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Putting floor down in attic

4K views 25 replies 8 participants last post by  concrete_joe 
#1 ·
This may be a really dumb question so bear with me. I'd like to put down a floor in my attic to increase storage space and ensure my wife doesn't fall through the ceiling when shes up there :laughing:

so i was just going to throw down 3/4" plywood across the joists. i guess my question is, can i do that without causing any issues related to the insulation, ventilation, etc?

i guess i'm mostly concerned about ventilation since i dont really understand how attic ventilation works...

thanks and sorry if this is a dumb question!

-marc
 
#2 · (Edited)
Very, very good question. This is what happened to me.

The first, is be sure you can have flooring. In my case, I have 2x6 joists in the attic, my house is fat, and my ceiling is plaster (heavy). Case in point, my 2x6 joists could not support a full 3/4 floor for its entirety else it would sag I could only put an attic floor where there were supporting walls underneath which in my case was pretty much only the middle (down the hallway).

I air sealed and insulated (I have 6" insulation between the joists kraft face down, then 10" fiberglass on top of that (unfaced) for 16" in total. For the flooring I put in some 2x12's so it sits above all my insulation and put the 3/4" floor on top. I was pretty darn surprised after a few years how warped the plywood was... there was a lot of condensation that forms underneath it in winter. Ironically I had placed a piece of cardboard over the insulation on the side of my partial flooring and lifting it up one winter to recycle I saw it had water droplets frozen underneath. I don't know if it's the amount of insulation I have or what's doing it.

I wish I knew the answer, I'm sure it depends on where you live and possibly the more insulation the more pronounced the issue. I live in a ranch with a wood stove and get my wood for free so I keep my house at 72-74F in winter and it's typically 20F outside in winter. My house was built in 1962 as well so it's not particularly air sealed though I did air seal the attic before adding so much insulation.

I don't know what the answer is, all I can say is me covering my attic floor with 3/4" plywood and seeing how warped my plywood floor is now ventilation/condensation is certainly an issue. I'm glad I used those little clips that can slide between plywood that keeps the edges of the plywood level if there's warping (as there's a whole lot of warping up there).
 
#3 ·
First off I don't know what you "air sealed" but if it was the attic you did the wrong thing. Here is how attic ventilation works. Picture your LIVING spaces below as a thermos bottle, sealed and insulated on the sides. Then in goes your attic insulation ( which you put down correctly,...but maybe not not enough depending on your zone. THEN there comes the UNINSULATED attic area. Should be vents along the soffits and along the. Ridge. Air enters through the. Soffits and exits through the ridge freely, no sealing of anything. The air flow handles moisture issues, adds life to your roof shingles, etc. in the very old days, people tried to get by with just a vent on each gable end but that doesn't work so well. Ron
 
#6 ·
Here is how attic ventilation works. Picture your LIVING spaces below as a thermos bottle, sealed and insulated on the sides. Then in goes your attic insulation ( which you put down correctly,...but maybe not not enough depending on your zone. THEN there comes the UNINSULATED attic area. Should be vents along the soffits and along the. Ridge. Air enters through the. Soffits and exits through the ridge freely, no sealing of anything. The air flow handles moisture issues, adds life to your roof shingles, etc. in the very old days, people tried to get by with just a vent on each gable end but that doesn't work so well. Ron
i recently had a new roof put on and i seem to remember the guy saying something to me like ridge vents are now either against building code or not recommended by shingle manufactures so they covered it up.

i do have vented gables on either side.
 
#4 · (Edited)
5/8" ply probably suffice here. is the attic already insulated? you dont want to squash down insulation, doing so reduces R value. example, squahing down R30 into a R19 space can bring R value lower than R19, etc. to avoid squashing i would perhaps replace any insulation that would be squashed with either properly size glass or loose blown type, (or reduce size or existing), etc.

the ply itself is a small R value. and as noted, unless you install a active dehumidifier in that space its best to have proper ventilation. if you need to seal things off because of what you are storing there then a dehumidifier is likely needed, etc.
 
#5 ·
yes there is insulation but its so old and disgusting that i'm replacing it completely. probably only 2 inches right now.

i'm not planning to do the entire floor - centainly not all the way to the edges where i would block the soffits. i'd probably leave like 6 feet of open space on either side. that should be fine right?
 
#7 · (Edited)
I air sealed the ceiling of the living space from the attic side (thought of another way the floor UNDER the joists in the attic). The more insulation one adds the more important it is that you seal air leaks before adding the insulation. I have more than usual attic ventilation because when redoing my roof I added full soffit and ridge ventilation plus the house had gable peak ventilation when built so I have both now. I've had no icicles come winter.

I see my attic floor is air sealed best I could, my attic has better than usual ventilation, and has more than usual insulation. Yet, the 3/4" plywood I added so I had a floor up there is warping from condensation. I think it has a lot to do with the amount of insulation I have which likely is so much I've moved the condensation point outside my insulation and it's now the plywood floor I added but I'm not sure.
 
#9 ·
so, you could use R30 as a redo, and then peel down the R30 so that the ply doesnt crush it, etc.

the insulation in attic should extend all the way to the ends of the bays, but use a soffit air vent against the roof to allow air flow. you definitely want air flow unless you seal it completely and add both temp and humidity controls, etc.
 
#12 ·
Nothing wrong with ridge vents in MA. HOWEVER, there limitations as to how much of the required ventilation can be provided by same. That is, JUST a ridge vent is not acceptable as there must be balance in the overall ventilation system. That is, New recommendations have something like between 40 and 50% of the ventilation can come from either ridge vents or gable end vents near the peak with the rest coming from the lower portion such as soffit vents. If your roofer said against code, he was not correct. Ron
 
#24 ·
One question you have an answer yet is do you have Soffit Vents? That would be movers in your eaves, you should be able to see them from outside and you also need to make sure they are covered with insulation on the inside.
 
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