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Installing insulation board over cedar siding on unsheathed & uninsulated (filled with brick) walls of 1890s farm house

Hi. I've been trying to read up on the best way to add insulation to the walls of an 1890s farmhouse but I haven't come across a similar situation/reply.

The farmouse has balloon framing with plaster interior and cedar siding exterior that was then wrapped in aluminum siding 40 years ago. The walls have brick nogging in them (all bays completely filled with low grade brick and unfinished mortar joints). As an fyi, the wall studs are 4.75" x 2". As you can see this wall has a very minimal R value. Since we are looking to replace our 36 windows I thought it would be a good time to replace the siding, rather than paint the aluminum.

The plan is to remove the aluminum siding, leave the cedar siding in place (since that is essentially the sheathing) and then add housewrap along with the thickest layer of EPS foam allowed by the siding company. We were looking at a cementicious siding product but that cannot be installed over existing siding and the cost to remove all of the cedar siding and resheath the entire house is cost prohibitive for us. Our search has led us to Royal's Celect siding product and currently they indicate a maximum insulation board thickness of 1" over existing siding. I'm trying to get them to approve 2" insulation board by using thicker/longer fasteners but I haven't heard back yet.

I was on GBA's calculator page for figuring out the maximum insulation board requirements and nothing relates to my situation. If I had the money I would tear off all of the siding, remove the brick nogging and insulate the stud cavities and resheath the house, etc..., but I cannot afford to do that. So my loaded question is: given my situation, am I doing this the best way to increase R value without compromising moisture problems (based upon GBA's recommendation that insulation board needs to be thicker) or would you recommend a better solution? Thank you.

Asked by Keith Miller
Posted May 22, 2014 8:36 AM ET


6 Answers

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I don't know where your house is located, but U.S. building codes require walls to have a minimum R-value of R-13 in southern states, or R-21 in northern states.

If you go ahead with your plan to install 1 inch of rigid foam, the insulation will have an R-value of R-4 (if it is EPS) or about R-6 or R-6.5 (if it is polyisocyanurate). That's less than code requirements, clearly, but better than nothing.

It is certainly possible to install 2 or 3 inches of rigid foam on the exterior side of your existing siding, followed by vertical furring strips and almost any kind of siding you want. But the work I have just described is more expensive than your plan.

For more information, see How to Install Rigid Foam Sheathing.

Answered by Martin Holladay
Posted May 22, 2014 8:51 AM ET


thank you for the quick reply. I'm located in zone 5, central NJ. At this point we have to take the approach that any insulation is an improvement. If we can do 2 inch board I think we would like to but since the Celect product is new, it doesn't seem like they know what thickness of insulation board they want to allow the fasteners to penetrate and still feel comfortable their siding will stay installed properly. I'm no expert but based upon everything I've read regarding the 2 most popular cementicious manufactures their warranty will be voided if their product is not installed against sheathing (no furring strips). Since Celect is a much lighter product they allow it to be installed over existing wood siding and a wall flattening system, like insulaiton board.

Your reply doesn't mention anything about vapor/moisture considerations depending on the thickness or type of insulation board. Would you believe that moisture is not an issue in my situation since there is (and will be) an air channel within the stud cavity - between cedar siding and brick nogging (although that air channel is capped with insulation at the top in the attic and bottom in the basement) ? Obviously as the walls exist now, they breathe too much so adding a WRB and Insulation board will change that dynamic. Thanks again!

Answered by Keith Miller
Posted May 22, 2014 9:14 AM ET
Edited May 22, 2014 9:15 AM ET.


There are two ways to install siding over rigid foam. One way without furring strips -- in which case the siding fasteners have to be long enough to penetrate the foam and grab the sheathing or studs. It sounds like your siding company limits foam thickness to 1 inch in this case.

The better way is to install vertical furring strips on the exterior side of the rigid foam. As long as the furring strips are securely screwed to the underlying studs, these furring strips can support almost any type of siding -- and the foam can be up to 3 or 4 inches thick.

For further discussion of these issues, see:

Fastening Furring Strips to a Foam-Sheathed Wall

Can I really use 2" of rigid insulation under fiber cement siding?

Exterior Rigid Insulation and Siding

The most value-engineered siding furring strips over thick rigid foam... but what width?

Should you use furring strips under Hardie siding?

Furring strips required over 1" rigid foam insulation?

Answered by Martin Holladay
Posted May 22, 2014 9:30 AM ET


THANK YOU! Very helpful - I will contact the manufacturers. If this is true, then it is amazing how many certified installers of fiber cement siding are misinformed.

I assume you are holding off on the moisture issue but I'm pretty thick headed - any thoughts on the moisture with my wall profile? With my walls being in between 2x4 and 2x6 I can approximate required insulation board thickness but my wall profile is still different than the wall profile what GBA's calculator refers to.

Answered by Keith Miller
Posted May 22, 2014 9:47 AM ET


You can use any thickness of rigid foam you want, without having to worry about moisture or condensation problems in your walls -- as long as your walls are properly flashed, of course.

There is no disadvantage to thin foam in this case -- other than the obvious disadvantage of the foam's low R-value.

Answered by Martin Holladay
Posted May 22, 2014 10:20 AM ET


excellent! thank you very much! So other than cost it would seem that I should use the insulation board that has the highest R value per inch, regardless of perm rating.

Answered by Keith Miller
Posted May 22, 2014 10:22 AM ET

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