Basement wall framing - Fitting rockwool insulation

[alkizmo]

I was recommended to use rockwool insulation (Roxul) for my basement wall insulation (exterior walls).

Now I know my wall framing must be 1" from the concrete wall to let air circulate, but with that air gap I'm wondering what's the best way to put in the rockwool insulation without having it fall back and touch the concrete.

I'm thinking of tacking a felt or paper membrane on the concrete-facing side of my frame (I'd be building it on the floor then raising it). That way the membrane would hold back the insulation. Don't worry I haven't forgotten about the vapor barrier between the insulation/frame and drywall.

However I'm wondering what's the proper most common way people hold that type of insulation inside the frame without it falling back.

I searched on google a lot but all I can find are guides using rigid XPS directly attached to the concrete walls. I'm not going with XPS as the rockwool would provide R14 by itself which is enough.


important edit I forgot to mention that while I'm doing my basement, it HAD 2x2 framing with 1" of EPS boards glued to the concrete wall. I removed one board to see the state of things and it was pretty darn clean (No water stains). Should I just leave the EPS boards glued to the walls (Replace the one I removed) and put up my 2x4 frame right up against it instead? The old 2x2 wood still looks brand new (Aside from being dry as a bone for age). So it looks like the EPS did a good job for the past 15+ odd years, considering there was no vapor barrier behind the old drywall and no tucktape between the EPS boards (Which I'd place if I keep the EPS).

 

[freddiemercurystwin]

Chicken wire.

 

[alkizmo]

Chicken wire.

Yeah I had something like that in mind too, basically a layer than lets air in/out.

But what about the EPS boards already in place? Could I just put my new frame directly over the EPS boards instead?

 

[freddiemercurystwin]

Err gotta admit I kinda speed read your post first time round. Anyhoo are you saying then you've got 2" timber frame against the basement wall face with 1" rigid insulation against the basement wall face? You then plan to build a further 4" timber frame with rockwool between that's 1" away from the 2" existing frame?

Also BTW I have never seen rockwool installed in such a situation, rigid insulation would always be preferable due to a) it being rigid and b) its a much better insulator than rockwool.

Are you really in Canada?

 

[alkizmo]

Err gotta admit I kinda speed read your post first time round. Anyhoo are you saying then you've got 2" timber frame against the basement wall face with 1" rigid insulation against the basement wall face? You then plan to build a further 4" timber frame with rockwool between that's 1" away from the 2" existing frame?

The 1" EPS is against the basement concrete wall. The 2" frame is against the EPS.
I plan on removing the 2" frame and replace with 2x4, but keeping the 1" EPS as a moisture retardant, if that's possible.

Also BTW I have never seen rockwool installed in such a situation, rigid insulation would always be preferable due to a) it being rigid and b) its a much better insulator than rockwool.

Rigid XPS is a better insulator than rockwool per inch, correct.
However, with 2x4 frame, I can put 3.5" thick rockwool of R14 insulation value. If I were to put up 2" XPS boards, I'd pay THREE times as much and only get R10.

So no, rigid insulation is not better if space isn't an issue, otherwise we wouldn't stuff our attics with 20 inches of fiberglass batts.

Are you really in Canada?

Yup, but I'd assume this type of topic is pretty standard. It's not like we're talking about electrical stuff

 

[freddiemercurystwin]

If the wall behind your existing xpx is dry then you could probably leave it place. Not sure why you'd bother with a void though if you've already got insulation hard up against the wall. A void, if you have one should go between the insulation and the wall.

I think you mean you'll get an R Value of 1.4 not 14!

Rockwool is rarely used in walls in the UK due to it sagging over time and the issues you have mentioned but mainly the fact that it is a poor insulator compared to the rigid insulations we commonly use in the UK. We would use something like 100mm Celotex which would have an R Value of 4.5.

 

[alkizmo]

If the wall behind your existing xpx is dry then you could probably leave it place. Not sure why you'd bother with a void though if you've already got insulation hard up against the wall. A void, if you have one should go between the insulation and the wall.

Well the void is so that the framing wood wouldn't be in direct contact with the concrete wall, but after writing my post, I realized I could still use the old EPS rigid insulation, thus not needing a void, the frame would be directly against it.

I think you mean you'll get an R Value of 1.4 not 14!

Roxul Stone Wool
Look at the Canadian specs on the R14. It's 3.5" thick and made to fit between studs.

Rockwool is rarely used in walls in the UK due to it sagging over time and the issues you have mentioned but mainly the fact that it is a poor insulator compared to the rigid insulations we commonly use in the UK. We would use something like 100mm Celotex which would have an R Value of 4.5.

Well, the Roxul insulation I linked does not sag over time. I've used a few of these in the past for a crawl space and they are quite blocky and solid.
They aren't bendy at all, unlike fiberglass batts.

I google image searched "rockwool" and I can see what you meant by sagging. The roxul insulation isn't the bendy kind that is rolled up. They're more like huge thick dry bread loafs.

 

[freddiemercurystwin]

Ahh well excuse me for not being familiar with Canadian insulation! In the UK 'R Values' are a measure of how good an insulator a material is and it would appear your R Values are calculated differently in Canada. How stupid of me but its not electrical installation is it as you say its all the same right?

I can't be bothered wasting anymore time in this thread.

 

[alkizmo]

I can't be bothered wasting anymore time in this thread.

I hope you didn't take offense to what I said or that I seemed condesending. I was just showing that it isn't R1.4 of total insulation value per "piece".

I guess your ratings are based on R per unit of thickness, which then yes, XPS boards are much better.

3.5" of Roxul is roughly 8.9cm
R14 / 8.9cm = R1.573 per 100mm
So you were pretty damn close with R1.4

Just for dicussion reasons I want to explain why I'll use Roxul rockwool instead of XPS polystyrene:

The end goal is to get a total insulation rating >R12
The 2x4 framing can accomodate rockwool batts
Roxul rockwool is perfectly rated for basement/damp areas

So since I can get R14 total insulation with Roxul, I have to compare the costs as XPS brings little more benefit other than being a vapor retardant and more R per thickness.

R12.5 XPS (2.5" thick) = 1£ per SQF
R14 Roxul (3.5" thick) = .42£ per SQF
That's a 0.58£ per SQF difference.

Now I have 66 feet of wall (8 feet high) to cover and it would be more if I were to finish all the exterior walls of my basement.
66 long x 8 high = 528 sqf
528 sqf x 0.58 £ = 306 £

306£ is quite a cost difference just to go XPS when Roxul can be used.
I think it's worth the extra hassle.