Stove install- foundation thermal mass vs insulation

[orionrising]

I am in the process of buying a house that I will be putting a woodstove back in. It is a split- level (4 foot basement poured walls. The stove will be in an alcove in one corner where the previous owners had one, and has an existing double walled pipe out the wall and up the side of the house.


The foundation wall is strapped and drywalled but not insulated on the inside (is poorly on the outside which will be a summer project) The previous owners had heat sheilds on the walls.

Would I be better off stripping the walls to the foundation to use the thermal mass ( slab will of course already be acting as thermal mass and a heat sink) or going with heat shields?

or should I strip the foundation, put 2 inches of foamboard in and then brick or stone the inside of that?


The stove will probably be an Englander NC 30

 

[pen]

Insulating those walls is something I keep debating. The problem with the concrete walls is that they conduct heat well to the ground outside.

I measured the temp on my poured walls back when it was -10F a few weeks back and 10 feet from the stove the wall was registering in the 40's, stove had been cranking for days and basement was in the upper 70's.

Insulating these walls would be a pain, but unless you have BTU's to spare, I believe it's a good move.

pen

 

[orionrising]

that is my thinking, however the slab is a huge thermal bridge as well that is connected.

I guess its really a question of heat loss % vs cost?

IE if I burn 5 chords a year and say lose 20% I am losing one chords worth of cost and effort?

 

[pen]

Exactly. I only have problems heating the house 100% with wood on the very coldest days. Not sure if insulating down there would make the difference or not, but for me, since I have enough power to overcome that loss and heat the house 95% of the time, I have not done the job. Also,,, it would force me to make other changes in the basement that would not be beneficial.

pen

 

[DougA]

I built my house with many tons of concrete thermal mass and it will work against you unless it is very well insulated. My experience is that mass on the floor is useless, except to help buffer extreme temperature swings or if you have sunlight heating it like I do. Easiest way to think of it is that your floor is always the coldest part of the house and mass, even if it's room temp, will always feel cold and will draw heat away from the rest of the room. Thermal mass absorbs heat very well. The secret is how to make it give up it's heat when you need it.

I think you should consider ripping down the drywall and using spray foam in the stud wall, then facing the wall near the stove with stone or brick. You must have the mass on the warm side. Exterior insulation is a waste of time. Spray foam will seal the wall, make a moisture barrier and it's the best insulation per inch you can buy without investing a fortune. You could use the extra heavy blue insulation and then use spray foam cans to seal the joints but that's a cheap substitute as it's not sealing the foundation wall and you will probably get mould between the foam board and the foundation.

 

[orionrising]

there is foam and waterproofing outside already ( the above grade stuff was not covered though and has broken down due to lack of UV protection.

The place has been empty and winterized all year so brining it up to temperature is going to be rough.

 

[Laurent Cyr]

I think you should consider ripping down the drywall and using spray foam in the stud wall, then facing the wall near the stove with stone or brick. You must have the mass on the warm side. Exterior insulation is a waste of time. Spray foam will seal the wall, make a moisture barrier and it's the best insulation per inch you can buy without investing a fortune. You could use the extra heavy blue insulation and then use spray foam cans to seal the joints but that's a cheap substitute as it's not sealing the foundation wall and you will probably get mould between the foam board and the foundation.

I agree with DougA that you need to insulate Inside. Spray foam is the best if you can afford it. If you can`t afford it, then pink styrofoam, 2 inches, glued to the foundation from the floor to the sill, is the best. Make sure you tuck tape all the joint, and spray foam at the top of the sill, the corners, and the bottom. You need to ensure there is no circulation of air behind the foam. Then install your 2X4`s for your wall. Depending on where you live, you may want to add fibregalss insulation between the studs, and then your dtry wall. DO NOT add plastic sheeting on the studs before installing the drywall. If you do, moisture might build up between the pink foam insulation and your drywall, and will lead to mold issues.

Then on your drtywall, you can install your brick or heatshield of whatever.

This is the way I`ve insulated my basement. I live in Québec, Canada, and I have noticed a HUGE difference in how much heat is retained in the basement, compared to before. You`ll see the difference when it is -30 outside. Your basement will be at +80F Inside.

Laurent

 

[DougA]

In our area, you can buy cannisters of foam spray at Lowes and that should be enough to do the job. Sealing the cement wall is very critical if you are going to heat the basement. Agree that if the basement is to be a living space, spend the time and money to insulate it well. You will not regret it.

 

[peakbagger]

Definitely insulate on the inside and forget insulating on the outside if you are in the woods. Carpenter ants love exterior foam insulation, they drill a hole in the foam and then tunnel inside it right up into the house. Took me several years and several ant infestations until I figured that out. I pity the folks who put in the foundations made with foam forms as I expect there is a long term ant problem.

 

[bmblank]

I've known many who used ICFs and haven't had any ant problems yet. Maybe they treat the icf gosh with something to make it unappealing to ants.

 

[DougA]

I didn't use foam in my house at all because I was warned that mice love burrowing into foam. I was told to use pink fibreglass batts because mice hate the scratchy texture. After tearing a wall out to renovate years late, I found that mice love burrowing into fibreglass also. I made up hundreds of small bags of rat poison and put them inside the insulation. Now, every winter we can smell the dead mice every few weeks. There's no winning.

 

[lindnova]

Nothing is perfect.

Yes mice love fiberglass insulation. I have not had problems yet with my foam, but someday I will probably get mice in there. I have T-Mass concrete walls in my house. 10" wall, 4" concrete, 2" polystyrene, 4" concrete. Works best in the summer for thermal mass, but good in the winter also. Also have 2" foam under the floors. This makes a big difference as you can feel where the floor is much cooler on top of the footings where there is no foam.

If I had to insulate existing conc walls I would use spray foam first and ridged foam board as a second option. I don't like fiberglass next to concrete as it will form mold with the coolness and even a little moisture over time.

Back to the subject. I assume you are talking about only the area by the stove. Yes, I would put the foam board in, seal the edges good and brick up a good fire stop with cement board & airspace between it and the foam. That will give you a good thermal mass and insulate from outside. You don't want the foam getting too hot as it will smoke and burn.

 

[razerface]

i always hoped the 9 inch thick walls of my basement would be a thermal mass that would benefit me. They do not seem cold to the touch,, i will measure temp. I put 2 inch pink foam on the outside of the walls before we backfilled with the dirt, hoping to insulate the house from the heat robbing ground around it.

 

[hugho]

I am in the process of buying a house that I will be putting a woodstove back in. It is a split- level (4 foot basement poured walls. The stove will be in an alcove in one corner where the previous owners had one, and has an existing double walled pipe out the wall and up the side of the house.


The foundation wall is strapped and drywalled but not insulated on the inside (is poorly on the outside which will be a summer project) The previous owners had heat sheilds on the walls.

Would I be better off stripping the walls to the foundation to use the thermal mass ( slab will of course already be acting as thermal mass and a heat sink) or going with heat shields?

or should I strip the foundation, put 2 inches of foamboard in and then brick or stone the inside of that?


The stove will probably be an Englander NC 30

Concrete is fabulous for thermal heat storage to be effective it has to be on the OUTSIDE and ideally during construction. We put 4" of blueboard glued outside all concrete perimeter walls and foundations (unless we are using ICFs) and 6" on the corners and 2-4" under slabs. I favor insulated concrete floors in fact using the solar slab technique of a poured slab over 12" concrete blocks laid sideways. Insulating on the inside is better than nothing but it is generally easier to add sheet foam to the outside of a shell. Pull off the siding and attach the sheet foam and put the old siding or new siding back on. No messing with wires and pipes and easier to trim out windows and doors. Spray foam is the cat's meow and it insulates perfectly and forever,: it adds amazing structural strength. Aircraft wings are built with foam sandwich construction.Fine homebuilding magazine has lots of articles on this technique. Be damn careful with heat shields because they are often done wrong. Insulated metabestos or equivalent quality pipe is mandatory and you have to stay with the same manufacturer so everything fits. Uninsulated double wall flue pipe allows closer clearances but you must respect clearances to a combustible surfaces!. I think heat shields should rarely or NEVER be used. Safety is fundamental. Trust no one else's installation. Get it inspected when you're done. If you incinerate your family, you'll never get over it.

 

[peakbagger]

Funny how carpenter ants seem to be a regional problem. I know of my issue and several homes that had foam added to the exterior that had serious ant issues. When I have asked spray foamers none have an additive to prevent ants. Borates seem to work in stud cavities but doesn't do anything when sprayed on foam.

 

[orionrising]

hmm actually looking at my photos, the area is wider then I remember... look like about 6 feet + which for a nc-30 is more then needed for combustible offset.

 

[Laurent Cyr]

Looking at your picture. Is that a cold air return above the stove, or is it a hole in your ceiling to get the hot air to rise to the first floor?

 

[orionrising]

it is both sortoff, house has a hot air oil system as well and that is the end of a line of duct in a t above it, so it serves as a vent to the second floor.

I will need to add some kind of outside air kit probably

 

[Laurent Cyr]

Well, I can`t help you with the OAK. I don`t have any knowledge on that. That is, assuming that you really need one. The duct above your stove is an issue if it is tied into your forced air heating system. If it is pulling air from above your stove, it might not be meeting code. It has the potential to draw fumes from your stove, which you might not want.
Some greater minds on this forum will certainly comment on that issue.

 

[orionrising]

dunno about the oak not sure how tight the place is. The duct above is the end of abot a 30' run from the stove, not a return, so should be fine

 

[razerface]

My basement walls are 9" thick. Right now they measure 77 deg, except the closet one to the stove is 84, and the walls next to the walkout doors are 52 deg. I have pink foam between them and the ground outside. I heat 3200 sq ft from the basement which is unfinished inside. If the fire goes down to small coals before I get home, which happens every day, the temp in the house never drops below 68 so far, in this weather.