Basement insulation question
Hi,
I am planning to finish my basement and need help with what materials I should use.
I live in Ohio (5A as per map). I have concrete walls with 8 ft ceiling.
I have been reading a lot and not sure what I should be using. I need good comfortable basement. I was reading best to use XPS but now after reading more I see it is not a green option. I also read there are few new products coming out with HFO blowing agents (where to buy them from or they are not available yet ?) I also looked at roxul comfortboard 80. Please advice.
Also what is the best space saving technique to insulate floors ? I don’t want to lose specially the ceiling height.
Thanks in advance.
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Replies
Bullet,
The first step is to read this article: How to Insulate a Basement Wall.
(Note that the article also has a section on insulating floors.)
The second step is to read this article: Choosing Rigid Foam.
The third step is to read this article: All About Basements.
Good luck with your project.
best & greenest methods we've found to quickly and effectively insulate basements:
1. 4" of polyiso foam with the joints taped, fastened directly to the concrete; the layer towards the living space must be a specific type of Thermax that is rated for indoor exposure. OR One layer of 2" polyiso with taped joints, and a 2x4 wall over it to hold wiring and 4" of batt insulation. The polyiso will help to keep moisture out of the basement.
2. One layer of 6 mil poly on the concrete, then 2" EPS installed on the floor with one layer of Advantech plywood for a subfloor. Probably gluing them down will work best.
> Thermax
Johns Manville CI Max appears to be similar at lower cost. But drywall is even lower cost.
In Ohio there are several vendors of used roofing foam, which the cheapest and greenest foam insulation solutions. Find one near you- they're out there!
https://columbus.craigslist.org/search/sss?query=rigid+insulation
https://toledo.craigslist.org/search/sss?query=rigid+insulation
https://cincinnati.craigslist.org/search/sss?query=rigid+insulation
From a design point of view, derate any used XPS to R4.2 inch, which is it's fully deplete performance after the HFCs have escaped. Assume fiber-faced roofing polyiso to be R5.5/inch if it's most of the insulation, or on the below-grade portion, R5/inch if its only half the R-value and on the above-grade part of the foundation.
IRC (International Residential Code) 2015 calls out R15 continuous insulation as code minimum for basement walls, which would take 3" of reclaimed roofing polyiso or 4" of reclaimed XPS, or U0.050 (R20 "whole wall" maximum. Using 1.5" of foam (any type) trapped to the foundation with an R13-R15 insulated studwall would still get you there on a U-factor basis. (With used foam it's often cheaper to go with 3" roofing polyiso strapped to the wall with 1x4 furring through-screwed to the foundation with 4.5-5" masonry screws, mounting the wallboard on the furring.)
https://up.codes/viewer/utah/irc-2015/chapter/11/re-energy-efficiency#N1102.1.2
https://up.codes/viewer/utah/irc-2015/chapter/11/re-energy-efficiency#N1102.1.4
With the hybrid approach with an insulated 2x4 studwall there needs to be at least R5 of foam on the exterior for wintertime dew point control on the above grade section, limiting the moisture accumulation in side the studwall over the winter months. Even low-density EPS would hit R5.7-R5.8 at 1.5" thickness, but you'd have more margin with polyiso or 1.5lb or denser EPS/XPS.
When using polyiso on the walls take care that the cut edge not rest on the slab where it can potentially wick moisture. Using an inch of XPS or EPS under both the bottom of the cut edge and under the bottom plate of any studwall provides a capillary break for the wood & polyiso
I read all the articles above but the one which is restricted for members only (Rigid Foam). I read other article related to rigid foam. Thanks for the links though.
I have read the polyiso is a no for basement? Previous owner of my house has 1" polyiso installed at the top portion 2' of the basement wall. Also he installed spray foam in rim joist.
What compressive strength rigid insulation should be used for a basement floor?
Polyiso is only a "NO" for direct soil contact (including even topside contact with concrete slabs that have no polyethylene vapor barrier below them.) On walls it's just fine, even below grade, with caveats about keeping the cut edge at the bottom from contacting the slab, or above the high-tide mark if there is a history of flooding.
Are you saying the previous owner installed the 1" polyiso on what is now YOUR house? Your sentence parses somewhat ambiguously in my dialect of 'merican English, since the previous owner probably doesn't still live there.
With an inch of pre-existing foam on the upper portion, first continue down with more 1" foam (any type). Then, add another half-inch to ensure dew-point control if building an insulated studwall up against it, or add 2" reclaimed roofing polyiso (R11+) over the 2" layer (stagger the seams of the new layer with those of the old) if going for a foam-only insulation solution.
Insulating just the top half or just the above-grade portion of unfinished basements was pretty popular in the 1990s but it's way below current code prescriptives.
For what it's worth, Ohio's residential building code is still anchored to the 2009 IRC, which has much leaner requirements for basements (http://codes.ohio.gov/oac/4101%3A8-11). Dana's recommendation based on the 2015 update is better, of course, but you're still legally good if you go with only 2" of continuous foam or a stud wall with fiberglass. It's what they call the 10/13. From the notes to Table 1102.1:
c. "10/13" means R-10 continuous insulated sheathing on the interior or exterior of the home or R-13 cavity insulation at the interior of the basement wall.
Obviously the cavity insulation alone is a BAD idea, for reasons discussed in the articles Martin referred to in the first answer.
Calum: The compressive strength of the foam doesn't matter if it's sandwiched between a subfloor and supportive concrete slab.
The subfloor distributes the weight quite well, and since it's uniformly supported by a slab there are no sections that will exceed the elastic deformation levels that would cause permanent depressions even with a 250lb Czech dancing a polka on it with a keg of beer on his shoulder. It's a much firmer floor than standard subfloor that has to span 16" on center joists, and the foam won't flex, or compress much at all, even 10psi rated foam is more than enough (not that the psi rating is meaningful in this context.
BULLET0770 / Andy: Building to lower 2009 R-values just because you're allowed to in Ohio doesn't mean that's the "right" R-value, or best bang/buck. It just means you're allowed to do it. It's not wrong to think of it as the crummiest thing that's still legal to build. In a zone 5A climate R15 foundations are financially rational even when using virgin-stock foam and paying somebody else to do it. If using recliamed foam and DIY labor it's something of a no-brainer to go better than the IRC 2015 code minimum R15. (At 3" roofing polyiso would be about R17-ish.)
Hi Dana,
Thanks for your reply. Previous owner is still living in the house and will be moving out shortly :) maybe the confusion to explain. Yes, you are correct, polyiso is already on the top 2' part of the wall. So you are saying continue polyiso below it and add half inch polyiso over the both ? What should I add to the bottom part which touches the concrete, XPS ? There will be stud walls as I am planning to run wires.
So what is my best bet here? 1.5/2"(total) of polyiso and then empty stud wall ? or have Roxul batt?
Do I need to install insulation on the flooring (Concrete slab) as well? or I can get away without it just don't want to lose height. I read you XPS -> sleepers -> plywood.
Thanks Andy for your reply!
Stud wall with fibreglass is NO for the basement ?
Thanks all for helping.
Bullet,
Q. "Stud wall with fibreglass is NO for the basement?"
A. It's not recommended if the stud wall is directly against the concrete or CMU wall. If there is an intervening layer of adequately thick continuous rigid foam (adhered to or fastened to the concrete wall), then it is OK. More information here: How to Insulate a Basement Wall.
Bullet,
Dana is right about the general compressive adequacy of foam under subfloors, but you will find the foam will compress slightly under the screws along the edges leaving a small depression where sheets join. Not a problem if you are installing an underlayment, but for some types of flooring it might be.
"So you are saying continue polyiso below it and add half inch polyiso over the both ? What should I add to the bottom part which touches the concrete, XPS ? There will be stud walls as I am planning to run wires."
You can continue the 1" layer down from the 1" with any type of foam. If all of the above-grade portion of the foundation down to about a foot or so below grade has the 1" polyiso it will be cheaper to continue the 1" layer using Type-I EPS with facers (typically R3.9 often the cheapest 1" foam found at box store) would be fine, and it can rest on the slab. A few blobs of foam board construction adhesive is fine for installing it, then tape the seams with housewrap tape if it's a plastic facer, or a decent quality foil tape if it's a foil facer. (Nashua 324a is a decent foil tape found at most box stores.)
If the existing polyiso stops right at grade level you'll have to use polyiso, (and not EPS), or you can add a second layer of half-inch foam (whatever is cheapest). The cut edge of the polyiso should stop above the slab level, whereas it's fine for EPS to rest on the slab. If using polyiso on the walls, take some 1" EPS and cut it into 4.5" wide strips, and tack it to the slab with about a 1/4" gap between the edge of the EPS and foundation wall (as a tiny drip cavity) to give the polyiso and bottom plate of the stud wall a capillary break, isolating it from the slab. The stud wall is not structural, not holding up the house, so it really doesn't matter what the compressive strength of the EPS is.
For better air & moisture tightness installing another layer of half-inch foam (any type) with the seams offset from those of the 1" layer gives it more dew point margin, and overall better performance, but it's optional.
When the studwall is up and wires run, insulating it with unfaced R13-R15 fiberglass or R15 rock wool, carefully trimmed and fitted around the electrical boxes will then bring the performance up to IRC 2015 code minimum. If you DON'T want to insulate the wall cavities with fiber, you'd need to put 2" of roofing polyiso (R11-ish) or 1.5" of foil faced polyiso (R9-R9.5 labeled, but R10-ish if the foil is facing an empty cavity) over the layer of 1" foam to hit IRC 2015 performance levels.
The polyiso continues foot or so below the grade, so I guess my best bet is to go with EPS- type one
something like R-Tech sheeting ?
and then stud walls with fiberglass or wool is better? It expensive but is it worth the price difference for my project ? I would save money if the difference is not that huge.
Will this work ?
Owens-Corning-R13-Unfaced-Fiberglass-Batt-Insulation-with-Sound-Barrier
Also what about the floors ?
Thanks for your help guys, much appreciated.
Dana,
Thanks.
Any help on Floor insulation ?
Bullet, Dana and Malcolm both provided good advice regarding floor insulation. The best material is EPS, either type 2 (15psi) or type 9 (25psi). Type 1 (10psi) or Type 11 (5psi) which you might find more easily at a big-box store is a bit soft for the application. Type 2 (15psi) will work great, unless you are introducing any loads such as a bearing wall, in which case you would want type 9 (25psi). By "best" I mean the best combination of performance and environmental impact.
Thanks for the answer Michael.
Will this work for my basement in OH, There is no visual dampness on the concrete basement slab.
Which option will work ?
- 1" EPS Type II 15 psi (Saving space) + 2 layers of 1/2" plywood (Which one's?)
- 1" EPS Type II 15psi (Saving space) + 1x4 sleepers attached to concrete slab and foam sealed + 1 layer of 1/2" plywood on top of both. Saving half inch here.
Thanks for your help.
Bullet, I don't understand your second assembly. Would you run the foam continuously, then 1x4 sleepers over the top of the foam? What do you mean by "...and foam sealed?" Why do you note that both are "saving space?" Do you mean using 1" foam instead of thicker foam?
If you run sleepers, I would use 3/4" Advantech as a subfloor. If your slab is very flat and you don't mind the possibility of some amount of bounce in the floor, two staggered layers of 1/2" Advantech or CDX, glued and screwed together, will also work. The version with sleepers will have slightly higher R-value because the air space will provide a bit of insulation, but not enough to make a noticeable difference.
There is rarely visible dampness on concrete slabs, even if the slabs are damp, because the surface layer dries to the air. It's a good idea to check the concrete with an appropriate moisture meter, or tape a square of poly sheeting over the slab, or run a continuous vapor barrier just in case there is (likely) some moisture accumulated.
I see some contractors run foam then furring 1x4 between the joins, then plywood on the top.
Yes, with space saving I mean using 1" instead of 2" foam.
I lost access to my email and cannot recover my account (bullet0770) can anyone (admin) help with this ?
I am starting my basement this week and I have couple of more questions:
So, after our discussion here, the basement is getting 3" reclaimed polyiso (Felt paper) and then vertical furring to pass wires from ceiling.
Then 1" outlet box and drywall on top.
One wall will be getting 2x4 on top of polyiso to hang TV (Or should I hand on concrete block wall with tapcpns ?)
I am planning on building a home theater in one side of the basement which is 12x12x12 nook and extending the length to 23ft. What is the best possible way to keep the width maximum ?
Can I do 1" polyiso on concrete walls, then 2"x2" vrtical furring and fill the space wth 2" polyiso/fiberglass ?
Thanks,
Nav
The thinnest IRC code-min (for zone 5) R15-ish wall would be 3" of reclaimed polyiso dadoed 3/4" to accomodate 1x furring, with the wallboard tight to the foam. At the dado the foam would only be 2.25" or about R 12-R13, but the furring itself adds about R1, bringing it to a stripe of R13-R14. The actual performance of the full thickness 3" foam will be R15.5-R17, and on a U-factor basis it would make it.
A 2x3 wall would be strong enough to hang even the heftiest TVs, if depth is an issue. A full sheet of 3/4" plywood TapConned to the foundation would be good enough too.
Thanks for your reply again Dana. I believe saving 3/4" from both sides will not make a huge difference and will go with 3" polyiso and then furring on top as I will need to pass wires.
So you are saying, plywood over 3" polyiso and then drywall for tv wall ?
>So you are saying, plywood over 3" polyiso and then drywall for tv wall ?
That would be sturdy enough.
Hi Dana,
Thanks for your reply, I have one more questions if you could help.
I want to know if I could use this method without issues, I will have to put less insulation on top of drywall. Saving width here.
1/2" polyiso/EPS and then frame 2x4 with rouxl Insulation ? Difference in depth of wall is 1/4" more but the Roxul in wall will have more STC that will help sound insulation. Appreciate your help.
A half inch of foam isn't sufficient dew point control on the above-grade portion of the foundation wall for R15 rock wool in zone 5. Over the winter the rock wool at the top of the wall would get wet. For fully above grade walls the IRC calls out for R5 for dew point control on R13, or about 28% of the total R. You can probably get away with a bit less for a wall that's 90% below grade but not a lot less. An 3/4" of foil faced would be the lowest I'd be willing to risk. An inch of foil faced polyiso & R15 rock wool would be fine even for above -grade walls.
Thanks Dana, Any difference in foil vs felt? I am using 3" felt polyiso in rest of the basement as I can not find foil face locally and shipping is getting way too expensive. I may end up going reclaimed or new from craigslist as recommended above. Thanks
Felt or fiberglass facers are fine here. Pretty much all roofing polyiso facer types are specified at less than 1 perm, making the whole sheet less than 1 perm independent of thickness, which is plenty for limiting moisture diffusion through the insulation layer.
Thanks Dana, appreciate your help. I am good to buy Polyiso for my basement now.
One more question:
I am not able to find psi rating of EPS in stores, where can I find 1" Type IX EPS for floor insulation use ? and can I just use one layer of OSB on top and then flooring ? My headroom is limited and want to save as much as possible.
Edit: Will the R-Tech from HD be fine ? Description says used for under slab, should be fine for floor insulation?
Thanks,
Nav
The HD stuff with facer rated R3.85/inch is Type I, fine for walls, not so great under anything less than a 3/4" subfloor. Most Type-I foam is 10-12 psi. It's cheaper (and greener) to go with used foam. There are multiple vendors of reclaimed foam board operating in OH:
https://columbus.craigslist.org/search/sss?query=rigid+insulation
https://akroncanton.craigslist.org/search/sss?query=rigid+insulation
https://toledo.craigslist.org/search/sss?query=rigid+insulation
https://cleveland.craigslist.org/search/sss?query=rigid+insulation
Most roofing EPS is 1.25lbs density Type VIII goods, noticeably more rugged than Type-I, but there will be some Type-II as well. You may have to weigh it. Type-VIII is usually 13-15 PSI sometimes more, Type-II is usually 15 psi minimum.
Thanks for your reply Dana, As per your recommendation earlier, I am buying 3" polyiso from the guy in Cleveland. He only has 1/2" fan folding EPS as of now, any other option ?
Also one 3/4" OSB is fine on top ? Attached with tapcons or floating ?
Used or factory seconds XPS would be a fine subsitute for EPS for an over-slab solution.
3/4" OSB subflooring is fine. Tapconned is also fine. If you're going to float the subfloor use two layers of half-inch, glue the bottom layer to the foam with foam board construction adhesive (seams staggered), and install the second layer with seams staggered over the first with ring shank nails or 1" bugle-head screws. Simply floating a single layer without fasteners to the slab is too likely to develop "potato-chip" curl waves & ridges from normal seasonal moisture cycling, but double-layered is fine.
Leave a full half-inch of expansion room at the perimeter if you do the floating double-layered half-inch OSB approach.
Thanks for you reply Dana, appreciate it. I am not able to find factory seconds or used XPS as well. Can I just use Platon and then luxury vinly planks on top of it (Recommendation on Platon website)? Will the room and floors be decent temp with walls being 3" polyiso ? or will the floor be cold for feet ?
Thanks!
Any input on my last question?
Thanks!
Found this article, looks like Delta FL type stuff is a "NO Go" in my case.
https://www.greenbuildingadvisor.com/article/whats-the-best-basement-flooring-system
Where can I find Type 2 or Type 9 EPS ? Please help. I am in Cleveland OH.
@Dana - you mentioned this tape: Nashua 324a good enough? o ruse something else ?
Thanks again.
Anyone knows where I can find type 2 EPS ?
Try a commerical insulation supply house. Type-II EPS isn’t an unusual product, any insulation supplier should be able to order it for you.
Bill
https://www.homedepot.com/p/R-Tech-1-in-x-4-ft-x-8-ft-R-3-85-Insulating-Sheathing-320821/202532854
will this work ? I don't see psi rating on this.
That is probably type-I EPS based on it’s density. There is some good info about density here: https://www.epsindustry.org/building-construction/compressive-strength
You might be able to special order type-II EPS through Home Depot, but be very specific with them since I suspect they probably won’t know the difference.
Bill
Thanks for your reply, I tried calling them and was on hold forever lol. I will probably stop by and check with them. So should I say order R-Tech type 2 EPS ?
Alright so I went and bought Polyiso 3" with felt backing on both sides (thick brown cardboard like material). Fine for my application (bottom 1" will be XPS/EPS).
Adding picture
Not sure why picture is not showing up, Is the polyiso fine for my application ?
I’m not entirely clear what you “application” is. For walls, I don’t think that polyiso will be any problem. For floors, I’m not so sure. Polyiso is not at all suitable for underslab use or where it would be exposed to moisture like on the outside of a foundation wall. Polyiso on top of a slab under a subfloor sounds risky to me, I personally wouldn’t use it for that application.
Bill
Thanks for your reply Zephyr. Yeah it will stop 1" above slab and EPS will go under it.
Maybe I will just paint walls with DryLock or maybe not needed, not sure.