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honusbam
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# Posted: 30 Mar 2011 11:41pm
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There is a lot of good information on this website regarding the construction of floors. However, to a noob (me), the information can be confusing, scattered, and overwhelming. So, based on self-interest, but also assuming that I am not the only one that has ever felt the same, I would like this to be the ultimate guide to building & insulating a wood-framed floor.
I welcome all your thoughts, advice, comments, lessons learned, etc... to help make this the last stop for anyone searching for help in constructing their wood-framed floors. For sake of discussion, let's assume worst-case scenarios (very wet environment, critter problems, very cold environment).
First off, this website gives a good introduction to building a floor using pressure-treated lumber. However, there is some information I wished this discussed in more detail.
For a cabin smaller than 200 sq.ft. (a.k.a. 'small cabin'), should we:
- use 16" or 24" spacing?
- use 2"x6", 2"x8", 2"x10", 2"x12"?
What about insulation? I have read that 2" foam insulation in the floor provides little to no R value. I have also read that if you don't enclose insulation underneath you will fall victim to critters making a new home using your insulation.
So, what is the end-all solution to insulating a new floor with a high R-value and also eliminating the possibility of critter damage? My thought is to use R30 batts in either a 8" or 10" floor, sealed on top with a 5/8" pressure treated sub-floor. For sealing underneath, pressure treated sheeting as well. As a final touch, covering floor sides and underneath in aluminum.
I welcome your thoughts and advice to make this the last stop for all us newbies who need to learn from you. Thanks!
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MikeOnBike
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# Posted: 31 Mar 2011 12:09am
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Quoting: honusbam I have read that 2" foam insulation in the floor provides little to no R value.
I had not heard this. This is exactly what I did for our 120sq ft. shed. Can you provide more detail on this line of thought? I'm curious because I planned on using it on my next shed project this spring.
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honusbam
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# Posted: 31 Mar 2011 12:17am
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I am certainly no expert, and I guess I was stating opinion over fact. I also read bobrok's post. I guess I should restate and say that 2" foam does have an r-value, but maybe when compared with R30 batting there is a big difference. Anyway, I really don't know, just basing my opinion on the opinion of others, hence this topic to remove all doubt. ;-)
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smitty
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# Posted: 31 Mar 2011 12:18am
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Just my opinions and thoughts, as I am thinking about this stuff as well for my own plans..
If the wood is not getting wet. I see no need for pressure treated wood. Especially sub flooring. As long as it stays dry.
Insulating the floor. I plan, to insulate with batt insulation. with a vapor barrier under it. The way they do mobile homes.
But to keep critters out, digging 12 inches around the foundation, and skirting, going below the ground surface, should give plenty of protection, against digging under, or chewing threw it.
They would have to tunnel 1 foot down and up 1 foot to get it. If that happens. A gopher trap would take care of them when they come out the next time.
Just things I have been planning. Look forward to opinions..
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honusbam
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# Posted: 31 Mar 2011 12:23am
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Quoting: smitty If the wood is not getting wet. I see no need for pressure treated wood.
I am not sure either, although this page says "I would stress again that you must use pressure treated timber for the cabin floor frame and subfloor sheeting. (And obviously support beams if you decided to go with this type of foundation/flooring structure)."
My thought is that while it may not get 'wet', there will still be moisture, whether it is from the ground 'sweating' or whatever it might be. This moisture/condensation will lead to rot & mold, hence our grand-wizard's suggestion to use pressure-treated throughout the flooring.
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MikeOnBike
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# Posted: 31 Mar 2011 12:38am
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Yes, the 2" foam is only 1/3 of the R30 batt. For a full on cabin I would probably do at least an R20+ floor, R21 walls and R30+ ceiling. For my sleeping shed and considering the seasonal use the R10 is a great upgrade from an uninsulated floor. It was cold on a Nov. morning but not icy.
I think the general recommendation for the pressure treated lumber is anything within 6" of ground contact or directly exposed. If you are enclosing the underside of the cabin and have a humidity issue you might have to worry about the floor joists and any other exposed lumber.
My shed is open underneath so no sweating. I used PT beams on top of the piers but used regular 2x6 lumber for the floor joists topped with regular T&G OSB 3/4 flooring.
I think the cabin discussed in that page is sitting very close to the ground on blocks and is with in the 6" rule.
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MtnDon
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# Posted: 31 Mar 2011 10:51am
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Building code (IRC) specifies PT wood for beams, girders and posts if closer than 12 inches to the ground, measured from lowest point to the ground. Floor joists to be PT wood if the bottom edge is closer than 18 inches to the ground. Those measurements also give enough room to crawl underneath if something needs to be done under there.
There is no hard fixed rule for joist spacing or sizing without considering the span length, the loads, the species of lumber, grade. Many small cabin floors will have the joists greatly over sized in order to insulate better.
R-value is R-value and does not matter much id its rigid foam, fiberglass or whatever, as long as it is installed correctly. Rigid foam (the blue or pink varieties) give greater R per inch than loose fills. They are superb when used to sheath walls, roofs and floors over the studs/joists/rafters. More expensive and may require extra sheathing over the foam to make rodent proof.
Note: when installing something like sheet metal or wire mesh to prevent rodents from digging down, under and up the other side it is a great help to bend the material outwards to extend flat away from the perimeter just below the surface. Rodents will try to dig down and hit the metal or mesh. They will stop when they find they can not dig or chew their way through. They are not smart enough to try backing up and digging down further away from the barrier. They will always try to dig down right at the vertical barrier.
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honusbam
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# Posted: 2 Apr 2011 10:52pm
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So, I am curious as to your thoughts. I am planning a 2"x10" floor using all PT wood. Shed will be 10'x10'. I am planning on using r30 batting (9 3/4") which will fill the 10" floor (i think the actual dimension is 9 1/4"). 16" o.c.
I will be putting PT sheeting on top as my sub-floor and PT sheeting underneath.
Any concerns here with humidity/vapor? I could definitely vapor-barrier under the sub floor and also underneath. Thoughts? Over-kill? Any potential problems? I am definitely a do it once and never worry about it/touch it again type of person, so I would rather over-do it.
Please let me know if I am missing anything or should do things differently.
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BlaineHill
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# Posted: 3 Apr 2011 09:20am
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I can't tell you the best way to do it, but I have given this problem some thought as I have been planning my own project. I intend to seal the bottom of each floor joist cavity with ply wood to seal the bottom of the insulation. I have seen round soffit vents that are round, 3 inches in diameter at the big home improvement stores. I intend to put one of these vents at each end of the bottom plywood of every floor joist cavity.
I have given some thought to vapor barrier under the sub-floor (top side of the floor). I can't see how to put a vapor barrier here and still have the ability to screw AND glue the sub-floor down as I would just be gluing the sub-floor to the vapor barrier and not the floor joists. Just something to think about if you want to glue the subfloor also.
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honusbam
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# Posted: 3 Apr 2011 01:35pm
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Well, I created this post to help sift through the information, and the more I dig into it the more confused I get! ;-)
Here are some good links I have found in researching this topic:
- http://www.shedking.net/shed-floors.html
- http://www.homeenvy.com/db/3/523.html (very interesting)
- http://saveenergy.owenscorning.com/2008/09/question_how_should_i_insulate_61.html
Still not sure what I am going to do. The more I read the more I am being steered away from using batting in the floor and using rigid foam, maybe in a floating floor setup.
However, I would like to use it year-round and I do want to walk around in socks. Still not sure.
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soundandfurycabin
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# Posted: 5 Apr 2011 04:40pm
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You don't need to put a vapor barrier under the subfloor because the plywood itself is a vapor barrier.
It's not a good idea to attach a vapor barrier to the underside of the floor. If a pipe leaks or liquid spills and seeps into the floor, the moisture will be trapped there. If the ground below the cabin is damp, you can lay plastic sheeting directly on the ground surface.
If you use batt insulation and you are not skirting around the cabin then cover the insulation with an air barrier (Tyvek or similar) or some kind of solid material. Otherwise the wind will blow through the insulation and leave you with cold floors (and feet).
5/8" subfloor sheathing might be ok if the joists are 16" oc, but for 24" oc I think you'd want at least 3/4" ply.
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honusbam
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# Posted: 5 Apr 2011 05:02pm
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I do like the idea of covering the ground under the cabin.
I can't quite explain what I am going through right now in trying to decide how to build my shed. It is kind of like going to a restaurant that offers too many choices. It would be a lot easier if there was only one way to do it. Part of the fun of the process!
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toyota_mdt_tech
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# Posted: 15 May 2011 01:35am
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2X8 or 2X10 will depend on the length. My plans allowed for 12 foot longest runs using 2X8 at 16" OC. I suppose you could go 16 or 18 feet by just going every 12OC or maybe going 2X10 and keeping them at 16" OC which makes it easier to insulate as insulation coems in 15 1/2 wide. . As for sheating, I used 3/4" tongue and grooved plywood glued and nailed. I think 3/4" or 1" would be the minimum I'd use for flooring. I have a full footing/stemwall with an inside crawl space access, under my cabin is critter free.
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