Timber Frame Wall setup . Condensation problem.Vapour permeability

[Patrick]

Maybe this post is in the wrong section, but I am trying to figure out if this works just fine or if it is going to give me problems:

 

 

I am trying to build it in that way, because it is to my limited knowledge, fairly cheap, compared to staggering different insulation Materials and Insulating the Outside of the Building.

Most of you on here who worked with Timber I-Beam construction opted for a thinner wall and to insulate on the outside .

Which is great, just an extra step of work, but thats just my opinion.

My main worry on this setup is condensation. Because the outer layer of OSB, even when left untaped is not very Vapour  permeable :

 

So following this logic, there could be a problem with condensation on the backside(looking from inside - outside) of the OSB sheets, as there is always going to be inperfections in the inner Airtight layer and Moisture going to get into the Insualtion .

 

Am I overcomplicating this or is that something that needs to be addressed?

 

 

 

[Big Jimbo]

All the smart people will be along soon. That thickness has got to be giving you a good "U" value though ?

[Patrick]

5 minutes ago, Big Jimbo said:

All the smart people will be along soon. That thickness has got to be giving you a good "U" value though ?

To both a yes, I hope .

[Russell griffiths]

What I will say is, have you priced up those I joists

thinking this will be a cheap way to build a wall I think is wrong. 

 

I have just  had my i joists for my roof delivered, the cost of the i joist instead of standard timber was far more than double the price. 

 

Looking at an mbc twin twin stud wall, this is a cheaper method, it sticks to standard cls sized timber with spacer blocks. 

 

I joists are exspensive. 

[Patrick]

@Russell griffiths that might be right. I-Joist is more expensive for sure.As of price, I was more thinking about the extra work involved switching between systems (like insulating I-Joists between and than putting a different/separate insualtion on the outside ) .

 

I might as well be wrong with this.Generally I can say that the Structural Timber for the House seems to be the least expensive of the overall costs . And seems to me so far that you can safe a great deal by carefully looking for cheaper suppliers.

Saying that, you still are right with something like the MBC twin stud . Material wise  a lot cheaper. I think price might get closer once constructed due to labour cost.

 

Would you be happy to share what you paid for your I-Joists? Either here or  PM .

 

[the_r_sole]

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Edited September 26, 2019 by the_r_sole

[Ed Davies]

2 hours ago, Patrick said:

Because the outer layer of OSB, even when left untaped is not very Vapour  permeable :

 

It's weird. The numbers for the permeability of OSB are all over the place, it's the sort of thing people do PhD's on, literally.

 

https://edavies.me.uk/2014/04/osb_vapour/

https://www.greenbuildingadvisor.com/article/is-oriented-strand-board-as-impermeable-as-they-say

 

Nobody (house designer/BCO) blinked at my using 9mm OSB on the gables of my house (cladding vertical I-beams). OTOH, they were very bothered about using it on the roof. Not really sure, why, perhaps because the roof would have needed thicker OSB (18mm, I assume). OTOH, I see plenty of houses going up with T&G OSB on the roof. I'm confused.

Edited March 8, 2019 by Ed Davies

[Patrick]

@Ed Davies Now thats a really usefull read. Thanks a lot.

 

Edited March 8, 2019 by Patrick
Spelling

[Russell griffiths]

I can’t really say what I paid for my i joists, as I had lots of other timber in with the same order

i just remember when i first started looking I priced up a 225x47 timber and it was under half what the same i joist would have been

 

however if you can use a wider but thinner i joist like a 300x45 these get cheaper as the cost is the top and bottom flange not the web. 

 

I could have saved £1500 on my roof if I wanted to use a 300 deep joist instead of 220. 

[Simplysimon]

with 360mm of i beams your u value will be at .1 so is the internal insulation really required and will make the fitting of insulation more awkward and leave cooler areas? i'm using a similar build up 350mm i beams, cellulose fill, egger dhf outside, osb internally either smartply osb or standard vcl (haven't decided yet), service cavity and pb.

[Patrick]

@Simplysimon
 

Sounds good. My internal insulation is only because there is empty space anyways. Might as well insulate the service cavity I thought. Just pushing down a few cheap rock wool pieces myself. Main insulation will be the 360mm. I wanted to do 350like you, but most ibeam companies only provide 360mm. Apart from 2, so wanted to have more choice getting quotes. I guess you are going with a certain timber frame company that likes to use the egger dhf outside  ??

 Any idea what bonus the outside insulation will give?

[Simplysimon]

nope, they didn't like the idea of steel after updating their quote. buy a load of i beams and make your own, massivly cheaper ? 1st quote expensive, updated quote 50% increase. seems like they've got a good base and therefore if you can afford the passive title you can afford the kit, ludicrous. i wouldn't have thought the int. ins. would really be worth the difference in u value especially as you're using solid timber strapping which will need either cutting or boring for services. batten horizontally leaving spaces for cabling for swtches and counter batten vertically for pb.

[Patrick]

Yep. You're right in regard to the timber frame company. Exactly my experience. So I'm doing the same than you. But I guess you copied their setup. Or what's your reason for outside insulation?

[Simplysimon]

if a system works there's not much point in changing it unless it can be improved, hence the smartply option. which outside insulation?

 

[Patrick]

2 hours ago, Simplysimon said:

i'm using a similar build up 350mm i beams, cellulose fill, egger dhf outside, osb internally either smartply osb 

Egger dhf is insulation board? Or not.

[Simplysimon]

similar to mdf looking at spec though maybe a bit coarser https://www.egger.com/shop/en_IE//DHF-DHF-underlay-board/p/DHF  will have some insulation properties, though i assume minimal. it's the use as a air tight membrane that's useful, putting it on the outside away from damage from follow on trades. d4 and seal all joints and d4 to i beams.