Elevated wood floor above grade - insulating options brainstorm

[Truefire]

Have a project pending and brainstorming on insulating means for an elevated wood floor. Needing the flooring insulated very well between the joists. Been thinking about mineral wool between joists largely due to it's breathability. However, also thought momentarily on rigid foam board for a few different reasons. One of which, would be the installation from the 'top side' of the floor would be much easier using the foam board as opposed to installing mineral wool. Would have ledger strips in place for foam board to sit upon. Could work from above with ease as opposed to the slightly more challenging installation mandate of installing mineral wool from above. If that was the route, thinking about installing hardware cloth-tacked in place- to hold insulation into position.

Entertained the thought of cut sections of foam tightly nestled between joists. However, I'm just not certain that would even be an option because of its resistance to breathability. However, if the foam R-value was appropriate, sections were cut to fit tightly and they were of an appropriate thickness would breathability even be a concern? Although the panels themselves would potentially prevent any meshing of cold to hot / vice versa...there are those wooden joists, which have a different r-value and possess the ability to move moisture, on each side of the panels.

How about my moisture membrane? What kind would be the best to use? Once the flooring is in place on top of these joists, I'd have no way to get back to the insulation. I'm just leaning really hard towards the mineral wool rolls but simply love the idea of installation practicals of the foam board.

 

[chris_goris]

Have a project pending and brainstorming on insulating means for an elevated wood floor. Needing the flooring insulated very well between the joists. Been thinking about mineral wool between joists largely due to it's breathability. However, also thought momentarily on rigid foam board for a few different reasons. One of which, would be the installation from the 'top side' of the floor would be much easier using the foam board as opposed to installing mineral wool. Would have ledger strips in place for foam board to sit upon. Could work from above with ease as opposed to the slightly more challenging installation mandate of installing mineral wool from above. If that was the route, thinking about installing hardware cloth-tacked in place- to hold insulation into position.

Entertained the thought of cut sections of foam tightly nestled between joists. However, I'm just not certain that would even be an option because of its resistance to breathability. However, if the foam R-value was appropriate, sections were cut to fit tightly and they were of an appropriate thickness would breathability even be a concern? Although the panels themselves would potentially prevent any meshing of cold to hot / vice versa...there are those wooden joists, which have a different r-value and possess the ability to move moisture, on each side of the panels.

How about my moisture membrane? What kind would be the best to use? Once the flooring is in place on top of these joists, I'd have no way to get back to the insulation. I'm just leaning really hard towards the mineral wool rolls but simply love the idea of installation practicals of the foam board.

Call in someone who does insulation. Youll be amazed at how inexpensive it is, not to mention, they know exactly what youll need to have done.

 

[KenOfCary]

You can just staple paper backed insulation to the bottom of the joists. Hardware cloth is not a bad idea to keep varmints out.

As Chris mentioned though, this might be the kind of job to let a professional do unless you enjoy crawling around in dark crawl spaces.

 

[gritz]

You can just staple paper backed insulation to the bottom of the joists. Hardware cloth is not a bad idea to keep varmints out.

Paper backing goes toward the warm side.
Have you considered a plywood bottom layer, filling the voids with your choice of cheapest and easiest installed insulation material?
Cutting foam cleanly is easiest with a heated wire foam cutter. Have you considered spray foam?

 

[SteveHall]

You didn't say specifically... are the joists over a crawlspace?

If so, you really want to seal off the humidity from the soil first. Green Building Advisor (actually more "building science" than "green") has tons of articles, but they made a very helpful PDF, Sealing a Crawlspace, that simply explains why and how.

In a nutshell, soil has very high relative humidity that drives through insulation and condenses wherever the dewpoint is... usually some spot in the middle of the joists, causing mold and rot. By sealing out humidity at the point of origin, moisture within the crawlspace is lowered considerably to avoid the problem.

To continue this aside, vapor retarders with higher permeability (Class II or III) are useful to allow moisture to flow and self-regulate in envelope sides where both sides of the barrier are subject to swings. Typically this includes walls, ceiling joists, and rafters. They can see higher humidity on either side due to wild daily and seasonal humidity and temperature swings both inside and outside the barrier. But this doesn't apply to a crawlspace, where moisture from the soil on one side is always higher than the adjacent, interior air. In ye ol' days before air conditioning, ventilated crawlspaces made sense because the house wasn't ever cool enough to have an interior dewpoint. But since A/C, all houses create dewpoints within the insulated walls, joists, and rafters without very good, scientific detailing. It's a big problem and we're finally starting to see vapor retarders on the exterior of walls (green membrane boards, NOT building wraps) to solve this for our regional climate. (Every region requires different solutions.)

As for the insulation, I also like mineral wool because it is dense, inflammable, and vapor permeable. But if you don't put in a Class I vapor retarder at the soil, I'd recommend avoiding any type of barrier at the joists. Just let moisture flow as much as possible, using wire insulation supports that are easy to snap in, adjust, and remove.

 

[Truefire]

You can just staple paper backed insulation to the bottom of the joists. Hardware cloth is not a bad idea to keep varmints out.

Paper backing goes toward the warm side.
Have you considered a plywood bottom layer, filling the voids with your choice of cheapest and easiest installed insulation material?
Cutting foam cleanly is easiest with a heated wire foam cutter. Have you considered spray foam?

Hello Robert, I have not seriously considered but I have thought about that. Seems like a good idea but I wonder about condensation internally in this closed box of the sorts.

Essentially that's the same construct as the flat roofs which are built without using joists and closed in. I would be concerned with warm shop air warming the plywood flooring to the extent that the space within that box, met cool air. I just don't know. I know with the roof structures built as such there are measures wrought to seal the box really well so that it is indeed a closed unit, without any leakage input or otherwise. I'm thinking if those guidelines were followed, it might be ok for the floor. I just wouldn't want this to be a 'learning experience' for me. I don't have even an ounce of time for another one of those right now. lol....way too many of them...lol

 

[Truefire]

Steve-- I'm very familiar with the moisture content resident in soil but its certainly worth reiterating; because I'm also very familiar with the damage that it does to joists when vapor barriers aren't in place or either not installed properly. So thanks for highlighting. I've certainly seen my share of damage through the years when in crawlspaces pulling wires and running circuitry. As we know, It can be BAD.

I had plans on breaking the hydraulic conductivity of the moisture rise by placing an inch or so of 57 grade stone or smaller upon the entire footprint of the building. To answer your question, no the crawlspace wouldn't be accessible, it would be too low for that. I was thinking, although I haven't spoken with the building inspector for the area yet, of going with an 8" block topped with a 4" solid topper sitting on top of poured footer cylindricals. So I would have at the least, 10" above the rock bed to bottom of joists.

What are your thoughts regarding a couple of layers of the foam board cut to fit snug within the joist spaces? Any thoughts on that? I was thinking, depending upon R value, of placing two pieces (2" thick), to give me 4" of foam, pressed up snugly against underside of flooring. I'm thinking the impervious nature would retard moisture rise. I would still need a moisture retarder underneath floor space but not sure which kind if I go that route.

I like the mineral wool a lot but am concerned with retention measures. Through the years, if any falls, I'll have no way of lifting it back in place without accessing through the flooring. The foam panel sections would provide me with the ability to be retained with ease.

 

[CrealBilly]

Couple of things... staple chicken wire to the bottom the the joists. You'll be glad you did, it prevents the insulation from falling down. Its that pesky little thing called gravity, plus there is a tremendous amount of vibration in a building (a lot more more than most people realize).
Ridgid foam in a crawl space? stay far far away... when it gets wet it will start stinking like rotten fish... :yuck dont ask me how i know this the fishy smell is mainly attributed to spray polyurethane foam but trust me it also happens with ridid polyurethane foam board. I know because it happened to me.

 

[JimD]

Why not just staple paper backed fiberglass in from above? The paper is a moisture barrier but also will support the fiberglass. I assume you will put plywood on top of the floor joists and it is also a vapor barrier. But you will need vents or something to address moisture coming up from the soil. I guess you could put plastic down and then rock but I worry the rock would puncture the plastic harming it's ability to block moisture from below. Better would be plastic over the soil then some sand. Or, if the space will effectively be sealed, you could just put plastic over the soil, put in some vents and call it done.

I think you would have leaky edges of foam board and it isn't cheap. You can seal it with expanding foam from a can - the $50 dispenser from HD works well. I think this will work - if you support it and seal the edges, but I think it will cost significantly more and be more trouble to install than paper backed fiberglass. Foam board will also sag, over time. I don't know if ledgers would be enough support.

 

[Truefire]

Couple of things... staple chicken wire to the bottom the the joists. You'll be glad you did, it prevents the insulation from falling down. Its that pesky little thing called gravity, plus there is a tremendous amount of vibration in a building (a lot more more than most people realize).
Ridgid foam in a crawl space? stay far far away... when it gets wet it will start stinking like rotten fish... :yuck dont ask me how i know this the fishy smell is mainly attributed to spray polyurethane foam but trust me it also happens with ridid polyurethane foam board. I know because it happened to me.

Jeff- Thank you. I certainly don't want that. Thank you so much for sharing, appreciate that. Leaning towards the mineral wool insulate more and more.

Jim- thank you. Love the idea of the paper backed being fastened on top of the joists. One concern, I'm thinking I might need a different type of vapor barrier though perhaps, considering its location. Most of the paper backed insulation is designed to be used in walls and not so much so for this particular arrangement. I'm just wondering if it would suffice for a vapor barrier when used in this capacity?

Considering all the great input and thoughts, I think I've decided on the way I will tackle this aspect of the build. I'm leaning more towards building my flooring arrangement and stapling chicken wire along the bottom of the joists, then calling in a couple of friends to invert the flooring and help me sit it upon the footer risers. Fill this joist's cavity with mineral insulation and topping with a vapor barrier like Steve highlighted before plywood flooring is screwed down.

 

[SteveHall]

What are your thoughts regarding a couple of layers of the foam board cut to fit snug within the joist spaces?

I like this if it is closed cell foam. Open cell foam won't act as a vapor barrier and will actually harbor it and the mold that will feast on it. It can be difficult to fit rigid foam between joists snuggly, but you can fill gaps with consumer cans of spray foam.

I wonder if you could install thin sheets or baffles between the joists and spray foam from above? It will price about the same as the rigid boards but will seal much better.

 

[CrealBilly]

Jeff- Thank you. I certainly don't want that. Thank you so much for sharing, appreciate that. Leaning towards the mineral wool insulate more and more.

Jim- thank you. Love the idea of the paper backed being fastened on top of the joists. One concern, I'm thinking I might need a different type of vapor barrier though perhaps, considering its location. Most of the paper backed insulation is designed to be used in walls and not so much so for this particular arrangement. I'm just wondering if it would suffice for a vapor barrier when used in this capacity?

Considering all the great input and thoughts, I think I've decided on the way I will tackle this aspect of the build. I'm leaning more towards building my flooring arrangement and stapling chicken wire along the bottom of the joists, then calling in a couple of friends to invert the flooring and help me sit it upon the footer risers. Fill this joist's cavity with mineral insulation and topping with a vapor barrier like Steve highlighted before plywood flooring is screwed down.

You'll still want to staple the aluminum foil flanges to the top of the joist. But eventually the insulation will pull away from the foil and fall to the ground. Not saying it will happen the day after you install it - but eventually it will... its that pesky thing called gravity. Gravity, vibration & age will do it. Chicken wire stapled to the bottom of the joists will help it stay in place and when it does fall - it will still be where it's supposed to be...

Rock wool is heavy all things considering and by sandwiching it inbetween galvanized chicken wire and the glued foil back. It should give you many many years of trouble free service.

I would opt for foil back instead of paper back (craft faced) - its in a crawlspace - where its moist most of the time - especially in the summer. For obvious reasons... I think you'll get more service years from foil vs paper. Just my opinon though.

Chicken wire is cheap... or should I say cheep - IDK...