Timber Frame External Wall Build-up Options

brophis

Hi all,

My partner and I are going the timber frame route for our self build. Been in touch with 10+ TF companies and it's coming to decision time. I've had them all quote for a similar spec in the 0.15/0.16 wall u-value range and there's 3 main types of wall build up I've to decide between that various companies are offering:

1) 140mm frame with semi rigid insulation (e.g. metac, frame therm, etc.) between studs with 40/50mm PIR insulation board inside
2) 184-220mm frame with semi rigid insulation and insulated service cavity
3) Same as above but with cellulose insulation instead

All have the usual membranes, OSB, plasterboard, etc. There'll be a block outer leave with cavity between block and frame. In your experience what would you say are the pros and cons of each build-up leaving out cost as the cellulose option is an extra €1k or so. All claim to have breathable walls and some have offered alternatives to OSB that are more breathable. How breathable is PIR and does it even matter when it's located on the inside??

Thanks
Brophis

BryanF

brophis wrote: »

Hi all,

My partner and I are going the timber frame route for our self build. Been in touch with 10+ TF companies and it's coming to decision time. I've had them all quote for a similar spec in the 0.15/0.16 wall u-value range and there's 3 main types of wall build up I've to decide between that various companies are offering:

1) 140mm frame with semi rigid insulation (e.g. metac, frame therm, etc.) between studs with 40/50mm PIR insulation board inside
2) 184-220mm frame with semi rigid insulation and insulated service cavity
3) Same as above but with cellulose insulation instead

All have the usual membranes, OSB, plasterboard, etc. There'll be a block outer leave with cavity between block and frame. In your experience what would you say are the pros and cons of each build-up leaving out cost as the cellulose option is an extra €1k or so. All claim to have breathable walls and some have offered alternatives to OSB that are more breathable. How breathable is PIR and does it even matter when it's located on the inside??

Thanks
Brophis

1. PIR is not very breathable, imo cellulose is better as its less carcinogenic and its production is less carbon intensive, it also offers whats known as a decrement delay. I would think the PIR should be offered on the outside, not the inside..
2. service cavity good - presuming the air-tightness is on the main structural wall so the sparkies can run cables etc without effecting performance
3. imho id go option 2. and wider frame with cellulose with maybe wood fibre or hemp fibre for the service cavity but forget the outside block and go rain-screen battened cement fibre board plaster painted

As a matter of interest what was offered instead of OSB. some are now offering a taped OSB or glued OSB as the air-tightness layer..

archtech

If it were me, I would put a suitable OSB board on the inside of the timber frame and use a wood fibre board to the vented cavity, omitting the insulated plasterboard altogether. The OSB board has the potential to act as the air tight membrane and all that needs to be done is to tape the joints with a suitable tape. Both OSB Board and plywood to the cold side of the timber frame can lead to interstitial condensation within the insulation, particularly when the thickness of insulation to the warm side of it is significant.

A service cavity is essential.

brophis

I should've noted that with option 1 there is a service cavity inside the PIR. All options have airtight membranes to the external side of the service cavity to prevent damage from services, naturally enough. As mentioned service cavity is a must. One manufacturer suggested that the PIR on the inside would reduce thermal bridging but would this even have much impact on the inside of the wall?

In relation to the external cement board, that would be my preference but from looking at posts on boards it appears to be a much more expensive option.

PM sent with OSB alternative. Not sure if I can name it here. Was a more vapour permeable sheathing board.

BryanF

brophis wrote: »

One manufacturer suggested that the PIR on the inside would reduce thermal bridging but would this even have much impact on the inside of the wall?

i cant find it now but i saw elsewhere that in the US their starting to clad the outside in PIR but as per Archtech's comments and my PM, consider the wood fibre board, although more expensive its is a better option.

In relation to the external cement board, that would be my preference but from looking at posts on boards it appears to be a much more expensive option.

more expensive than a block outer-leaf, interesting..

PM sent with OSB alternative. Not sure if I can name it here. Was a more vapour permeable sheathing board.

yes, that's what the website states, but there is mixed views around on its performance. I've been lead to believe its in the same category as OSB for racking purposes.. BTW try to get an OSB that's low in formaldehyde.

the wood fibre board would be a better breathable option and will reduce the cold bridging externally, which is better than the PIR inside

sas

BryanF wrote: »

more expensive than a block outer-leaf, interesting..

The NSAI certified one (I'm only aware of one) is astonishingly expensive.

When pricing build options I got a quote for supply and fit it was approx 36k.
That didn't include the battening out for the cement board, I'd have to get that done. That is completely unacceptable in my opinion.

If I was building TF, I'd do everything I could to accomadate a block outer leaf. It's definitely cheaper and definitely more rebust.

carnotcycle

For what its worth, we have settled on, and are happy with, a TF stick build structure consisting of (inside to outside); 50 mm service cavity insulated with Metac or rockwool, vapour control barrier, 225mm stud with cellulose insulation, a water vapour open sheathing board to enhance breathabilty and give racking strength, 50mm service cavity, 100mm block.

I would have preferred to use woodfibre insulation on the external wall surface against the sheathing board, but the cost is prohibitive for our budget hence the use of the external block, which I agree contributes zeros to the thermal performance of the wall structure.

The build up I believe is a good breathable structure which was a priority (after a low thermal conductivity value) at the design stage. I never considered the use of PIR insulation as an option for a TF structure. Have I been missing something?

drum11

Hi there guys.

I am hoping you can help me out a bit. I am studying Quantity Surveying and am in my final year in collage.
I am doing my dissertation on the differences in cost and construction between timber frame and masonry houses based on the Irish industry.
One area i have to look into is a cost comparison and the way i hope to do this is based on a case study.In order to complete the case study I need to get the cost to construct a house that was priced in both timber frame and block built.

Any information would be helpful and much appreciated.

BryanF

drum11
may i ask, will you be considering or at least explaining the limitations of your study. outlining:

1. the time differences in site works, savings/expenses in labour, scaffolding hire, slow or quick to weather tightness etc
2. including or excluding air-tightness and associated costs
3. the differences between off-site v on-site tolerances and quality
4. what standard of specification/provisional BER rating/mvhr or wall vents etc (basically what the level of info on compliance with current regs is in the tender documents and what whether accredited details are used)
5. is the build time tabled for the summer or winter

the list could go on and on... you see my point this is often the archs or PM's issue with a black and white price between timber and Conc


best of luck with the study
ill pm you with some info (it might be a few days)

PJMCKE

hi ,
I see the pain you are going through.
I went down this route and cellulose and decrement delay is important. So is the diffusion openness of your material choice. I put a blogspot of my build that is now finished if you find it of any use.
www.knocksaxonblogspot.com
I used boards for my build and so far so good. I don't like giving out the blogspot but its all finished now so Its old news. . Best of luck. P.S. we have put in 500 euro since february in oil and still have loads of oil left.

brophis

Thanks, interesting read