Helpful? 0

What is the best way to insulate the exterior of an old house?

We live in central Alberta, Canada. Our house has R-12 insulation in the walls with vapor barrier.

I would like to add some sort of insulation on the outside and put on new siding. I would like some suggestions on what type of foam product is best, etc.

Thanks.

Asked by Stephen Mitchell
Posted Tue, 09/13/2011 - 22:01
Edited Wed, 09/14/2011 - 08:05

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7 Answers

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1.
Helpful? 1

Dow Thermax is terrific. We 've used quite a bit and have been very successful

Answered by Christopher Hall CAPS CGP
Posted Tue, 09/13/2011 - 22:52
Edited Tue, 09/13/2011 - 22:53.

2.
Helpful? 1

Stephen,
I hope that your interior vapor barrier is not polyethylene; kraft paper or vapor-retarder paint would be preferable. But since you live in Alberta, I'm guessing that your house does have interior poly.

The standard way to improve the wall insulation in a home like yours is to install rigid foam on the exterior. Any foam will work -- EPS, XPS, or polyisocyanurate. Of these three types of foam, polyisocyanurate is the most environmentally benign.

Many energy experts have worried whether it's a good idea to install exterior foam on a house with interior poly. Although it would be better if the poly wasn't there, the fact is that tens of thousands of Canadian homes with interior poly have been retrofitted with exterior rigid foam, and there haven't been any reports of widespread problems. So you are probably OK.

Here are four caveats:
1. Be sure that your foam is thick enough to keep the wall sheathing above the dew point in winter. Read more on this topic here: Calculating the Minimum Thickness of Rigid Foam Sheathing.

2. When the siding is being removed, inspect the existing sheathing carefully for any signs of water intrusion, and correct any flashing or housewrap problems.

3. Install rainscreen strapping so that there is a ventilated gap between the new exterior foam and the siding.

4. Keep your interior humidity under control during the winter; if the interior humidity gets to high, operate your ventilation fan more frequently.

Answered by Martin Holladay, GBA Advisor
Posted Wed, 09/14/2011 - 08:21

3.
Helpful? 0

Thanks. However I have lots of more questions. Is polyiso insulation the type of insulation with a foil back? Is it ok to install when we a vapour barrier on the inside of our house? If so, how exactly do you install it? Vertically or horizontally? How do you seal the joints? What is rain strapping, and how do you install it? Is it ok just to install the siding on top of the foam insulation using those special long nails/screws with some sort of washer on them? When does the housewrap go on, before or after the foam insulation? Should I put wood on the corners and around the bottom of house to butt the insulation against thereby giving me backing for my siding corners and starter strip? Thanks for the help.

Answered by Stephen Mitchell
Posted Thu, 09/15/2011 - 23:18

4.
Helpful? 1

Stephen,
Considering how many questions you have, you may wish to hire an experienced contractor for this job.

Since it sounds like this type of work is something you are unfamiliar with, you may want to watch these two GBA videos:

Video: How to Install Rigid Foam Insulation Outside a House

Video: Superinsulating a Home With Rigid Foam

Here are my answers to your questions:

Q. "Is polyiso insulation the type of insulation with a foil back?"

A. Sometimes, but not always. Some types of EPS are also sold with a foil facing, and some types of polyiso are sold without foil facing. If you are buying the insulation yourself, ask for polyisocyanurate.

Q. " Is it OK to install when we have a vapor barrier on the inside of our house?"

A. I answered that question the first time. I'll repeat the answer here. It depends on what you mean by a "vapor barrier." If the vapor barrier is polyethylene, that's not ideal, but it is still possible to proceed with exterior foam, as long as you take certain precautions. The precautions I recommend are the "four caveats" I listed in my first answer.

Q. "How exactly do you install it? Vertically or horizontally?"

A. Either way.

Q. "How do you seal the joints?"

A. With caulk, canned spray foam, or a compatible tape.

Q. "What is rain strapping, and how do you install it?"

A. The strapping or furring strips used by most builders are vertical 1x4s. These are usually installed 16 inches on center, and are fastened with long screws through the foam to the studs.

Q. "Is it OK just to install the siding on top of the foam insulation using those special long nails/screws with some sort of washer on them?"

A. It depends on what type of siding you are using, but in general, the answer is no.

Q. "When does the housewrap go on, before or after the foam insulation?"

A. Either way, as long as the housewrap is integrated with the window flashing and the flashing used at penetrations. If you don't know how to install flashing on walls, hire a contractor.

Q. "Should I put wood on the corners and around the bottom of house to butt the insulation against thereby giving me backing for my siding corners and starter strip?"

A. Yes.

Answered by Martin Holladay, GBA Advisor
Posted Fri, 09/16/2011 - 05:23
Edited Fri, 09/16/2011 - 06:05.

5.
Helpful? 1

Stephen
IKO Enerfoil is a foil backed polyiso insulation that is made in Calgary.
http://www.iko.com/shared/commercial/brochures/BrochEnerfoil.pdf

Answered by Garth Sproule 7B
Posted Fri, 09/16/2011 - 11:40

6.
Helpful? 0

Thanks again. Very helpful. If I don't install the rain strapping and just put on vinyl siding on top of the foam, what could be the problem? I also noticed in the videos they put a flashing around the bottom of the house first, that would stick out about 3 inches, what do you do to finish from there down to the ground? Thanks for the help.

Answered by Stephen Mitchell
Posted Fri, 09/16/2011 - 13:45

7.
Helpful? 0

Stephen,
It's possible to install vinyl siding directly over foam sheathing. More information: http://www.nbnnews.com/NBN/issues/2008-07-21/codes+and+standards/index.html

If you plan to install exterior rigid foam on your walls, followed by new siding, then your walls will be thicker than they used to be, and the walls will overhang your foundation. There is no reason to install any material "to finish from there down to the ground." However, if you want to insulate the exterior of your foundation with rigid foam, you can always do so.

Answered by Martin Holladay, GBA Advisor
Posted Fri, 09/16/2011 - 14:01

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