energy efficiency and plywood for attic floors

[birgco]

I was thinking about some new construction framing details and (IMO),
the importance of capping the second floor ceiling joists /attic floor joists with a layer of solid plywood before the roof is framed. There are several benefits to using this method of construction including improved energy efficiency, virtually eliminating ice damming, hurricane and
high wind resistance, adding more usable attic space, and greatly countering the tendancy of the roof to exert an
outward force on the exterior walls.
I believe if this method was required code for all new construction and additions, we would greatly improve the quality of newly constructed buildings. The additional cost per building would be approx. $2000-3000 but spread out over the life of the structure, it should be worth it, especially with potential energy savings. It doesn't seem to make much sense to insulate an attic floor with nothing as a backing for the insulation . An enclosed cavity (plywood backed) should greatly improve energy efficiency. Don't know if this method is standard in other areas?

[phansford]

A couple of issues here -

1. Most roof systems are pre-engineered trusses. (At least in this area) In otherwords, the ceiling joist is the bottom chord of the truss. So adding a plywood floor would be fairly impossible. You would be cutting the plywood into 24" strips and cutting around all of the diagonal truss members. I am also concerning about creating an unvented, unconditioned space between the batts and the plywood.

You COULD use an attic truss, which are designed to carry a floor load. The bottom chord is typical a 2x6 or 2x8. Then you add the floor decking. This is very common.

2. What does the sheet of plywood add to the thermal qualities of the assembly? Why or how do you think it will add to the thermal envelop. Properly installed - with eave baffles - insulation (blown or batt) will stay in position and provide your thermal control. Attics are just that - they are vented - unconditioned spaces that are not intended for storage or habitation. (Thus the creation of the Attic Truss - allows some access and usable space - and properly designed to take loads).

Explain in more detail your thoughts?

[csintexas]

Certainly it would add something to the rigidity of the structure but I don't see that as a major issue and it would be more cost effective to brace walls better. For high wind resistance the key is to have a strong connection between the roof and the walls (hurricane clips, etc.) and I don't think decking the attic floor joist would accomplish this. Plywood has very little R value and trying to get an air tight seal in most attics would be very difficult.

In a conventionally framed structure the ceiling joist depth does not allow for enough insulation. Generally unsealed attics are not good for storage because of the extremes in temperature and humidity. Ice dams should not be a problem in any well insulated structure. A properly framed roof will exert force downward and not out.

There are many better things we could spend 2-3 thousand dollars on.

[kmapro]

Around our parts, inspectors will not allow blown in insulation in areas where there is floor covering above.

I don't think many people around here would go the extra mile for the decking when they could easily have the convenience of the blown in insulation.

I like the idea of a deccked celing, but most ceiling joists are not "designed" for carrying much load at all. We assume a 20/10 load pattern where access is sufficient enough for light storage. By adding the plywood, you might give off the impression - to the home owner - that they can store more and more crap out there....then you have a safety factor involved.

[birgco]

Ok, some great points, here are my thoughts in further detail....

-the attic floor joists/second floor ceiling joists are designed from scratch to be a minimum of 2x12 DF or 2x4 spf chord trusses of greater height if desired.

-what bothers me about an attic floor (open on top with sheetrock on the bottom) is that you wouldn't design an exterior wall that way, would you?. Ever place your hand on a kneewall with just insulation stapled between the studs with no solid backing on the exterior of the studs? It's feels about the same temperature as the outside winter air. It seems a similar condition is created with an attic floor "open" on top. It would be an interesting test to measure heat loss with an open attic floor versus a closed (plywood covered floor). Creating a enclosed cavity, I think, is an important consideration.

-I lived in a house with this design for seven years and found the attic to be extremely functional for storage and access to mechanicals. (I wouldn't store chocolate there in the summer, but with a radiant panel on the underside of the roof and an attic fan, summer temps are minimized. The key is good attic ventilation.

Another point, when you think about constructing a 2ft x 2ft. plywood box, think about the rigidity of one built with 4 sides of plywood vs. one built with 3 sides?

[phansford]

You need to think of the entire assembly. You seem to be thinking of separate components of the assembly. The joist/truss is the 4th side of the box, which is further stiffened by the gypsum board ceiling and the plywood roof decking. Adding the plywood at the top of the bottom chord or the top of the ceiling joist does not necessarily add to the assembly's bracing capacity.

You can actually make a structure too stiff and thus causing problems with the structure not being able to move as needed when a load is applied. ie - If a building is too stiff and can't move when a wind load is applied, you can actual have the member failure. Or think of it this way - The John Hancock Tower in Chicago sways (per design) 36" in either direciton. If it was stiff, the building would experience internal stresses.

A flexible tree bends, but does not break - an old stiff tree splits and falls.

[birgco]

phansford,
I agree with your point about the skyscraper, but I don't see how it applies to a 32 ft. tall house.
Consider a solid stone house. I have seen some with standard sized rafters and joists (some with substandard) and have never heard of one having a structural failure. Such a constructed house could not move or flex a fraction of an inch under wind or other stresses.
Also have experienced 70 mph wind gusts in a "plywood boxed" house and was glad to be there.
Forgot to mention another benefit of designing a structural attic floor in new construction; inexpensive future expansion potential.
And I'm still thinking about the energy efficiency question regarding open air space above the ceiling/attic insulation.