Attic instulation and storage improvement questions

[Greebo]

1) My attic hasn't got many soffets - maybe none - but it does have two vents in the actual roof (not ridgebeam) - one is a fan the other is just open air. There might even be 3 vents, I need to poke around more. Will that do for ventillation? I dont see any problems in the attic, so I think so, but I'm not 100% certain.

2) Right now, we have about 3-4 inches of loose fill insulation in the attic. I want to build up the joists in the attic by 6" to support 3/4 plywood for storage and then fill it with loose fill cellulose. Should I run the new floor supports parallel or perpendicular to the existing joists? The plan is - build up the floor 6", fill to level with loose fill insulation, cover an area with plywood and use the attic as storage.

3) Would I gain much by adding insulation to the underside of the roof or is it not worth it if I'm already adding enough insulation to achieve R-38?

 

[JohnR]

What is your roof stucture? If it's pre-engineered wood trusses, I'd recommend asking a truss manufacturer to help you with it. If the trusses were designed for attic storage loads, all you need to do is nail down the plywood and go; if they were designed NOT for storage, they'll benefit from reinforcement. A truss manufacturer can tell you how to do that. Your local building department MAY have copies of the original truss drawings on file, from when the building permit was issued.

The attic ventilation requirements depend on your climate and local building codes. In Florida, there's a formula to determine the total area for vents. What you do NOT want to do is insulate up under the sloping part and have it vented - defeats the purpose of the insulation. R38 should be more than adequate at the attic "floor".

And that's my free professional advice.

John Reep, Architect (FL 0014860)

 

[Greebo]

The house is a fairly standard colonial style built in 1940 in Maryland. The trusses are not engineered. The purpose of building up the joists is to allow for the added insulation with plywood on top for storage. There's only 3" of insulation up there now, so the storage area is going to get built up either way to make room for the insulation under the plywood. The question is whether I should build up perpendicular or parallel.

Personally I'm leaning to perpindicular. The existing joists run front to back, and there is only one wall on either side of the house which runs perpendicular to them. I'm thinking that by running perpendicular joists across the top in the area I intend to use for storage (ie: the attic center, call it oh 15 by 30), it will be much stronger than if I ran in parallel.

On the venting - as I said, there are currently two vents, both on the west side of the roof, with the fan vent near the ridge and the other roof vent about halfway along the roof from the fan, halfway up the roof. The result is the main airflow in the attic is following a path of about 15 feet before leaving.

I'm thinking about adding two gable vents closer to the base of the attic, one at either end, and closing off the existing vent, so that the fan has to pull air through the whole of the attic.

 

[JohnR]

Your best bet would be to find a structural engineer to take a look and give some advice. There are too many variables, like the span, member size, spacing, and so on. You may be right that joists 16" on center perpendicular to the ceiling members would spread the load pretty evenly, but without knowing details I can't say for sure.

Do you have eave vents (is this what you mean by soffits? The roof overhang that shades the outside walls) If not, then adding a gable vent is the best way and more leak-"resistant" than a hole in the roof. Even older houses usually have holes drilled in the eaves to let air in the low part of the attic, to be let out up high.

 

[mikea]

The current "Ask This Old House" has Tom Silva installing a solar-powered gable fan to help get rid of condensing moisture in the attic. He says they need two. He also tells the homeowner to run a second layer of insulation 90 degrees to the joists, after they rearrange the lower batts to fill in the voids.

I know that doesn't mesh with your desire to put in the plywood floor for storage but my vote is to run your support joists 90 degrees for the area you want the storage platform.

 

[Greebo]

As far as I can tell, no there are no soffit vents (you call them eave vents, DIY and Lowes and Home Depot call them soffit vents, me, I just can't spell...)

As for bringing in an engineer - if we were going to convert this to living space, yeah, I'd do that. But we're just going to be adding some storage space for smaller and lighter items that we can manage to lift up an attic ladder.

 

[Greebo]

The current "Ask This Old House" has Tom Silva installing a solar-powered gable fan to help get rid of condensing moisture in the attic. He says they need two. He also tells the homeowner to run a second layer of insulation 90 degrees to the joists, after they rearrange the lower batts to fill in the voids.

I know that doesn't mesh with your desire to put in the plywood floor for storage but my vote is to run your support joists 90 degrees for the area you want the storage platform.

We've considered batting vs. loose fill, and while I'd prefer the batting for convenience, the loose fill has a better R per inch value and is more cost effective. (So I understand, anyway...)

I don't have a moisture problem in the attic and I know that more insulation will help keep that from being a problem, but the attic gets dangerously hot in the summer, to the point that the attic fan runs all night, which is why I think more venting is called for.

 

[JohnR]

Attics without storage are designed to 10 lbs./sq. foot live load, attics with storage are designed to 20 lbs/sq. ft. (Florida Building Code).

I don't think you need to hire an engineer, but if you happen to know one he could take a look at your attic and probably tell right away if it makes sense. Buy him a 6-pack or something.

 

[Skip Miller]

I've done what you are thining about, Chuck. First, do NOT put insulation on the underside of the attic roof. Insulation belongs as close to the heated space as possible. When you lay on more insulation, regardless of the type you choose, put some kind of vapor barrier (I used Tyvek) on the hot side of the insulation.

I chose to run my new floor joists perpendicular to the existing rafters. I thought the new floor would be more stable that way. Then, after adding batts of insulation, I decked it with plywood.

-Skip

 

[Greebo]

When you lay on more insulation, regardless of the type you choose, put some kind of vapor barrier (I used Tyvek) on the hot side of the insulation.

This is contradictory to everything I've read/seen about laying insulation in the attic, especially with adding to existing insulation.

 

[JRitt]

Chuck. For ventilation you should have 1sqft of ventilation for each 300sqft of cealing area. so if you have a 1500sqft house you need 5sqft (720 sqin) of ventilation split into 2 with half in the eaves (soffit) and half in gable venting. Adding soffit ventilation is easy. If you need more in the roof you can add luvered vents. When you measure the vent size multiply it by .66 to compensate for the screen.

For insulation in your attic and I would blow in insulation (the blower is free at lowes wih the purchase of insulation) to bring the depth up to 12 inches (R40) and because you want to put storage in the attic you need to have the storage floor above the insulation level I would install 2x12's and nail them next to the current 2x4s (use a nail gun to minimize damage to the ceiling drywall). where the attic access is biuld a box around the opening to keep the insulation from falling in the house when it is open. If you blow in insulation you will need to install rafter baffles where you have soffit vents to keep the blown insulation from blocking the flow of air.

If you can add a vapor barrior against the ceiling (by moving the old insulation away and putting in a 4 mil plastic or better vapor barrior)it will help keep moisture from exiting the living area and going into the attic but do not put a vaper barrier between insulation lawers in the attic. This will cause moisture buildup under the barrier and cause mold and other moisture damage to the ceiling

EDIT: since your ceiling joist run is on the long section I would run 2x8's perpendicular.

 

[Skip Miller]

This is contradictory to everything I've read/seen about laying insulation in the attic, especially with adding to existing insulation.

I should have mentioned that the attic I treated this way had no vapor barrier in the original installation. You are right... you don't want two vapor barriers. And the Tyvek, being somewhat permeable to vapor, was more of a draft barrier than a vapor barrier. So I really shouldn't have called it a vapor barrier. Sorry, long time ago and typing too fast.

-Skip