, if you take Kurt's advice and use spray foam, there is no movement of air, no cavity, and therefore, no moisture.. unless the deck develops a leak.

Also, it is best to give that deck just enough slope to prevent puddles on your nice new Duradeck. Puddles leave dark spots when they dry and puddles at a seam are bad news.

Finally, you might want to install gutters, so consider that when you install the deck cover. if you don't add gutters, water will drip off the deck at random places and you end up with green slime growing there.

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What a great forum! Jim and Kurt, thanks for your quick replies. I've done some more digging around the internet and see that you opinion is common knowledge among the informed. I've assembled a set of documents and will share a couple snippets from them in a post; perhaps you'll be able to help me with applying them to my situation.

Great to be able to hear from experienced building inspectors [:-thumbu]

Randy

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John, thanks for the note. I see we're in essentially the same climate (as is Kurt, too).

Will the deck have overhangs? That is, will the joists overhang the walls at each end? They should have about 2 feet to keep your walls dry.

I'm hoping to use virtually no overhang on this "box" additioni/extension; just enough to have proper rainproof overlap and flashings. The house has horizontal cedar siding, so would use the same on this extension, or perhaps cedar shingles as a contrast.[?]

I often see perforated soffit covers installed under those overhangs. Good or bad, that is what we see here. A bit of air can flow through the joist cavities that way . . . However, if you take Kurt's advice and use spray foam, there is no movement of air, no cavity, and therefore, no moisture.. unless the deck develops a leak.

Right. That's what was installed over the original stucco soffiting on the main house. But with no overhangs on this additioin, no soffiting. My readings so far suggest that I would have very little air flow in this addition from overhand/vented soffits anyway. The joists would run out perpendicularly from the back of the house. Each air space between pairs of joists would be sealed against the house and only available for venting at the opposite end. No way to have thru-flow.

Also, it is best to give that deck just enough slope to prevent puddles on your nice new Duradeck. Puddles leave dark spots when they dry and puddles at a seam are bad news.

Right. Building codes seem to give ample guidance on how much slope to build in. My current deck suffers from puddling in places (sagging here and there over the years). And puddles collect not just water but all the dirt; when the water dries, you've got quite an accumulation.

Finally, you might want to install gutters, so consider that when you install the deck cover. if you don't add gutters, water will drip off the deck at random places and you end up with green slime growing there.

Agreed. Again, on my current deck, the water runs off the end opposite the house (the direction of the slope), but especially in the low area, and it's rotting the "facia"? board that runs across the end of all the joists. And of course "green factor" is a constant reality in this rainforest! I had planned to use a gutter along this one side (at least), but also to clad the facia boards all round the deck in enamelled tin. I might still get some slime if I leave it long enough between cleanings, but at least it won't rot soon. [;)]

Randy

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2% slope is required by code, cross venting is achieved by perpendicular strapping, you need cross air flow. if the deck is being built to a 90 degree wall there is a way to have a deck vent out of a box mounted on top of the deck that looks similar to a base board heater. Add a 6'' overhang to allow venting and keep that wall dry

From Victoria

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Thanks, Lee. "Cool" way to achieve cross-flow ventilation in the joist space of my "deck/roof". And I haven't fully decided on overhang yet.

But here's what I've found so far regarding venting, and it concurs with Jim and Kurt. (I understand that what applies to "attics" and cathedral ceiling roof assemblies applies to flat roof assemblies like I would be building). Enjoy . . . and hope to hear from people what they think [:-wiltel]

http://www.strunk-albert.com/index2.php ... &Itemid=58

Ventilated Attics and Un-Vented Roof Assemblies

David B. Strunk, P.E., C.E.M. Strunk-Albert Engineering

Attic ventilation is common practice, and if installed properly, will function without a problem. However, attics and roofs can be designed and constructed without ventilation. In fact un-vented roof assemblies, such as conditioned attics and un-vented cathedral ceilings, are generating more interest and are becoming a more common construction technique. The decision to vent or not to vent is a design and construction choice not a requirement by the building code. Model building codes allow both vented and unvented roof assemblies.

From the International Residential Code (IRC) 2009:

SECTION R806

ROOF VENTILATION

R806.4 Unvented attic assemblies. Unvented attic assemblies (spaces between the ceiling joists of the top story and the roof rafters) shall be permitted if all the following conditions are met:

1. The unvented attic space is completely contained within the building thermal envelope.

2. No interior vapor retarders are installed on the ceiling side (attic floor) of the unvented attic assembly.

3. Where wood shingles or shakes are used, a minimum 1/4 inch (6 mm) vented air space separates the shingles or shakes and the roofing underlayment above the structural sheathing.

4. In Climate Zones 5, 6, 7, and 8, any air-impermeable insulation shall be a vapor retarder, or shall have a vapor retarder coating or covering in direct contact with the underside of the insulation.

5. Either Items 5.1, 5.2, or 5.3 shall be met, depending on the air permeability of the insulation directly under the structural roof sheathing.

5.1. Air-impermeable insulation only. Insulation shall be applied in direct contact with the underside of the structural roof sheathing.

5.2. Air-permeable insulation only. In addition to the air-permeable installed directly below the structural sheathing, rigid board or sheet insulation shall be installed directly above the structural roof sheathing as specified in Table R806.4 for condensation control.

5.3. Air-impermeable and air-permeable insulation. The air-impermeable insulation shall be applied in direct contact with the underside of the structural roof sheathing as specified in Table R806.4 for condensation control. The air-permeable insulation shall be installed directly under the air-impermeable insulation.

http://www.greenhorizon.com/files/JLC%2 ... 20Foam.pdf

Insulating Unvented Attics With Spray Foam

Closed-cell polyurethane foam provides the insulation, air barrier, and vapor retarder necessary for an unvented attic assembly

by James Morshead MARCH 2007 I JLC I 1

Provisions for Unvented Attics

Every state except California and Hawaii has adopted some version of the IRC. And California is expected to adopt it in 2008. Until recently, the IRC required all attics and enclosed rafter spaces to be vented. But the latest version allows unvented attic assemblies if certain conditions are met. According to Section R806.4 of the 2006 IRC, unvented assemblies are allowed if "no interior vapor retarders are installed on the ceiling side (attic floor) of the unvented attic assembly" and if "air-impermeable insulation is applied in direct contact with the underside/interior of the structural roof deck."

Space below the foam: most were taught that it's bad to leave an air space below insulation. This is true of fiber insulations because convection currents can form in gaps and degrade the insulation's thermal performance. But it is not true of foam, which can't be infiltrated and is relatively unaffected by surrounding air currents. Any space left below the foam is considered conditioned space (Figure 6).

http://www.buildingscience.com/document ... oof-design

Roof Design

Research Report - 0404, 2004. Joseph Lstiburek

Abstract: Roofs can be designed and constructed to be either vented or unvented in any hygro-thermal zone. Air barrier systems are typically the most common approach, however, air pressure control approaches are be-coming more common especially in cases involving remedial work on existing structures. Vapor diffusion should be considered as a secondary moisture transport mechanism when designing and building roofs. Specific vapor retarders are often unnecessary if appropriate air movement control is provided or if control of condensing surface temperatures is provided.

Randy

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