Insulation job: from the inside or the outside?

kit_813March 8, 2009

Hi to everyone, thanks in advance for helping me out on this one.

I bought an old house 2 years ago and I started renovating it last year. The house was built in 1941, which is about the oldest houses you can get in my area of northern Quebec. The original insulation is sawdust, and it has become compressed in the lower 4 feet of the walls, leaving the rest of the walls with no insulation.

I have an evaluation from a governemental energy agency that suggests somes things to be done in priority to save energy, and the best way to do them. They suggest I get rid of the sawdust and fill the walls with projected polyurethane by the outside (which I cannot do for financial reasons), or let the sawdust in place and project polyurethane above it (which would be fine for me). Then, complete the insulation with projected polyurethane on the outside onf the house, or polyisocyanurate sheets.

Before getting the report, my local renovation center suggested I placed the polyisocyanurate sheets directly on the outside wall, without concerning myself with the inner walls. It cost much, much, much less than putting projected polyurethane in the walls.

I phoned the inspector and talked to him about that, and here is what he told me. He said that I need to put an insulation inside the walls to cut the vapor from entering the sawdust and the wood structure. If I only put insulation on the outside, humidity is sure to find its way inside my walls, creating a mold problem in no time. That's the reason why he suggested to fill up the walls before adding outside insulation. My local renovation center says it's overkill.

I know next to nothing about insulation, and now I think I am more mixed-up than ever. If someone in here is experienced in this aspect of renovating, I would be immensely grateful for your opinion on the matter.

THanks a lot

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If it were me and I didn't want to go to the great expense and inconvenience of tearing my walls up, I'd have cellulose or foam injected from the outside. They drill a 2" hole at the top of the wall for every stud space, fill the cavity, and cap the hole with a plug. There are all kinds of wonderful insulation products out there, but many of them are only reasonable to install during new construction, or during a remodel where you're opening walls. I'm a firm believer in energy conservation, but don't get talked into spending huge amounts of money for something that is only incrementally better. If you simply get your wall voids filled, you'll get most of the benefit at a much lower cost. Don't let the perfect be the enemy of the good...

    Bookmark   March 8, 2009 at 7:00PM
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Why couldn't you insulate such walls from the inside? Assuming there is a floor plate, you could drill a hole near the floor, vacuum out sawdust as best as possible, drill hole at top of wall, inject foam in (the lower-expansion stuff so your walls wouldn't burst.

Would that work? Easier to repair walls inside, I would think.

    Bookmark   March 8, 2009 at 8:19PM
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I can't remove the sawdust from inside the house simply because of the mess it would create. I am highly allergic to molds and dust, and 70 years old sawdust is assured to have both. And I can't remove it from outside either because of the prohibitive cost. I have to leave it in place. The only thing that has to be decided is if I want the foam to be injected in the wall atop the sawdust, or simply projected upon the outer wall before I (they...) do the siding. And it is a big decision, for me anyway, because I certainly don't want my walls to rot because of the insulation! :o(

    Bookmark   March 8, 2009 at 8:37PM
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Your walls haven't rotted from the sawdust, and they won't rot from the foam, so this shouldn't be a difficult decision. Just go ahead and leave the sawdust in place, top it off with foam, and don't worry about it any more. That's the most economical decision, and the easiest, and will accomplish your objective of improving the energy efficiency at a reasonable cost.

    Bookmark   March 8, 2009 at 11:27PM
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You might ask for "Worthy" over at the building a home forum and run it by him. He is a high end builder based in
Toronto. He has great advise concerning insulation techniques, especially in colder climates and im sure the polyisocyanurate panels will be one of the recommended priorities.

    Bookmark   March 9, 2009 at 11:56AM
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What I don't see in this post are R values for the various scenarios. There is also no information as to the recommended values to have.
The only information that can be determined is that the, "local remodeling center" should refrain from speaking as the information is inane. Leaving an empty wall cavity and just installing exterior panels is of little use. One inch of polyisocyanurate board has an R-7 rating. Hardly enough for a Canadian environment.
What sort of exterior siding is planned?

    Bookmark   March 9, 2009 at 12:38PM
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You probably can't 'overkill' with insulation here in Canada, if it's effective.

If you can afford it, go with the injected stuff as well as the panels on the outside - that is going to be a huge improvement. No doubt you're replacing or refurbishing your windows which will help as well.

In terms of the moisture, I assume you will be adding tyvek-type paper since you're re-cladding. Keep in mind, it's a lot more effective than what you've had, so your house should breathe a lot better (I suppose you have to consider whether the outside insulation will interfere with the tyvek paper breathing...)

    Bookmark   March 9, 2009 at 5:36PM
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Yes Ron you are right, I haven't posted any R values simply because I don't really know them. What I know is that the polyurethane foam does have the highest R value of all the projected stuff available here (vs cellulose and others), and the polyisocyanurate sheets have the highest R value of all the sheets available. But, for the outside of the house they are both pretty similar so I think cost will determine which one I will use.

So you think it's true that I can't leave the wall empty? Seemed logical to me, but I wanted some confirmation of that. My house now is frigid and cost tons of money for energy, I want to improve it the best way possible while staying economical and the least messy possible...

Thanks for the help!

    Bookmark   March 10, 2009 at 9:36AM
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I think the confirmation preceded the research if the house is frigid and energy costs are that high.
Does Canada have energy credit dollars available for this upgrade? Here in the US, you can get tax credit for energy upgrades. Some local utilities were giving away free gas boilers if you converted from oil.
Check to see if this is available to you.
The wall cavity is probably between 3 1/2" -4" depending on the age of the house. Pre WW 2 houses here have actual 2x4's.
The effectiveness of the insulation is based upon the entire wall envelope, not just the area above the sawdust. So if you use the poly foam only above the sawdust, you'll still have substantial heat loss below.
I would pick a scenario that insulates the wall uniformly. This might mean using a less effective product, but the heat loss below that will be more of a loss then, say, using cellulose overall.
This is not the time to insulate anyway. The winter is almost over and these guys are very busy. I would wait until late spring/early summer and if need be move out for the few days it will take to do the job correctly.

    Bookmark   March 10, 2009 at 10:22AM
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Ignore R factors for a few days, until you get your mind comfortable with a couple other bigger concerns.

kit, when you insulate better and better you transform your building into more and more like a plastic bag. In winter, if you breathe into a plastic bag for a couple seconds you will see dew in it. Or frost.

kit, learn about "dew point" ("rosée") in Your Climate, in your walls, in buidings like yours. It's deep inside your walls. It's probably not where the average person posting in this forum thinks it is. Even people from the warmer parts of Canada. In cold climates, it is a serious concern. Then when you insulate well it becomes a big thing to "manage". Ask the government guys for help in understanding this. They will know a lot about it. This is the Key to understanding why some houses rot and others don't, and why some have mold everywhere in their walls and others don't. The dew point is inside the walls; it causes rot. There is a lot of overnight freezing and thawing in walls, for many months in your climate. When the walls have higher humidity in them that before, this problem is worsened.

Polyisocyanurate is good, and spray-on is good, and sheets too.
Foil-faced polyisocyanurate sheets are great too: do you have a fireplace?


    Bookmark   March 10, 2009 at 3:31PM
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Hi David,

No I don't have a fireplace. I have a chimney, which I do not use. The governement agent has tested my house for "loss of air" sorry don't know the english term. He figured that my house was full of holes where air leeked, especially around the windows and electrical outlets. I will change the windows next summer before doing the insulation.

About the dew point: are you telling me that wherever I put the insulation, there will be condensation on it, even if I put it inside the walls?

I am already on a program for energy savings, which gives me up to 3000$ if I increase the energy coefficient of my house. According to the agent who came and did the inspection, it will be easy for me to get that money. I started last year by having a drain and a membrane put around my foundations, and the basement was insulated with polyurethane. Now the only hot place in the house is the basement! But you are right, I will call them back and ask for more guidance in choosing the type of material and the place to put it, I prefer to take my time, insulate this summer and do the esthetic siding next year, rather than do it wrong and having to do it again...

Thanks, very helpful interventions.

    Bookmark   March 10, 2009 at 9:10PM
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you will be able to answer your own question about condensation when you read more about the dew point falling halfway between the cold side (exterior) and the warm side (interior edge of wall). So, essentially yes, condensation _can_ occur inside the wall and lead to mold fast... but I'm sure you won't panic because of my saying this. The Key thing to know is that you are the one who will decide where the dew point will fall, and whether it will fall inside the vapor barrier or outside it, and you will make these decisions based on your research. Note that the foams mentioned above are closed-cell and therefore dry inside their air bubbles. Some other foams let humidity cross through.


Here is a link that might be useful: sealing a house; air infiltration and exfilitration

    Bookmark   March 10, 2009 at 11:01PM
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I would reiterate the idea of removing the sawdust. I understand OP has allergy type issues, as do I, if you wear proper protection and clean up well, it should not be such an issue, besides, if the sawdust IS mouldy, then leaving it in there will possibly make it worse. If the spot where the sawdust has accumulated becomes the cold spot, moisture will continue to collect there, and with the nice insulation in the rest of the wall, it might stay damper.

Best to get it, plus all the mice droppings/skeletons/bird nests etc all out.

You might be able to employ a commercial service to do it, the people who vacuum out attics for example.

If you are re-cladding, then what is to stop you from drilling the holes on the outside? Then there will be no infiltration of dust to the inside, and, again, wearing disposable coveralls and a REAL respirator/face mask (not those fifty-cent paper things, they don't work well and only until they become moist from your breath, then they don't stop much dust at all.

    Bookmark   March 11, 2009 at 12:38AM
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