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Hexar 03-05-2010 07:06 PM

Rigid Foam Insulation over existing wood siding
Hi guys,
First post here.

I live in Calgary, Alberta, Canada, I have an old one story bunglow (built in around 1965), the exterior wall is 2X4 and the existing siding is wood, the windows are two single paned sliders.

I am thinking to add insulations to the walls and retrofit the windows at the same time. I am thinking to use either 1" or 1.5" rigid foam insulation board over existing wood sidings, and then apply a layer of house wraps, then apply vinyl sidings.

Will this work? And for the windows, especially the "edge" between windows and the walls, I have not figured out how to do. All the "how to" sites I found are for either windows or insulations, not the combinations of the two.

I think I need to:

add a 1" or 1.5" wood frame around the outside of window rough opening, so that this new "frame" is flash with the foam insulation board added to the siding, then I need to apply sill flashing and etc to the "frame" and opening, then install the window.

Seems like the window will be installed "outside" of the original 2x4 wall.

Will the above steps sound correct?


CustomBuild 03-05-2010 08:30 PM

Are replacement windows an option for you? If so, you can focus on making a jamb extension for each window that would serve a s a nailer for the ends of your siding and also for "j" channel around the windows.

lemon714 03-05-2010 08:47 PM

I would remove the existing siding and then install foam and vinyl siding. I give you points for creativity with the new frame on the outside. By the time you do all that, it would have been cheaper to just start from scratch then to retrofit. Plus youll be able to address any problems with the sheathing etc. if there are any....

good luck

Brian Fox

Hexar 03-07-2010 10:43 PM

Thanks CustomBuild and lemon714, I appreciate the advices.
lemon 714, I was thinking tearing down the existing wood siding was too much trouble, but I guess doing so will make things much easier... Thanks for that.

I searched the topic, and found this:

It seems that dow's method of applying the rigid foam is like this:

But it seems to me that nailing the window fin through the rigid foam seems a little bit too weak? I mean, the foam is "soft" right?

I am thinking doing this: adding a 2x2 (or whatever the thickness of foam I am going to use, 1.5 or 2") around the window opening so that this frame's face is flash with the foam, and I can then apply flashing tapes and etc. And I can nail the window fin to this new frame.

What do you think? Is this ok or this is flawed?


ccarlisle 03-08-2010 07:28 AM

I'm glad you found that info on the house wrap because I think it will guide you to the other aspect you don't seem to be addressing in you olans: air infiltration. Consider this: about 30-40% of your heating dollars is being lost because of a lack of proper air infiltration barrier.

This has nothing to do with the amount of insulation you're putting in and you can't just increase the level of insulation to compensate for it. You have a leaky house - and that will cost you heating $$$.

You don't say why you are leaving the siding...another poster mentioned that that would be a good move to inspect your sheathing - and I agree - but to add to that, I'd say that is the best approach to add an air barrier just like the diagram you showed us. Then insulate.

The fact is that you could get by equally with less insulation+an air barrier than more insulation+ no air barrier. In other words you could get the same performance (if not better) from air barrier + one inch polyisocyanurate foam board than you could from siding+two inches styrofoam and nbo air barrier...

The bonus to this is you don't have to reframe your windows...

Hexar 03-08-2010 11:47 AM

ccarlisle, Thanks for the reply.

I am planning to do now is:
1. take off the existing wood siding, exterior window trims, inspect and fix any minor problems with sheathings.
2. apply 1.5 or 2" thick rigid foam insulations onto the sheathing, using nails with 1" fishers.
3. seal all the foam edges/joints with tapes.
4. wrap the whole house with house wraps.
5. nail 1x4 boards through the foams into the sheathing, so that sidings can be nailed to
6. install vinyl sidings.

The thing I don't know for sure is: should I use dow's technique when installing windows: nail the window's fin through foam to the sheathing (maybe further deeper into the window frame?), that I need at least 3" nails.

Or, I install an window frame extension around the rough opening, using wood with the same thickness as the foam insulation, so that this frame extension's face is flash with the foam.

Which way is better?


ccarlisle 03-08-2010 12:41 PM

You may have to rethink the order of installation, at least from what you said; the housewrap goes on against the sheathing, then any insulation. I would never use 3" nails to go through foam, in order to secure my windows. They go against the wrap. So sheathing, wrap window, then insulation.

Frostbite 03-08-2010 01:26 PM

Agreed with above, the foam insulation have no structural rigidity at all and the process you suggested will have the 1x4 board and your siding help up by cantilevered nails. You'll need some wood rails in between the insulation to nail your siding to. Also the wrap needs to be supported on both side so it should be sandwiched between the sheathing and the insulation.

Hexar 03-08-2010 02:07 PM

Thanks guys. I guess i did not express myself too well.

Let me try again:

I am planning to insulate walls and retrofit my windows at the same time. Initially I did not plan to tear down the existing wood sidings, (that thinking affected my "original" plan and made things more confusing), right now, I think I will tear down the existing wood sidings.

The things I don't understand is: how to deal with the area around window openings, how to insulate that areas.

So I searched google, and found has several PDF files detailing how to do exactly that.

Please take a look this: is suggesting that I apply the foam board to the opening, and install sill pan, and flash the window opening directly against the foam.

I found the above method strange.

Like you mentioned, I agree with foam being non-structural, that is why I think's method is kind of strange. that is why I was planning to add an extension to the window frame, and install foam "side-by-side" to this frame extension, so that the exterior faces of foam and the extension are flush. I will install sill pan over the wood frame extension, install the window and nail the window fin against this new wood frame.

So basically, the method 1 is's method, and method 2 is "my planning", but I don't know if my method is the right one. Therefore my previous questions. :)

Sorry for the confusions !

Tom Struble 03-08-2010 10:59 PM

either method will work fine,main thing is getting the window flashed

1-1.5'' dow board over a flat surface is as firm as it needs to be

Frostbite 03-09-2010 10:04 AM

I think what Dow suggested is fine, the flashing weights very little and the window frame is being supported by the wood frame around it. Although rigid insulation is none structural it can take some load if it is spread evenly, just don't nail through it since the nail will both act aas a thermal bridge, rip the foam and be a point of condensation.

Skuce 03-09-2010 10:49 AM

Foam over wood.

There is a rot situation if I ever heard of one.

Tom Struble 03-09-2010 12:06 PM

it been and being done millions of times with no problems

whats your point?

Frostbite 03-09-2010 04:08 PM

I generally dislike having rigid insulation on the outside of a house, but it is waterproof. I have heard that it is prone to insect infestation though.

Tom Struble 03-09-2010 05:58 PM

thermal break is a good thing,no more prone to insects than any other sheathing

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