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~Coach Dave~ 09-07-2011 09:17 AM

Foam board vs Styrofoam-backed siding
Hello All:

New to the forum, not new to hundreds of DIY house projects :)

This is actually my first post and first home project that I'm not doing myself. I am looking to have my house insulated and resided. Thus far I've had 2 estimates with two more in the works.

The first estimate includes installing P38 extruded polystyrene foam board with tyvek on top.

The second estimate includes installing "Generations" thermal core siding with tyvek.

I'm partial to the foamboard covered by the tyvek but heard from the second contractor that adding foamboard can actually hold in moisture not allowing the house to breathe; installing the styrofoam-backed siding does allow the house to breathe.

Couple of notes:

We live in a middle-unit townhouse.
I'm most concerned with residing the west-facing side of our house (i.e., only the backside).

Does anyone have any thoughts on this? The two estimates are pretty close in price. Wind pours through the back of our house making most of the areas very cold during the winter months so I'm really looking for the best bang for my buck as far as insulation goes.

Thanks everyone!


Gary in WA 09-08-2011 10:39 PM

This topic has been covered many times, try a "search" in the white box above every page....

One of those sited:


Tom Struble 09-08-2011 11:29 PM

is the existing siding being removed?

~Coach Dave~ 09-09-2011 03:37 PM

Thanks for the feedback!

I've done some searches but, as always, the Internet can be as useful as it is detrimental or confusing. Lots of contradictory information out there - all of it interesting reading though :yes:

To answer your question, yes, the old siding is coming off. I just had another person tell me today that I should put Tyvek the foam board. I always thought of Tyvek going on the outside since it's really just a weather/water barrier... He stated that if moisture (e.g., condensation) builds up behind the tyvek that it will actually cause the tyvek to stick to the foam board. If it's put on first, any moisture that may build-up between the tyvek and foam board would still be on the outside of the tyvek and away from the plywood (or whatever God-awful cheap board).

Thanks for the DOW doc, interesting comparison. I guess what I'm looking for is personal experience with one or the other. I need to do some checking around re: tyvek under or over.

Off to mix a drink! Happy Friday! :drink:

Tom Struble 09-10-2011 06:10 AM

has nothing to do with tyvek sticking or whatever:no:it has to do with where the drainage plane is

the drainage plane starts under the bottom window flange or sill and must be continuous,wrb can be either foam or paper but it needs to be integrated not just applied

~Coach Dave~ 09-15-2011 02:37 PM


Originally Posted by Tom Struble (Post 725087)
has nothing to do with tyvek sticking or whatever:no:it has to do with where the drainage plane is

the drainage plane starts under the bottom window flange or sill and must be continuous,wrb can be either foam or paper but it needs to be integrated not just applied

Hmm, okay so if it has to be continuous how should the Tyvek be installed? I've always seen it run horizontal to the ground. Presumably you're talking about having the Tyvek overlap by six or so inches thus providing a water resistant barrier? I'm not replacing the windows so I can't wrap the window frame as I've seen in new construction.

I guess I'm stuck on the word integrated. Sounds like you're trying to sell me a car, "The paint isn't applied, it's integrated into the frame" :) Seriously though, should I be taping the seams of the foam board to ensure that if any water does get in behind the siding that it runs down across the tape and not into the seams of the foam board?

Tom Struble 09-15-2011 06:09 PM

3 Attachment(s)
notice the foam on this house that is ''not integrated'' into the drainage plane

Tom Struble 09-15-2011 06:21 PM

2 Attachment(s)
notice this one is,now hopefully you understand the importance of integrated wrb installations

it's not some salesman jargon:wink:

just because your not changing your windows does not mean you can just forget about it,there are techniques you can use in retro fit applications

tape alone will not keep out water long term imo
it's the over lapping and direction the tape is applied [shingle fashion] that keeps water out of the wall assembly

~Coach Dave~ 09-16-2011 07:16 AM

WOW! It never ceases to amaze me how shoddy some workmanship can be! With your first set of pics I'm not sure why the person even bothered with the foam :eek:

I like the overlapping and taping of the Tyvek at the seams - really seems to bring it all together. Your window pic is EXACTLY what I was looking for when trying to imagine what my house will look like before the Tyvek and foam is installed.

I've been told that my windows should be "caulked". Initially I imagined that caulking the windows meant doing so underneath the siding where it wouldn't be visible once the siding was up. Although from your picture I really don't know how one would caulk windows without completely removing them and exposing the frame where caulk could be added...

I had one person tell me to caulk the edge of the window where it contacts the J-channel. Presumably the J-channel is going to stick out further b/c of the extra padding (e.g., foam layer). I'm hoping that caulking the reveal will look good. The same person recommended actually caulking the seems on the window itself using OSI Quad caulking. Again, I don't know if this will look good. I'm all about stopping the air flow but adding caulking to the window itself seems pretty slip-shod.

I'll take a picture of my window and post it as I'm not sure I'm describing the recommendation correctly...

I can see how adding some insulation tape around each window would effectively close off the gap along the nailer. Your pics of work done correctly are really helpful :thumbup:

Tom Struble 09-16-2011 05:06 PM

if your windows have a flange on them pull the nails on the bottom flange and get some paper up behind it then renail it,only do this with the bottom flange your paper should be over the flanges on the sides and top

and think continuous/integrated and shingle fashion with the house wrap and install it exactly how the maker says:wink:

Tom Struble 09-18-2011 06:04 AM

caulk will do little doing it the way the person is describing it
do not caulk the top j to the window [actually you should install a metal drip there]but you can caulk the sides and bottom for cosmetics if you wish

the water and air proofing is done at sheathing/window intersection

~Coach Dave~ 09-20-2011 07:23 AM

Here is a picture of the window:

"A" is the seam where the caulk would be applied. Doesn't seem like adding caulk to that tiny slit would do much in the way of providing protection against windflow. Moreover, I don't know if that's the chase that allows any water to flow around the window...

"B" is the J-channel and I could see how caulking this would help provide some protection against wind/water and gives it a more finished look.

To your point, I agree that the wind/water proofing really is done behind the scenes and simply adding caulk is more protection eye-wash. I guess doing both can only be a good thing.

The J-channel as you see it runs all the way around the window - I don't think there is a plan to add a metal drip edge at the top. I'm curious why I would caulk the bottom J and not the top. Usually it's the other way around, caulk/seal at the top and leave the bottom open to allow for drainage.

~Coach Dave~ 09-20-2011 07:49 AM

As a follow-up I'm considering removing my deck and building a new one (with the help of a couple buddies). Do you feel it better to j-channel down-to & up-to the existing ledger board and add flashing or completely remove the ledger board and side straight down the house and install the ledger over the new siding?

Not sure when I'll be able to get around to building a new deck but I certainly don’t want to spend a ton of money on new siding only to learn that I should have demo’d the deck before installation. Driving rains lead to intermittent leaks in my basement (above the door and window only) and I think the water is finding its way in either because of a lack of flashing and/or via the lag bolts…

The deck is going to be replaced regardless so going with the first I could demo the deck leaving the existing ledger board and have the siding/flashing installed. Once the siding is up I can remove the ledger board and replace it with a new and build off of that (flashing already in place).


I could demo the deck completely removing the ledger board prior to the siding going up and then install the new ledger board against the siding and go from there. The main problem I see with this option is that there won't be any flashing in place and I may end up getting water in my basement again. I know there's a compound I can add to the lag bolts that will completely seal the holes but then there's the issue of water getting in behind the siding (via the holes I'd have to shoot to attach the ledger board).

woodworkbykirk 09-25-2011 06:33 PM

tom is on the ball here, trust me he knows his stuff.

the key thing to realize when it comes to waterproofing the outside of the house is that the house should be waterproof before the siding goes on. the siding is just the lipstick and mascara.. if your relying on the siding to keep the weather out your in for problems. you always have to start from the bottom up making sure that every row above the previous with foam, or tyvek overlaps the lower one so the rain will run down and stay out

~Coach Dave~ 09-30-2011 07:57 AM

Thanks for the input - it seemed strange to me that additional caulking on the outside of the siding would do the trick. I agree that siding is really the final touch but there seems to be some value in the thickness of the siding (e.g., .41, .46) so it must offer some protection against the elements.

And yes, Tom does seem to know his stuff - his feedback and pics are worth a thousand words :)

The weather is starting to turn here in MD so I'm anxious to get this project underway. Looking forward to using the rooms on the backside of our house in the winter months sans a sweatshirt, blanket or space heater!!!

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