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Can polyiso be added to interior finished walls?

CrawfordGene's picture

We are looking at a house to buy that was built in 2003, but has insufficient (R13) insulation in the walls.  The windows don't look so great either.  The interior is finished in drywall.  Can polyiso be glued and/or nailed to the interior walls and then paneled over?  The paneling would be glued and nailed too.  Would the polyiso provide enough of a backing for the paneling?  Would there be a condensation problem in the walls?  It appears that house wrap was used under the Hardiboard siding (no passage for rain there though).  There may be cellulose in the walls, but I'm unsure of that.  What is the best way to seal the T&G—bead of caulk in the groove?  Glue at the back of the tongue?  What glue is used on polyiso?—I understand commonly used glues dissolve it.

I am thinking of 1/2 polyiso and 3/4 paneling, or 1" polyiso and those 3/8" panels that come in overpriced kits, but would provide a lot more R value.

There are 6 fixed thermopane windows on the north wall in the living room.  They look like cheap vinyl, so I thought of getting more thermopane glass and installing them on the inside of the window openings.  This would substantially increase R value by having one sealed unit and an airspace between the 2 thermopane units.  With the paneling, I can make this look good and hide the vinyl around the original windows.

This house has a lot going for it but some things have been done badly and I can fix them fairly easily (famous last words remembered after 5 or 10 years fixing things).  The 700 sq. ft. shop in a walkout basement certainly would make me happy and provide a place where I can work on things for the house and the big kitchen makes my wife happy.

Are these crazy ideas or am I onto something?


Where? (post #207396, reply #1 of 22)


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Dan, South of Grand (post #207396, reply #2 of 22)


South of Grand Junction at 6,900'.  Not as wintry or cold as the Front Range or central mountains, but much cooler and wintry than Grand Junction.


So this house was built in (post #207396, reply #3 of 22)

So this house was built in 2003 with 3.5" stud walls?  Is that code where you are?

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Dan, I assume that is (or (post #207396, reply #4 of 22)


I assume that is (or was) code in that county.  The walls indicate 3.5" studs.  I realize I could tear out the drywall and do it better, but am trying to avoid that.  Drywall dust and living don't go well together.


The system ate my last (post #207396, reply #6 of 22)

The system ate my last post.

You can add the foam.  In your climate there should not be a condensation problem with it on the inside.  (On the outside would be another matter.)  You need to cover the foam with an appropriate fire-resistant covering -- I don't think wood paneling is sufficient.  Keep in mind that the treatment around windows and doors will be a problem, and electrical outlets, etc, will have to be repositioned.

It's unclear what you want to do to the windows.  The second layer of glass is almost certain to result in condensation.

I'd put the effort into air sealing instead.

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

I thinkyou are better off (post #207396, reply #5 of 22)

I thinkyou are better off putting the insulation on the outside, but facing the inerior walls isn't out of the question either.  I would use EPS rather than XPS or polyiso as it breaths a little.  As foam is flamable you would have to cover it with something fire resistant... like drywall.

Honestly I think you need to look at air sealing in that house if it seems drafty, that may make everything much more livable.

YAY!  I love WYSISYG editing!  And Spellcheck!


I'm looking for a relatively (post #207396, reply #7 of 22)

I'm looking for a relatively simple way to improve imsulation and soften the stark look of the house.  It was not designed well as far as looking warm and inviting, so paneling a couple of rooms will soften the look.  Utility bills seem high, so increasing insulation is one way to improve things.  Removing the Hardiboard siding is not simple nor cost effective.  The house appears to be built more to 1980's standards.  

Figuring I would panel it anyway, I thought of ways to improve insulation and polyiso has the highest R value, so it might make a project like this sensible.  I suspect, but don't know, that the walls are filled with cellulose and for that reason removing the drywall doesn't make a lot of sense to me.  If it starts to make sense as I think this through, I could fill the walls with polyiso after improving the sealing of the sheathing from the inside, and then add 1/4" drywall and paneling thus making the adjustments for windows, doors and receptacles easier.  I could take the cellulose and add it to the cellulose in the attic.  If I find the walls have fiberglass (the owner told the realtor he did the walls himself making me wonder if he did it right), same deal.  Removing the walls would probably make my wife (and me) very unhappy, but we remodel every house we buy, so I guess we could get through another mess.

I have the same concerns about the windows and condensation between two themopane units.  These fixed windows are a bank of six to take advantage of the view.  My guess is the vinyl windows are cheap ones and contribute a lot to the utility bills.  Given the expense and trouble of replacing them, I'm looking for a way to do that simply too.  The rest of the main floor has only 6 more windows, replacing those would not be cost prohibitive if done over time, but replacing 12 windows starts to get to big money.  That's why I thought of a second fixed window on the inside, but how to prevent condensation between the windows has me stumped.

This house has lots of potential or I wouldn't be interested in going through this.  I will call the bulding dep't and see what they say about installing paneling on top of foam.  Other threads say it is ok, but I understand the concern about fire safety.  


Interior rigid foam (post #207396, reply #8 of 22)

Hi Gene,

I've been contemplating this as well.  The best resource I've found is Martin Hollaway's Green Building Advisor, which you can find on the Fine Homebuilding homepage, on the top tabs, under Blogs.  He has articles, discussions, and links to building science articles that relate to the subject. 

If I understand the discussions in the GBA, the issue has to do with where condensation will form inside your wall, based on the temperatures of the structures inside it.  For example, if you add rigid foam insulation to the exterior of the wall, the framing and sheathing now are more likely to be warmer than your condensing temperatures and will have a reduced tendency to rot.  OTOH, adding rigid foam on the interior increases the potential for condensation within your wall. 

Read up on air tight drywall.  That is your best interior air barrier.  Lots of info about it in GBA.  Your concept of sealing T&G won't work.

And check in with a licensed contractor to ensure you have an appropriate fire rated barrier over your rigid foam.

Remember, rotted walls are the price you will pay if you do it wrong.  Even Building Science master engineer Joe Lstiburek found rotted walls in his house, which he had insulated early in his career.  Your climate is on your side, since the house will likely dry in the summer, but that doesn't preclude mold if there are extended periods of moisture in your walls.

Soooo.... vapor seal the interior of the wall, allow the wall to dry to the exterior ( no exterior vapor seal) , remember that the wall can suck up moisture from the crawl space (if that is what you have).  If you're seeing condensation on the lower edge of first floor windows, your wall may be moving moisture from the crawl space.  Pay attention to your eaves and gutters and drainage to reduce crawl space moisture, and consider a vapor barrier over the ground (again assuming a crawl space).

On very cold mornings, I prowl the exterior of my house to find where warm air is exiting.  I can see the frost form at each air leak.  Remember that each leak you see on the exterior is coming from warm, moisture-laden interior air that is moving through the wall.   I also had a blower door test (free from the electric utility) which helped find the huge leaks (the wall behind the shower enclosure for one).

Your approach (interior foam) is higher risk than exterior foam insulation, so learn as much as you can, both about the building science involved, and about your house.  Your dry summers will help, but the winter cold and moisture increase your risk. 

As for windows, the glass doesn't leak air, but in my house the rough framing around the windows does (built in 1978).  Replacement windows are the most buck for your bang.  I'd work on air leak sealing instead.

I'm sorry to say that to do it well isn't simple or easy.   I'm still working on my house, and it's a long way from tight. 



Bummer (post #207396, reply #9 of 22)

Bummer to buy a house and then basically have to rebuild it.  Seems like a lot of work and expense, foam and drywall or panneling, and then need to retrime all your windows.  WOW.  How bad are the heating billings?  My parents 2000 square foot house wiht electric heat and R 13 walls costs about $150 a month to heat.  How much lower could one make that bill.  I doubt you could cut it in half by adding R-3 (3/4 inch foam). 

Spring for the $200 in extra heating.


Yes, a bummer to do these (post #207396, reply #11 of 22)

Yes, a bummer to do these things, but if I can get the house fairly cheaply, it can be worth it.  It uses about 1,000 gal. of propane a year, so even at summer propane rates, that's around $2,000 for the main floor—about 1,500'.  The basement (also 1,500') has electric heat and I can't tell how much goes to heat.

I checked with the building department and they have no requirement to put drywall on top of foam board; wood paneling is ok except between the house and garage.  The garage wall is not a problem anyway.  The guy I talked to said the only way he could think to eliminate condensation between two thermopane fixed windows would be to have the inner one open—a slider unit may be available (ugly perhaps) or I could buy a fixed window, frame it in wood and put a hinge on it.  When condensation appears, open it, dry it off, close it.  Might not be the perfect insulating solution, but would help.

He also said the county required R 19 insulation when this house was built, but the owner only put R 13 in (a bargaining point on price if we buy it).

Humidity here is very low.  Before I turned on the humidifier in our house, it was 22% last week (even with the humidified on full blast, humidy it only in the low 30's).  Outside it can be 10% or less.  The house we are interested in does not have forced air heat, so I doubt it has positive air pressure inside to push water vapor outside.  


Actually, to prevent (post #207396, reply #15 of 22)

Actually, to prevent condensation the OUTER pain needs to be (slightly) "open".

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Why am I talking about my parent's house? (post #207396, reply #10 of 22)

My house has 2 x 4 walls!  I need to add insulation in my attic, so my gas heating bill is not as low as it could be, but even so it is no more than $130 in January.

I think building with effiency in mind is great and teh right thing to do, to a point.  But not to get hesterical about.  To retro-fit? To retro fit at what expense to save what?  Do the math.


You're right about not (post #207396, reply #12 of 22)

You're right about not getting histerical.  I am retired, so major projects keep me off the streets.  I'm looking at all possible ideas and appreciate the responses—they make me think.  A retired building inspector and friend is stopping by later to pick up some stuff, so I'll see what he says too.

I haven't been in this house when it is windy, so I can't say if it is drafty, but I tend to doubt it.  

I thnk the big heat loss may be the living/dining/kitchen great room with 9 windows, 2 doors with windows and the R 13 walls.  If I took out the approx. 60' of drywall there, sealed the sheathing at the studs with spray foam, put in polyiso and sealed the edges I would get R 23 (doubling ormaybe close to tripling the R value). The basement has fiberglass, so the main floor may be fiberglass too.  I can assume it is more like R 8-10 if the owner installed it and the sheathing isn't sealed either.  In the basement I can find no spray foam sealing above the concrete walls (another project), so I doubt they sealed anything with spray foam or caulk.  Put 1/4" drywall over the polyiso and then panel with 3/8" paneling and the interior trim and receptacles would not be such a problem.  Maybe polyiso the basement walls that are exposed—another project!

This would upgrade roughly half the main floor exterior walls.  We keep the master bedroom cool anyway, so maybe just panel it and eventually replace the windows.  

And I'll look up that tab about green building too.


Polyiso - not suitable! (post #207396, reply #13 of 22)

Polyiso is not suitable for internal insulation, as it shrinks over time!

This means that the water vapor you produce by breathing, sweating, cooking, washing etc will slip through the gaps and will condense inside your walls - leading to possible mould and wood rot.

You will do better, to use polystyrene or polyurethane or any other closed cell sheet insulation.

For the best result, fix a water vapor proof membrane between the polystyrene and the drywall, to keep the water vapor out of any cracks. Note: water vapor will move through a crack so small that you cannot see it, a litre and a half of water vapor can move through an 8x4 sheet of drywall in 24 hours, when the indoor temperature is 30C at 100% humidity and the other side is below dew point. Water vapor always moves from warm to cold. Thats drywall, plastic sheet, polystyrene, frame. The polystyrene will keep the surface of the plastic sheet and the drywall above dew point.

At this point confusion (post #207396, reply #14 of 22)

At this point confusion reigns.  My friend who is a former building inspector mentioned blown in blanket for retrofit, but I'm not sure that is the answer.  The drywall stays and I just panel over it but it isn't clear to me I gain that much in reducing air infiltration or increasing R value.  And I haven't got enough information about blowing open cell foam through holes in the drywall though it doesn't look worthwhile either.  

Adding a layer of foam board on top of the drywall is looking more and more like it can't provide enough of a difference to be worth the hassles.  If polyiso shrinks and other board has somewhat lower R value, no point in it.

Spraying urethane foam in the bays after removing the drywall would seal against air infiltration and 2" would provide about R 13.  I could add 1 1/2" of foam board (not sure which) and the thinest drywall I could find on top, then panel.  I'm unsure that gains enough either.

Insulated window coverings may provide more value than anything else, but I sure like to look out the windows.  They could be used for the fixed windows when we are asleep or not home and I could replace the 3 other windows with better ones.  I don't know what I would do with the other 25' of north facing wall (office and master bedroom) in the house either.  

Thanks again for your comments.


I'd say seal the heck out of (post #207396, reply #16 of 22)

I'd say seal the heck out of things, blow another foot of cells into the attic, and call it good.

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

I'm currently taking a house (post #207396, reply #17 of 22)

I'm currently taking a house apart, and putting it back together. It has been, well, educational.

Before you do anything, check your assumptions. Do you KNOW where the heat is escaping? I doubt it.

First step, it seems to me, is to have an infra-red exam of the house done. The colder the night, the better. Then you can see, in picture form, just where the heat leaks are.  Then you can plan your action.

You'll probably be surprised at what you find. I've found fist-sized holes direct to the outside, waterlogged insulation, missing insulation, and missing wall sheathing. I've found framing errors and newspaper shims. Major gaps around homeowner-replaced windows and doors. Missing sections of top plate. The steel front door was only slightly better at keeping the heat in than the screen door in front of it.

Sugestion: Any place that was tampered with by the previous owner is suspect.

Suggestion #2: Any place that's inaccessible is probably concealing something.

Now, as for adding a layer of foamboard ....

Foam insulation should ALWAYS be covered with a fire-protective barrier. Period. Even if it claims to be fire-resistant, self extinguishing, etc. That means drywall indoors and cement siding outdoors.

Making walls thicker complicates things. Your window and door frames were sized for the existing walls; what will you do when you get to them? Your electrical receptacles and switches will have to be extended out. Moulding will have to be replaced.

That's why it's so popular to add the layer of foam to the outside of the house, usually under new siding. As long as the window casings project enough, and proper attention is paid to flashing, this is a much easier option.

Inside the house, adding foam and drywall adds about 1-1/2" to the walls. This means re-framing the windows, and

Having been somewhat educated (post #207396, reply #18 of 22)

Having been somewhat educated by past remodeling projects, I've learned a few things—every project takes 2 or 3 times more time and somewhat more money than I planned on, my imagination is stronger than my body and there are, no matter how thoroughly I plan, surprises.

I don't want to take a house apart—I've done some gutting (a kitchen that took 5 years while we both worked and proved our marriage is strong).  Our 2nd kitchen remodel took a month, so I guess I'm getting better at it (we were retired by then).  But if the house is right for many reasons, I have to consider that some creative destruction may be necessary.  

Having paneled rooms before, I know about receptacles, switches, windows and doors.  I am not daunted by adding 3/4" panels to the walls, but 1 1/2" is more challenging.  I would replace the moldings anyway because they look and are the cheapest you can get at big box stores.  Windows and doors are unattractive and will be either replaced, or in the case of the fixed windows, the cheap looking vinyl can be hidden by wood trim.  The rustic look (paneling, natural wood trim, for ex.) is easiest for me as an amateur carpenter.  I can also do some arts and crafts touches which works with rustic.

I think the idea of increasing the attic insulation is worth a good look—I have to see just what is up there (a foot of cellulose is claimed by owner).  I also have to check whether the attic is properly sealed.  While paneling, I can improve sealing on windows that I keep and install new ones better than the old.  Better window coverings seem to be a necessity.  In the basement, there is access to some of the top of the foundation and I can seal that better. If the paneling weren't almost new and made to last a very long time, I would look at adding insulation under new siding.  While paneling, I can make sure all electrical boxes are sealed properly.   

I am suspect of any previous work whether by an owner or a contractor.  From what I can see with limited chance to explore, the owner did some work himself and did not do it well.  And I can see the subs and the general cut corners when they could.  Why even think of buying it?  It is in an area we like, there are few houses in the area, we think we can get it cheap, it has many things we like and some that are not so good.  While construction is not the best, much can be solved and it is a lot better than the 100 year old Victorian I once owned and the 75 year old cabin we bought after we were married.  Our present house wasn't finished after the original owner lived in it 17 years, so we finished it.  I made a good profit on the first 2 and hope to on our present house.  So this doesn't seem so bad after those houses.  

This forum gives me a chance to test my sometimes crazy ideas.  I haven't found any simple way to improve the exterior walls (R value and air infiltration) and have to look for a fairly recent FH article I vaguely remember (newer than the sidebar in June/July '09, p. 37).  I'll plan on using an infrared unit—I can use the small one I have for a quick look and see if the power co. does it for free.


The utility may offer blower door testing as well. (post #207396, reply #19 of 22)

That's a good way to find air infiltration sources.

Good point Jim. We went (post #207396, reply #21 of 22)

Good point Jim.

We went back to look at the house again and the windows look better than I thought and I got some numbers off of them and will check out their quality when I can look up the information on them.  The attic cellulose insulation is very uneven—some places look 2' and others 6" or 7".  Originally it was supposed to be 12.5" and R 30.  It has settled and some more was blown in after the initial job.  I could see light down towards one corner and 2 vents.  If we make an offer, the inspector can crawl around up there.  


Two links to useful articles, (post #207396, reply #22 of 22)

Two links to useful articles, re: issues in wall design: