EWI, roof insulation, air tightness. Sanity check.

Lumen

I think I've read all the threads on these subjects, just trying to bring it all together.

Detached 14x11m two-storey, 100/100/100 cavity with ~40mm polystyrene, roof is 23.5 degree pitch with no attic insulation. Crappy cement tiles which I'd like to replace, single glazed windows.

Place is not comfortable below 5C despite big new boiler, walls are cold and hallways are bordering on draughty.

I was going to pump the walls and replace windows but from everything I've read that seems futile with all the cold bridging and lack of air tightness, not to mention possible long term effects on wall ties and irreversibility if products degrade.

So new plan is: remove windows, doors, roof tiles, make the whole thing airtight (wrap?), add 200mm EWI down to foundations, solid insulation over the rafters, fit new triple glazed windows and doors into the EWI layer, french drain round the outside, new tiles on the roof (can't do natural slate due to low pitch, grrr), new gutters, downpipes, facia, soffits etc, add ventilation (DCV or baffled passive for simplicity?).

I'd rather not touch the cavity (no pumping) and keep all work external, using the cavity for services (I expect electrics are run through there already).

I'm ignoring cold floors (mix of solid and suspended) as I think that can be fixed in phase 2 if it's still cold, but maybe that's stupid if I'm trying to get airtightness.

Rough budget (incl. VAT):
Windows: 25k for 36sqm.
Doors: 15k for 16sqm (porch area, 2 side doors, patio slider).
EWI: 25k for 245sqm.
Roof: 20k for 170sqm (15m ridgeline), including external insulation and tiles.
---
Total 85k + contingency. Prob 100k.

Is there anything wrong with this plan?

How "normal" is this approach now? (i.e. will a good ordinary architect be able to run it, or do I need someone with near-passive retrofit experience).

Metric Tensor

I had a whole long post written and then I spotted your "airtightness - wrap" comment - which is what my long post was getting at.

How do you intend to accomplish this "wrapping effect" - that will be key because if you keep the cavity open the entire cavity has to be inside the air-tightness envelope of the very expensive EWI will be a total waste.

BryanF

Look for an arch/ arch tech who is willing to solve the detailing challenges. Your budget sounds reasonable. it’ll be any extras to the internala that will drive up the costs - assuming the structure is sound (eng to check once opening up works begin)

Are you sure the wiring is in the cavity? Does this suggest its time for a rewire? I’d pump the cavity if possible.

Lumen

Metric Tensor wrote: »

I had a whole long post written and then I spotted your "airtightness - wrap" comment - which is what my long post was getting at.

How do you intend to accomplish this "wrapping effect" - that will be key because if you keep the cavity open the entire cavity has to be inside the air-tightness envelope of the very expensive EWI will be a total waste.

I was thinking of a continuous VCL (Intello or similar) running under the EWI and under the rigid external roof insulation (over the outer leaf and over the existing rafters). Like a Goretex tea cosy.

With the windows and doors fitted on the EWI layer, the whole of the existing masonry structure would be warm and dry.

I'm probably missing something fundamental.

For the sake of clarity I'm not intending to spec or manage this myself, I'm just trying to get my head around best practice.

Lumen

BryanF wrote: »

Are you sure the wiring is in the cavity? Does this suggest its time for a rewire? I’d pump the cavity if possible.

No, I'm not sure. I'll check tonight (I can peer into the gable end cavity from the attic).

I just like the idea of being able to use the existing cavity as a giant service duct. Like if I want to throw a load of CAT5 around the house without pulling up the flooring.

If my electrician reads this he'll probably have a coronary.

I guess you're suggesting that best practice is to keep wiring out of the cavity and pump it.

sydthebeat

http://www.seai.ie/resources/publications/20170804_Code-of-practice-technical-Specification-rev-7-2.pdf

page 25 - 30 is a good starter

MicktheMan

How will you mechanically fix the external insulation batts to the wall without significantly compromising the AT layer? Same question for the roof.

Lumen

MicktheMan wrote: »

How will you mechanically fix the external insulation batts to the wall without significantly compromising the AT layer? Same question for the roof.

Good point. Mechanical fixings through the membrane, I guess. But looking at a few system descriptions it seems like mechanical fixing is secondary to adhesive.

So the question is: in a retrofit EWI project on a block CW, how to do you ensure sufficient airtightness?

I've read in a few places that EWI on its own does not give airtightness. I assumed this means I have to achieve airtightness with a membrane, since
bonded bead in the cavity doesn't give airtightness and closed cell foam in the cavity isn't certified by NSAI or BBA (although some installers advertise this use).

Or is it just a question of detailing around the windows, doors, and roof junctions?

Or is this the point I just give up asking for free answers to dumb questions and engage a professional? :pac:

MicktheMan

Lumen wrote: »

1. Good point. Mechanical fixings through the membrane, I guess. But looking at a few system descriptions it seems like mechanical fixing is secondary to adhesive.

2. So the question is: in a retrofit EWI project on a block CW, how to do you ensure sufficient airtightness?

3. I've read in a few places that EWI on its own does not give airtightness. I assumed this means I have to achieve airtightness with a membrane, since
bonded bead in the cavity doesn't give airtightness and closed cell foam in the cavity isn't certified by NSAI or BBA (although some installers advertise this use).

4. Or is it just a question of detailing around the windows, doors, and roof junctions?

5. Or is this the point I just give up asking for free answers to dumb questions and engage a professional? :pac:

1. Adhesive to what? The membrane?
2. From the inside
3. See 2
4. Depends on what's there now; for example sand/cement internal render on external walls is good AT layer whereas insulated slab on bare block is very poor.
5. No such thing as dumb q's;). Quite often, it's better to have done some research before engaging professionals.

Lumen

MicktheMan wrote: »

1. Adhesive to what? The membrane?

Exactly, I get your point.

MicktheMan wrote: »

2. From the inside
3. See 2
4. Depends on what's there now; for example sand/cement internal render on external walls is good AT layer whereas insulated slab on bare block is very poor.

OK, I don't know. I think it's regular skimmed plasterboard, 1987 vintage.

I'm less concerned about the plaster itself and more concerned with circulation of air around the structure, e.g.

Typical two storey masonry houses also face common problems. Kyne continues: “The most common problem here is also in the floor void. Joists run across the width of the house where they are butted into the wall, where they usually rest on the inner leaf of the cavity wall. The most common method is to butter up around the joists with mortar, but if they aren’t sealed up correctly any air free in the cavity can get into the floor void, and the problem is especially bad with partial fill cavity walls.” .

Source: https://passivehouseplus.ie/articles/ventilation/air-tight-case

MicktheMan wrote: »

5. No such thing as dumb q's;). Quite often, it's better to have done some research before engaging professionals.

Thanks!

Lumen

I suppose what I really don't get is: what's the big deal with airtightness in an EWI'd block cavity house? There's three coats of cement render/plaster and a load of continuous insulation across a two-foot thick build up. Isn't that sufficient?

Metric Tensor

I'm with Micktheman all the way on this.

If the inside of your wall is slabbed it's a disaster for air-tightness. The worst performing houses from an AT point of view are the 1970s/80s houses with open cavities and internal "dry-lining"*

So:

1. Rip off the internal dry lining and roofing.
(2. With any luck your wiring is behind the dry lining!)
3. Check and fix wiring layout.
4. Pump cavity
5. 2 coat sand and cement on the internal leaf with AT tapes and membranes at doors, windows, floors, ceilings, etc.
6. Continue AT layer to underside of rafters.
7. External insulate roof and walls.
8. New roofing, fascia, soffit, etc.
8. New windows and doors taped to sand cement layer previously discussed.
9. Install ventilation carefully!
10. Let herself free in the hardware with the remaining cash!!!




*I don't have the reference to the study to hand but I will try to find it tomorrow if I have time.

MicktheMan

Lumen wrote: »

I suppose what I really don't get is: what's the big deal with airtightness in an EWI'd block cavity house? There's three coats of cement render/plaster and a load of continuous insulation across a two-foot thick build up. Isn't that sufficient?

The ewi addresses the conduction heat losses only. Conduction heat losses are driven by temperature diff from inside to outside. The larger this diff the more important the insulation detailing.
The AT barrier addresses convection heat losses aka draughts. This barrier needs ideally to be continuous to be effective and, imo, is THE most important part of the thermal envelope for reducing heat loss and comfort in our mild / windy climate. No amount of bits or layers of air barriers are any good if a continuous layer is not achieved and this continuous layer needs to be on the warm side of the insulation.

Lumen

Metric Tensor wrote: »

*I don't have the reference to the study to hand but I will try to find it tomorrow if I have time.

This?

http://www.energyaction.ie/doc/2012/EnergyActionconferenceEWIworkshop2012.pdf

Metric Tensor

I recall a slightly different one that specifically mentioned dry lining but I could be mistaken remembering. It's a few years since I saw it.

Lumen

Ok, this is what I was imagining to give airtight layer external to existing wall and rafters:

http://europhit.eu/products-focus#ROOF%20TO%20WALL%20CONNECTIONS



Design brief:
http://europhit.eu/sites/europhit.eu/files/Design%20briefs/EuroPHit_D5.1.5a_Roof_Wall%20Junction_LMP.pdf

edit: That design is for a single leaf masonry wall, I have a cavity, I have no idea what the detailing is like at the bottom or what that means for airtightness.